[v5,4/4] rust: add abstraction for `struct page`

Commit Message

Alice Ryhl April 15, 2024, 7:13 a.m. UTC

Adds a new struct called `Page` that wraps a pointer to `struct page`.
This struct is assumed to hold ownership over the page, so that Rust
code can allocate and manage pages directly.

The page type has various methods for reading and writing into the page.
These methods will temporarily map the page to allow the operation. All
of these methods use a helper that takes an offset and length, performs
bounds checks, and returns a pointer to the given offset in the page.

This patch only adds support for pages of order zero, as that is all
Rust Binder needs. However, it is written to make it easy to add support
for higher-order pages in the future. To do that, you would add a const
generic parameter to `Page` that specifies the order. Most of the
methods do not need to be adjusted, as the logic for dealing with
mapping multiple pages at once can be isolated to just the
`with_pointer_into_page` method.

Rust Binder needs to manage pages directly as that is how transactions
are delivered: Each process has an mmap'd region for incoming
transactions. When an incoming transaction arrives, the Binder driver
will choose a region in the mmap, allocate and map the relevant pages
manually, and copy the incoming transaction directly into the page. This
architecture allows the driver to copy transactions directly from the
address space of one process to another, without an intermediate copy
to a kernel buffer.

This code is based on Wedson's page abstractions from the old rust
branch, but it has been modified by Alice by removing the incomplete
support for higher-order pages, by introducing the `with_*` helpers
to consolidate the bounds checking logic into a single place, and by
introducing gfp flags.

Co-developed-by: Wedson Almeida Filho <[e-mail]>
Signed-off-by: Wedson Almeida Filho <[e-mail]>
Signed-off-by: Alice Ryhl <[e-mail]>
---
 rust/bindings/bindings_helper.h |   2 +
 rust/helpers.c                  |  20 ++++
 rust/kernel/lib.rs              |   1 +
 rust/kernel/page.rs             | 240 ++++++++++++++++++++++++++++++++++++++++
 4 files changed, 263 insertions(+)

Comments

Andreas Hindborg April 15, 2024, 8:10 a.m. UTC | #1

Alice Ryhl <[e-mail]> writes:

> Adds a new struct called `Page` that wraps a pointer to `struct page`.
> This struct is assumed to hold ownership over the page, so that Rust
> code can allocate and manage pages directly.
>
> The page type has various methods for reading and writing into the page.
> These methods will temporarily map the page to allow the operation. All
> of these methods use a helper that takes an offset and length, performs
> bounds checks, and returns a pointer to the given offset in the page.
>
> This patch only adds support for pages of order zero, as that is all
> Rust Binder needs. However, it is written to make it easy to add support
> for higher-order pages in the future. To do that, you would add a const
> generic parameter to `Page` that specifies the order. Most of the
> methods do not need to be adjusted, as the logic for dealing with
> mapping multiple pages at once can be isolated to just the
> `with_pointer_into_page` method.
>
> Rust Binder needs to manage pages directly as that is how transactions
> are delivered: Each process has an mmap'd region for incoming
> transactions. When an incoming transaction arrives, the Binder driver
> will choose a region in the mmap, allocate and map the relevant pages
> manually, and copy the incoming transaction directly into the page. This
> architecture allows the driver to copy transactions directly from the
> address space of one process to another, without an intermediate copy
> to a kernel buffer.
>
> This code is based on Wedson's page abstractions from the old rust
> branch, but it has been modified by Alice by removing the incomplete
> support for higher-order pages, by introducing the `with_*` helpers
> to consolidate the bounds checking logic into a single place, and by
> introducing gfp flags.
>
> Co-developed-by: Wedson Almeida Filho <[e-mail]>
> Signed-off-by: Wedson Almeida Filho <[e-mail]>
> Signed-off-by: Alice Ryhl <[e-mail]>
> ---

Reviewed-by: Andreas Hindborg <[e-mail]>

Benno Lossin April 15, 2024, 9:54 a.m. UTC | #2

On 15.04.24 09:13, Alice Ryhl wrote:
> Adds a new struct called `Page` that wraps a pointer to `struct page`.
> This struct is assumed to hold ownership over the page, so that Rust
> code can allocate and manage pages directly.
> 
> The page type has various methods for reading and writing into the page.
> These methods will temporarily map the page to allow the operation. All
> of these methods use a helper that takes an offset and length, performs
> bounds checks, and returns a pointer to the given offset in the page.
> 
> This patch only adds support for pages of order zero, as that is all
> Rust Binder needs. However, it is written to make it easy to add support
> for higher-order pages in the future. To do that, you would add a const
> generic parameter to `Page` that specifies the order. Most of the
> methods do not need to be adjusted, as the logic for dealing with
> mapping multiple pages at once can be isolated to just the
> `with_pointer_into_page` method.
> 
> Rust Binder needs to manage pages directly as that is how transactions
> are delivered: Each process has an mmap'd region for incoming
> transactions. When an incoming transaction arrives, the Binder driver
> will choose a region in the mmap, allocate and map the relevant pages
> manually, and copy the incoming transaction directly into the page. This
> architecture allows the driver to copy transactions directly from the
> address space of one process to another, without an intermediate copy
> to a kernel buffer.
> 
> This code is based on Wedson's page abstractions from the old rust
> branch, but it has been modified by Alice by removing the incomplete
> support for higher-order pages, by introducing the `with_*` helpers
> to consolidate the bounds checking logic into a single place, and by
> introducing gfp flags.
> 
> Co-developed-by: Wedson Almeida Filho <[e-mail]>
> Signed-off-by: Wedson Almeida Filho <[e-mail]>
> Signed-off-by: Alice Ryhl <[e-mail]>
> ---
>  rust/bindings/bindings_helper.h |   2 +
>  rust/helpers.c                  |  20 ++++
>  rust/kernel/lib.rs              |   1 +
>  rust/kernel/page.rs             | 240 ++++++++++++++++++++++++++++++++++++++++
>  4 files changed, 263 insertions(+)

Reviewed-by: Benno Lossin <[e-mail]>

Trevor Gross April 16, 2024, 5:40 a.m. UTC | #3

On Mon, Apr 15, 2024 at 3:15 AM Alice Ryhl <[e-mail]> wrote:
>
> Adds a new struct called `Page` that wraps a pointer to `struct page`.
> This struct is assumed to hold ownership over the page, so that Rust
> code can allocate and manage pages directly.
>
> The page type has various methods for reading and writing into the page.
> These methods will temporarily map the page to allow the operation. All
> of these methods use a helper that takes an offset and length, performs
> bounds checks, and returns a pointer to the given offset in the page.
>
> This patch only adds support for pages of order zero, as that is all
> Rust Binder needs. However, it is written to make it easy to add support
> for higher-order pages in the future. To do that, you would add a const
> generic parameter to `Page` that specifies the order. Most of the
> methods do not need to be adjusted, as the logic for dealing with
> mapping multiple pages at once can be isolated to just the
> `with_pointer_into_page` method.
>
> Rust Binder needs to manage pages directly as that is how transactions
> are delivered: Each process has an mmap'd region for incoming
> transactions. When an incoming transaction arrives, the Binder driver
> will choose a region in the mmap, allocate and map the relevant pages
> manually, and copy the incoming transaction directly into the page. This
> architecture allows the driver to copy transactions directly from the
> address space of one process to another, without an intermediate copy
> to a kernel buffer.
>
> This code is based on Wedson's page abstractions from the old rust
> branch, but it has been modified by Alice by removing the incomplete
> support for higher-order pages, by introducing the `with_*` helpers
> to consolidate the bounds checking logic into a single place, and by
> introducing gfp flags.
>
> Co-developed-by: Wedson Almeida Filho <[e-mail]>
> Signed-off-by: Wedson Almeida Filho <[e-mail]>
> Signed-off-by: Alice Ryhl <[e-mail]>

I have a couple questions about naming, and think an example would be
good for the functions that are trickier to use correctly. But I
wouldn't block on this, implementation looks good to me.

Reviewed-by: Trevor Gross <[e-mail]>

> +++ b/rust/kernel/page.rs
> @@ -0,0 +1,240 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +//! Kernel page allocation and management.
> +
> +use crate::{bindings, error::code::*, error::Result, uaccess::UserSliceReader};
> +use core::{
> +    alloc::AllocError,
> +    ptr::{self, NonNull},
> +};
> +
> +/// A bitwise shift for the page size.
> +pub const PAGE_SHIFT: usize = bindings::PAGE_SHIFT as usize;
> +
> +/// The number of bytes in a page.
> +pub const PAGE_SIZE: usize = bindings::PAGE_SIZE;
> +
> +/// A bitmask that gives the page containing a given address.
> +pub const PAGE_MASK: usize = !(PAGE_SIZE - 1);
> +
> +/// Flags for the "get free page" function that underlies all memory allocations.
> +pub mod flags {
> +    /// gfp flags.

Uppercase acronym, maybe with a description:

    GFP (Get Free Page) flags.

> +    #[allow(non_camel_case_types)]
> +    pub type gfp_t = bindings::gfp_t;

Why not GfpFlags, do we do this elsewhere?

> +    /// `GFP_KERNEL` is typical for kernel-internal allocations. The caller requires `ZONE_NORMAL`
> +    /// or a lower zone for direct access but can direct reclaim.
> +    pub const GFP_KERNEL: gfp_t = bindings::GFP_KERNEL;
> +    /// `GFP_ZERO` returns a zeroed page on success.
> +    pub const __GFP_ZERO: gfp_t = bindings::__GFP_ZERO;
> +    /// `GFP_HIGHMEM` indicates that the allocated memory may be located in high memory.
> +    pub const __GFP_HIGHMEM: gfp_t = bindings::__GFP_HIGHMEM;

It feels a bit weird to have dunder constants on the rust side that
aren't also `#[doc(hidden)]` or just nonpublic. Makes me think they
are an implementation detail or not really meant to be used - could
you update the docs if this is the case?

> +
> +impl Page {
> +    /// Allocates a new page.

Could you add a small example here?

> +    pub fn alloc_page(gfp_flags: flags::gfp_t) -> Result<Self, AllocError> {
> [...]
> +    }
> +
> +    /// Returns a raw pointer to the page.

Could you add a note about how the pointer needs to be used correctly,
if it is for anything more than interfacing with kernel APIs?

> +    pub fn as_ptr(&self) -> *mut bindings::page {
> +        self.page.as_ptr()
> +    }
> +
> +    /// Runs a piece of code with this page mapped to an address.
> +    ///
> +    /// The page is unmapped when this call returns.
> +    ///
> +    /// # Using the raw pointer
> +    ///
> +    /// It is up to the caller to use the provided raw pointer correctly. The pointer is valid for
> +    /// `PAGE_SIZE` bytes and for the duration in which the closure is called. The pointer might
> +    /// only be mapped on the current thread, and when that is the case, dereferencing it on other
> +    /// threads is UB. Other than that, the usual rules for dereferencing a raw pointer apply: don't
> +    /// cause data races, the memory may be uninitialized, and so on.
> +    ///
> +    /// If multiple threads map the same page at the same time, then they may reference with
> +    /// different addresses. However, even if the addresses are different, the underlying memory is
> +    /// still the same for these purposes (e.g., it's still a data race if they both write to the
> +    /// same underlying byte at the same time).
> +    fn with_page_mapped<T>(&self, f: impl FnOnce(*mut u8) -> T) -> T {
> [...]
> +    }

Could you add an example of how to use this correctly?

> +    /// Runs a piece of code with a raw pointer to a slice of this page, with bounds checking.
> +    ///
> +    /// If `f` is called, then it will be called with a pointer that points at `off` bytes into the
> +    /// page, and the pointer will be valid for at least `len` bytes. The pointer is only valid on
> +    /// this task, as this method uses a local mapping.
> +    ///
> +    /// If `off` and `len` refers to a region outside of this page, then this method returns
> +    /// `EINVAL` and does not call `f`.
> +    ///
> +    /// # Using the raw pointer
> +    ///
> +    /// It is up to the caller to use the provided raw pointer correctly. The pointer is valid for
> +    /// `len` bytes and for the duration in which the closure is called. The pointer might only be
> +    /// mapped on the current thread, and when that is the case, dereferencing it on other threads
> +    /// is UB. Other than that, the usual rules for dereferencing a raw pointer apply: don't cause
> +    /// data races, the memory may be uninitialized, and so on.
> +    ///
> +    /// If multiple threads map the same page at the same time, then they may reference with
> +    /// different addresses. However, even if the addresses are different, the underlying memory is
> +    /// still the same for these purposes (e.g., it's still a data race if they both write to the
> +    /// same underlying byte at the same time).

This could probably also use an example. A note about how to select
between with_pointer_into_page and with_page_mapped would also be nice
to guide usage, e.g. "prefer with_pointer_into_page for all cases
except when..."

> +    fn with_pointer_into_page<T>(
> +        &self,
> +        off: usize,
> +        len: usize,
> +        f: impl FnOnce(*mut u8) -> Result<T>,
> +    ) -> Result<T> {
> [...]
> +    /// Maps the page and zeroes the given slice.
> +    ///
> +    /// This method will perform bounds checks on the page offset. If `offset .. offset+len` goes
> +    /// outside ot the page, then this call returns `EINVAL`.
> +    ///
> +    /// # Safety
> +    ///
> +    /// Callers must ensure that this call does not race with a read or write to the same page that
> +    /// overlaps with this write.
> +    pub unsafe fn fill_zero(&self, offset: usize, len: usize) -> Result {
> +        self.with_pointer_into_page(offset, len, move |dst| {
> +            // SAFETY: If `with_pointer_into_page` calls into this closure, then it has performed a
> +            // bounds check and guarantees that `dst` is valid for `len` bytes.
> +            //
> +            // There caller guarantees that there is no data race.
> +            unsafe { ptr::write_bytes(dst, 0u8, len) };
> +            Ok(())
> +        })
> +    }

Could this be named `fill_zero_raw` to leave room for a safe
`fill_zero(&mut self, ...)`?

> +    /// Copies data from userspace into this page.
> +    ///
> +    /// This method will perform bounds checks on the page offset. If `offset .. offset+len` goes
> +    /// outside ot the page, then this call returns `EINVAL`.
> +    ///
> +    /// Like the other `UserSliceReader` methods, data races are allowed on the userspace address.
> +    /// However, they are not allowed on the page you are copying into.
> +    ///
> +    /// # Safety
> +    ///
> +    /// Callers must ensure that this call does not race with a read or write to the same page that
> +    /// overlaps with this write.
> +    pub unsafe fn copy_from_user_slice(
> +        &self,
> +        reader: &mut UserSliceReader,
> +        offset: usize,
> +        len: usize,
> +    ) -> Result {
> +        self.with_pointer_into_page(offset, len, move |dst| {
> +            // SAFETY: If `with_pointer_into_page` calls into this closure, then it has performed a
> +            // bounds check and guarantees that `dst` is valid for `len` bytes. Furthermore, we have
> +            // exclusive access to the slice since the caller guarantees that there are no races.
> +            reader.read_raw(unsafe { core::slice::from_raw_parts_mut(dst.cast(), len) })
> +        })
> +    }
> +}

Same as above, `copy_from_user_slice_raw` would leave room for a safe API.

Alice Ryhl April 16, 2024, 9:53 a.m. UTC | #4

Trevor Gross <[e-mail]> writes:
> On Mon, Apr 15, 2024 at 3:15 AM Alice Ryhl <[e-mail]> wrote:
>>
>> Adds a new struct called `Page` that wraps a pointer to `struct page`.
>> This struct is assumed to hold ownership over the page, so that Rust
>> code can allocate and manage pages directly.
>>
>> The page type has various methods for reading and writing into the page.
>> These methods will temporarily map the page to allow the operation. All
>> of these methods use a helper that takes an offset and length, performs
>> bounds checks, and returns a pointer to the given offset in the page.
>>
>> This patch only adds support for pages of order zero, as that is all
>> Rust Binder needs. However, it is written to make it easy to add support
>> for higher-order pages in the future. To do that, you would add a const
>> generic parameter to `Page` that specifies the order. Most of the
>> methods do not need to be adjusted, as the logic for dealing with
>> mapping multiple pages at once can be isolated to just the
>> `with_pointer_into_page` method.
>>
>> Rust Binder needs to manage pages directly as that is how transactions
>> are delivered: Each process has an mmap'd region for incoming
>> transactions. When an incoming transaction arrives, the Binder driver
>> will choose a region in the mmap, allocate and map the relevant pages
>> manually, and copy the incoming transaction directly into the page. This
>> architecture allows the driver to copy transactions directly from the
>> address space of one process to another, without an intermediate copy
>> to a kernel buffer.
>>
>> This code is based on Wedson's page abstractions from the old rust
>> branch, but it has been modified by Alice by removing the incomplete
>> support for higher-order pages, by introducing the `with_*` helpers
>> to consolidate the bounds checking logic into a single place, and by
>> introducing gfp flags.
>>
>> Co-developed-by: Wedson Almeida Filho <[e-mail]>
>> Signed-off-by: Wedson Almeida Filho <[e-mail]>
>> Signed-off-by: Alice Ryhl <[e-mail]>
> 
> I have a couple questions about naming, and think an example would be
> good for the functions that are trickier to use correctly. But I
> wouldn't block on this, implementation looks good to me.
> 
> Reviewed-by: Trevor Gross <[e-mail]>

Thanks for taking a look!

>> +/// Flags for the "get free page" function that underlies all memory allocations.
>> +pub mod flags {
>> +    /// gfp flags.
> 
> Uppercase acronym, maybe with a description:
> 
>     GFP (Get Free Page) flags.
> 
>> +    #[allow(non_camel_case_types)]
>> +    pub type gfp_t = bindings::gfp_t;
> 
> Why not GfpFlags, do we do this elsewhere?
> 
>> +    /// `GFP_KERNEL` is typical for kernel-internal allocations. The caller requires `ZONE_NORMAL`
>> +    /// or a lower zone for direct access but can direct reclaim.
>> +    pub const GFP_KERNEL: gfp_t = bindings::GFP_KERNEL;
>> +    /// `GFP_ZERO` returns a zeroed page on success.
>> +    pub const __GFP_ZERO: gfp_t = bindings::__GFP_ZERO;
>> +    /// `GFP_HIGHMEM` indicates that the allocated memory may be located in high memory.
>> +    pub const __GFP_HIGHMEM: gfp_t = bindings::__GFP_HIGHMEM;
> 
> It feels a bit weird to have dunder constants on the rust side that
> aren't also `#[doc(hidden)]` or just nonpublic. Makes me think they
> are an implementation detail or not really meant to be used - could
> you update the docs if this is the case?

All of this is going away in the next version because it will be based
on [1], which defines the gfp flags type for us.

[1]: https://lore.kernel.org/rust-for-linux/[e-mail]/

>> +
>> +impl Page {
>> +    /// Allocates a new page.
> 
> Could you add a small example here?

I can add an example that shows how to pass gfp flags.

>> +    pub fn alloc_page(gfp_flags: flags::gfp_t) -> Result<Self, AllocError> {
>> [...]
>> +    }
>> +
>> +    /// Returns a raw pointer to the page.
> 
> Could you add a note about how the pointer needs to be used correctly,
> if it is for anything more than interfacing with kernel APIs?

I can clarify that it's a pointer to the `struct page` and not the
actual PAGE_SIZE bytes stored in the page, and that it's for use with
the raw C apis. I won't go into more details than that.

>> +    pub fn as_ptr(&self) -> *mut bindings::page {
>> +        self.page.as_ptr()
>> +    }
>> +
>> +    /// Runs a piece of code with this page mapped to an address.
>> +    ///
>> +    /// The page is unmapped when this call returns.
>> +    ///
>> +    /// # Using the raw pointer
>> +    ///
>> +    /// It is up to the caller to use the provided raw pointer correctly. The pointer is valid for
>> +    /// `PAGE_SIZE` bytes and for the duration in which the closure is called. The pointer might
>> +    /// only be mapped on the current thread, and when that is the case, dereferencing it on other
>> +    /// threads is UB. Other than that, the usual rules for dereferencing a raw pointer apply: don't
>> +    /// cause data races, the memory may be uninitialized, and so on.
>> +    ///
>> +    /// If multiple threads map the same page at the same time, then they may reference with
>> +    /// different addresses. However, even if the addresses are different, the underlying memory is
>> +    /// still the same for these purposes (e.g., it's still a data race if they both write to the
>> +    /// same underlying byte at the same time).
>> +    fn with_page_mapped<T>(&self, f: impl FnOnce(*mut u8) -> T) -> T {
>> [...]
>> +    }
> 
> Could you add an example of how to use this correctly?

This is a private function, you're not supposed to use it directly.
Anyone who is modifying this file directly can look at the existing
users for examples.

>> +    /// Runs a piece of code with a raw pointer to a slice of this page, with bounds checking.
>> +    ///
>> +    /// If `f` is called, then it will be called with a pointer that points at `off` bytes into the
>> +    /// page, and the pointer will be valid for at least `len` bytes. The pointer is only valid on
>> +    /// this task, as this method uses a local mapping.
>> +    ///
>> +    /// If `off` and `len` refers to a region outside of this page, then this method returns
>> +    /// `EINVAL` and does not call `f`.
>> +    ///
>> +    /// # Using the raw pointer
>> +    ///
>> +    /// It is up to the caller to use the provided raw pointer correctly. The pointer is valid for
>> +    /// `len` bytes and for the duration in which the closure is called. The pointer might only be
>> +    /// mapped on the current thread, and when that is the case, dereferencing it on other threads
>> +    /// is UB. Other than that, the usual rules for dereferencing a raw pointer apply: don't cause
>> +    /// data races, the memory may be uninitialized, and so on.
>> +    ///
>> +    /// If multiple threads map the same page at the same time, then they may reference with
>> +    /// different addresses. However, even if the addresses are different, the underlying memory is
>> +    /// still the same for these purposes (e.g., it's still a data race if they both write to the
>> +    /// same underlying byte at the same time).
> 
> This could probably also use an example. A note about how to select
> between with_pointer_into_page and with_page_mapped would also be nice
> to guide usage, e.g. "prefer with_pointer_into_page for all cases
> except when..."

Same as above. This is a private function.

>> +    fn with_pointer_into_page<T>(
>> +        &self,
>> +        off: usize,
>> +        len: usize,
>> +        f: impl FnOnce(*mut u8) -> Result<T>,
>> +    ) -> Result<T> {
>> [...]
>> +    /// Maps the page and zeroes the given slice.
>> +    ///
>> +    /// This method will perform bounds checks on the page offset. If `offset .. offset+len` goes
>> +    /// outside ot the page, then this call returns `EINVAL`.
>> +    ///
>> +    /// # Safety
>> +    ///
>> +    /// Callers must ensure that this call does not race with a read or write to the same page that
>> +    /// overlaps with this write.
>> +    pub unsafe fn fill_zero(&self, offset: usize, len: usize) -> Result {
>> +        self.with_pointer_into_page(offset, len, move |dst| {
>> +            // SAFETY: If `with_pointer_into_page` calls into this closure, then it has performed a
>> +            // bounds check and guarantees that `dst` is valid for `len` bytes.
>> +            //
>> +            // There caller guarantees that there is no data race.
>> +            unsafe { ptr::write_bytes(dst, 0u8, len) };
>> +            Ok(())
>> +        })
>> +    }
> 
> Could this be named `fill_zero_raw` to leave room for a safe
> `fill_zero(&mut self, ...)`?

I suppose I can rename these _raw as well.

>> +    /// Copies data from userspace into this page.
>> +    ///
>> +    /// This method will perform bounds checks on the page offset. If `offset .. offset+len` goes
>> +    /// outside ot the page, then this call returns `EINVAL`.
>> +    ///
>> +    /// Like the other `UserSliceReader` methods, data races are allowed on the userspace address.
>> +    /// However, they are not allowed on the page you are copying into.
>> +    ///
>> +    /// # Safety
>> +    ///
>> +    /// Callers must ensure that this call does not race with a read or write to the same page that
>> +    /// overlaps with this write.
>> +    pub unsafe fn copy_from_user_slice(
>> +        &self,
>> +        reader: &mut UserSliceReader,
>> +        offset: usize,
>> +        len: usize,
>> +    ) -> Result {
>> +        self.with_pointer_into_page(offset, len, move |dst| {
>> +            // SAFETY: If `with_pointer_into_page` calls into this closure, then it has performed a
>> +            // bounds check and guarantees that `dst` is valid for `len` bytes. Furthermore, we have
>> +            // exclusive access to the slice since the caller guarantees that there are no races.
>> +            reader.read_raw(unsafe { core::slice::from_raw_parts_mut(dst.cast(), len) })
>> +        })
>> +    }
>> +}
> 
> Same as above, `copy_from_user_slice_raw` would leave room for a safe API.

Okay

Alice

Trevor Gross April 16, 2024, 5:47 p.m. UTC | #5

On Tue, Apr 16, 2024 at 5:53 AM Alice Ryhl <[e-mail]> wrote:
>
> >> +/// Flags for the "get free page" function that underlies all memory allocations.
> >> +pub mod flags {
> >> +    /// gfp flags.
> >
> > Uppercase acronym, maybe with a description:
> >
> >     GFP (Get Free Page) flags.
> >
> >> +    #[allow(non_camel_case_types)]
> >> +    pub type gfp_t = bindings::gfp_t;
> >
> > Why not GfpFlags, do we do this elsewhere?
> >
> >> +    /// `GFP_KERNEL` is typical for kernel-internal allocations. The caller requires `ZONE_NORMAL`
> >> +    /// or a lower zone for direct access but can direct reclaim.
> >> +    pub const GFP_KERNEL: gfp_t = bindings::GFP_KERNEL;
> >> +    /// `GFP_ZERO` returns a zeroed page on success.
> >> +    pub const __GFP_ZERO: gfp_t = bindings::__GFP_ZERO;
> >> +    /// `GFP_HIGHMEM` indicates that the allocated memory may be located in high memory.
> >> +    pub const __GFP_HIGHMEM: gfp_t = bindings::__GFP_HIGHMEM;
> >
> > It feels a bit weird to have dunder constants on the rust side that
> > aren't also `#[doc(hidden)]` or just nonpublic. Makes me think they
> > are an implementation detail or not really meant to be used - could
> > you update the docs if this is the case?
>
> All of this is going away in the next version because it will be based
> on [1], which defines the gfp flags type for us.
>
> [1]: https://lore.kernel.org/rust-for-linux/[e-mail]/

Great, thanks for the link.

> > Could you add an example of how to use this correctly?
>
> This is a private function, you're not supposed to use it directly.
> Anyone who is modifying this file directly can look at the existing
> users for examples.

Ah you're right, missed this bit.

Thanks for the followup.

Trevor

Patch

diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
index 65b98831b975..da1e97871419 100644
--- a/rust/bindings/bindings_helper.h
+++ b/rust/bindings/bindings_helper.h
@@ -20,5 +20,7 @@ 
 
 /* `bindgen` gets confused at certain things. */
 const size_t RUST_CONST_HELPER_ARCH_SLAB_MINALIGN = ARCH_SLAB_MINALIGN;
+const size_t RUST_CONST_HELPER_PAGE_SIZE = PAGE_SIZE;
 const gfp_t RUST_CONST_HELPER_GFP_KERNEL = GFP_KERNEL;
 const gfp_t RUST_CONST_HELPER___GFP_ZERO = __GFP_ZERO;
+const gfp_t RUST_CONST_HELPER___GFP_HIGHMEM = ___GFP_HIGHMEM;
diff --git a/rust/helpers.c b/rust/helpers.c
index 312b6fcb49d5..72361003ba91 100644
--- a/rust/helpers.c
+++ b/rust/helpers.c
@@ -25,6 +25,8 @@ 
 #include <linux/build_bug.h>
 #include <linux/err.h>
 #include <linux/errname.h>
+#include <linux/gfp.h>
+#include <linux/highmem.h>
 #include <linux/mutex.h>
 #include <linux/refcount.h>
 #include <linux/sched/signal.h>
@@ -93,6 +95,24 @@  int rust_helper_signal_pending(struct task_struct *t)
 }
 EXPORT_SYMBOL_GPL(rust_helper_signal_pending);
 
+struct page *rust_helper_alloc_pages(gfp_t gfp_mask, unsigned int order)
+{
+	return alloc_pages(gfp_mask, order);
+}
+EXPORT_SYMBOL_GPL(rust_helper_alloc_pages);
+
+void *rust_helper_kmap_local_page(struct page *page)
+{
+	return kmap_local_page(page);
+}
+EXPORT_SYMBOL_GPL(rust_helper_kmap_local_page);
+
+void rust_helper_kunmap_local(const void *addr)
+{
+	kunmap_local(addr);
+}
+EXPORT_SYMBOL_GPL(rust_helper_kunmap_local);
+
 refcount_t rust_helper_REFCOUNT_INIT(int n)
 {
 	return (refcount_t)REFCOUNT_INIT(n);
diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index 37f84223b83f..667fc67fa24f 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -39,6 +39,7 @@ 
 pub mod kunit;
 #[cfg(CONFIG_NET)]
 pub mod net;
+pub mod page;
 pub mod prelude;
 pub mod print;
 mod static_assert;
diff --git a/rust/kernel/page.rs b/rust/kernel/page.rs
new file mode 100644
index 000000000000..f7f8870ddb66
--- /dev/null
+++ b/rust/kernel/page.rs
@@ -0,0 +1,240 @@ 
+// SPDX-License-Identifier: GPL-2.0
+
+//! Kernel page allocation and management.
+
+use crate::{bindings, error::code::*, error::Result, uaccess::UserSliceReader};
+use core::{
+    alloc::AllocError,
+    ptr::{self, NonNull},
+};
+
+/// A bitwise shift for the page size.
+pub const PAGE_SHIFT: usize = bindings::PAGE_SHIFT as usize;
+
+/// The number of bytes in a page.
+pub const PAGE_SIZE: usize = bindings::PAGE_SIZE;
+
+/// A bitmask that gives the page containing a given address.
+pub const PAGE_MASK: usize = !(PAGE_SIZE - 1);
+
+/// Flags for the "get free page" function that underlies all memory allocations.
+pub mod flags {
+    /// gfp flags.
+    #[allow(non_camel_case_types)]
+    pub type gfp_t = bindings::gfp_t;
+
+    /// `GFP_KERNEL` is typical for kernel-internal allocations. The caller requires `ZONE_NORMAL`
+    /// or a lower zone for direct access but can direct reclaim.
+    pub const GFP_KERNEL: gfp_t = bindings::GFP_KERNEL;
+    /// `GFP_ZERO` returns a zeroed page on success.
+    pub const __GFP_ZERO: gfp_t = bindings::__GFP_ZERO;
+    /// `GFP_HIGHMEM` indicates that the allocated memory may be located in high memory.
+    pub const __GFP_HIGHMEM: gfp_t = bindings::__GFP_HIGHMEM;
+}
+
+/// A pointer to a page that owns the page allocation.
+///
+/// # Invariants
+///
+/// The pointer is valid, and has ownership over the page.
+pub struct Page {
+    page: NonNull<bindings::page>,
+}
+
+// SAFETY: Pages have no logic that relies on them staying on a given thread, so moving them across
+// threads is safe.
+unsafe impl Send for Page {}
+
+// SAFETY: Pages have no logic that relies on them not being accessed concurrently, so accessing
+// them concurrently is safe.
+unsafe impl Sync for Page {}
+
+impl Page {
+    /// Allocates a new page.
+    pub fn alloc_page(gfp_flags: flags::gfp_t) -> Result<Self, AllocError> {
+        // SAFETY: Depending on the value of `gfp_flags`, this call may sleep. Other than that, it
+        // is always safe to call this method.
+        let page = unsafe { bindings::alloc_pages(gfp_flags, 0) };
+        let page = NonNull::new(page).ok_or(AllocError)?;
+        // INVARIANT: We just successfully allocated a page, so we now have ownership of the newly
+        // allocated page. We transfer that ownership to the new `Page` object.
+        Ok(Self { page })
+    }
+
+    /// Returns a raw pointer to the page.
+    pub fn as_ptr(&self) -> *mut bindings::page {
+        self.page.as_ptr()
+    }
+
+    /// Runs a piece of code with this page mapped to an address.
+    ///
+    /// The page is unmapped when this call returns.
+    ///
+    /// # Using the raw pointer
+    ///
+    /// It is up to the caller to use the provided raw pointer correctly. The pointer is valid for
+    /// `PAGE_SIZE` bytes and for the duration in which the closure is called. The pointer might
+    /// only be mapped on the current thread, and when that is the case, dereferencing it on other
+    /// threads is UB. Other than that, the usual rules for dereferencing a raw pointer apply: don't
+    /// cause data races, the memory may be uninitialized, and so on.
+    ///
+    /// If multiple threads map the same page at the same time, then they may reference with
+    /// different addresses. However, even if the addresses are different, the underlying memory is
+    /// still the same for these purposes (e.g., it's still a data race if they both write to the
+    /// same underlying byte at the same time).
+    fn with_page_mapped<T>(&self, f: impl FnOnce(*mut u8) -> T) -> T {
+        // SAFETY: `page` is valid due to the type invariants on `Page`.
+        let mapped_addr = unsafe { bindings::kmap_local_page(self.as_ptr()) };
+
+        let res = f(mapped_addr.cast());
+
+        // This unmaps the page mapped above.
+        //
+        // SAFETY: Since this API takes the user code as a closure, it can only be used in a manner
+        // where the pages are unmapped in reverse order. This is as required by `kunmap_local`.
+        //
+        // In other words, if this call to `kunmap_local` happens when a different page should be
+        // unmapped first, then there must necessarily be a call to `kmap_local_page` other than the
+        // call just above in `with_page_mapped` that made that possible. In this case, it is the
+        // unsafe block that wraps that other call that is incorrect.
+        unsafe { bindings::kunmap_local(mapped_addr) };
+
+        res
+    }
+
+    /// Runs a piece of code with a raw pointer to a slice of this page, with bounds checking.
+    ///
+    /// If `f` is called, then it will be called with a pointer that points at `off` bytes into the
+    /// page, and the pointer will be valid for at least `len` bytes. The pointer is only valid on
+    /// this task, as this method uses a local mapping.
+    ///
+    /// If `off` and `len` refers to a region outside of this page, then this method returns
+    /// `EINVAL` and does not call `f`.
+    ///
+    /// # Using the raw pointer
+    ///
+    /// It is up to the caller to use the provided raw pointer correctly. The pointer is valid for
+    /// `len` bytes and for the duration in which the closure is called. The pointer might only be
+    /// mapped on the current thread, and when that is the case, dereferencing it on other threads
+    /// is UB. Other than that, the usual rules for dereferencing a raw pointer apply: don't cause
+    /// data races, the memory may be uninitialized, and so on.
+    ///
+    /// If multiple threads map the same page at the same time, then they may reference with
+    /// different addresses. However, even if the addresses are different, the underlying memory is
+    /// still the same for these purposes (e.g., it's still a data race if they both write to the
+    /// same underlying byte at the same time).
+    fn with_pointer_into_page<T>(
+        &self,
+        off: usize,
+        len: usize,
+        f: impl FnOnce(*mut u8) -> Result<T>,
+    ) -> Result<T> {
+        let bounds_ok = off <= PAGE_SIZE && len <= PAGE_SIZE && (off + len) <= PAGE_SIZE;
+
+        if bounds_ok {
+            self.with_page_mapped(move |page_addr| {
+                // SAFETY: The `off` integer is at most `PAGE_SIZE`, so this pointer offset will
+                // result in a pointer that is in bounds or one off the end of the page.
+                f(unsafe { page_addr.add(off) })
+            })
+        } else {
+            Err(EINVAL)
+        }
+    }
+
+    /// Maps the page and reads from it into the given buffer.
+    ///
+    /// This method will perform bounds checks on the page offset. If `offset .. offset+len` goes
+    /// outside ot the page, then this call returns `EINVAL`.
+    ///
+    /// # Safety
+    ///
+    /// * Callers must ensure that `dst` is valid for writing `len` bytes.
+    /// * Callers must ensure that this call does not race with a write to the same page that
+    ///   overlaps with this read.
+    pub unsafe fn read_raw(&self, dst: *mut u8, offset: usize, len: usize) -> Result {
+        self.with_pointer_into_page(offset, len, move |src| {
+            // SAFETY: If `with_pointer_into_page` calls into this closure, then
+            // it has performed a bounds check and guarantees that `src` is
+            // valid for `len` bytes.
+            //
+            // There caller guarantees that there is no data race.
+            unsafe { ptr::copy_nonoverlapping(src, dst, len) };
+            Ok(())
+        })
+    }
+
+    /// Maps the page and writes into it from the given buffer.
+    ///
+    /// This method will perform bounds checks on the page offset. If `offset .. offset+len` goes
+    /// outside ot the page, then this call returns `EINVAL`.
+    ///
+    /// # Safety
+    ///
+    /// * Callers must ensure that `src` is valid for reading `len` bytes.
+    /// * Callers must ensure that this call does not race with a read or write to the same page
+    ///   that overlaps with this write.
+    pub unsafe fn write_raw(&self, src: *const u8, offset: usize, len: usize) -> Result {
+        self.with_pointer_into_page(offset, len, move |dst| {
+            // SAFETY: If `with_pointer_into_page` calls into this closure, then it has performed a
+            // bounds check and guarantees that `dst` is valid for `len` bytes.
+            //
+            // There caller guarantees that there is no data race.
+            unsafe { ptr::copy_nonoverlapping(src, dst, len) };
+            Ok(())
+        })
+    }
+
+    /// Maps the page and zeroes the given slice.
+    ///
+    /// This method will perform bounds checks on the page offset. If `offset .. offset+len` goes
+    /// outside ot the page, then this call returns `EINVAL`.
+    ///
+    /// # Safety
+    ///
+    /// Callers must ensure that this call does not race with a read or write to the same page that
+    /// overlaps with this write.
+    pub unsafe fn fill_zero(&self, offset: usize, len: usize) -> Result {
+        self.with_pointer_into_page(offset, len, move |dst| {
+            // SAFETY: If `with_pointer_into_page` calls into this closure, then it has performed a
+            // bounds check and guarantees that `dst` is valid for `len` bytes.
+            //
+            // There caller guarantees that there is no data race.
+            unsafe { ptr::write_bytes(dst, 0u8, len) };
+            Ok(())
+        })
+    }
+
+    /// Copies data from userspace into this page.
+    ///
+    /// This method will perform bounds checks on the page offset. If `offset .. offset+len` goes
+    /// outside ot the page, then this call returns `EINVAL`.
+    ///
+    /// Like the other `UserSliceReader` methods, data races are allowed on the userspace address.
+    /// However, they are not allowed on the page you are copying into.
+    ///
+    /// # Safety
+    ///
+    /// Callers must ensure that this call does not race with a read or write to the same page that
+    /// overlaps with this write.
+    pub unsafe fn copy_from_user_slice(
+        &self,
+        reader: &mut UserSliceReader,
+        offset: usize,
+        len: usize,
+    ) -> Result {
+        self.with_pointer_into_page(offset, len, move |dst| {
+            // SAFETY: If `with_pointer_into_page` calls into this closure, then it has performed a
+            // bounds check and guarantees that `dst` is valid for `len` bytes. Furthermore, we have
+            // exclusive access to the slice since the caller guarantees that there are no races.
+            reader.read_raw(unsafe { core::slice::from_raw_parts_mut(dst.cast(), len) })
+        })
+    }
+}
+
+impl Drop for Page {
+    fn drop(&mut self) {
+        // SAFETY: By the type invariants, we have ownership of the page and can free it.
+        unsafe { bindings::__free_pages(self.page.as_ptr(), 0) };
+    }
+}