| Message ID | 20240306232339.29659-2-richard@nod.at (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | [1/2,RFC] proc: pagemap: Expose whether a PTE is writable | expand |
On 07.03.24 00:23, Richard Weinberger wrote: > Bit 58 denotes that a PTE is writable. > The main use case is detecting CoW mappings. > > Signed-off-by: Richard Weinberger <richard@nod.at> > --- > Documentation/admin-guide/mm/pagemap.rst | 8 +++++++- > 1 file changed, 7 insertions(+), 1 deletion(-) > > diff --git a/Documentation/admin-guide/mm/pagemap.rst b/Documentation/admin-guide/mm/pagemap.rst > index f5f065c67615..81ffe3601b96 100644 > --- a/Documentation/admin-guide/mm/pagemap.rst > +++ b/Documentation/admin-guide/mm/pagemap.rst > @@ -21,7 +21,8 @@ There are four components to pagemap: > * Bit 56 page exclusively mapped (since 4.2) > * Bit 57 pte is uffd-wp write-protected (since 5.13) (see > Documentation/admin-guide/mm/userfaultfd.rst) > - * Bits 58-60 zero > + * Bit 58 pte is writable (since 6.10) > + * Bits 59-60 zero > * Bit 61 page is file-page or shared-anon (since 3.5) > * Bit 62 page swapped > * Bit 63 page present > @@ -37,6 +38,11 @@ There are four components to pagemap: > precisely which pages are mapped (or in swap) and comparing mapped > pages between processes. > > + Bit 58 is useful to detect CoW mappings; however, it does not indicate > + whether the page mapping is writable or not. If an anonymous mapping is > + writable but the write bit is not set, it means that the next write access > + will cause a page fault, and copy-on-write will happen. That is not true.
----- Ursprüngliche Mail ----- > Von: "David Hildenbrand" <david@redhat.com> > An: "richard" <richard@nod.at>, "linux-mm" <linux-mm@kvack.org> >> + Bit 58 is useful to detect CoW mappings; however, it does not indicate >> + whether the page mapping is writable or not. If an anonymous mapping is >> + writable but the write bit is not set, it means that the next write access >> + will cause a page fault, and copy-on-write will happen. > > That is not true. Can you please help me correct my obvious misunderstanding?
On 07.03.24 12:10, Richard Weinberger wrote: > ----- Ursprüngliche Mail ----- >> Von: "David Hildenbrand" <david@redhat.com> >> An: "richard" <richard@nod.at>, "linux-mm" <linux-mm@kvack.org> >>> + Bit 58 is useful to detect CoW mappings; however, it does not indicate >>> + whether the page mapping is writable or not. If an anonymous mapping is >>> + writable but the write bit is not set, it means that the next write access >>> + will cause a page fault, and copy-on-write will happen. >> >> That is not true. > > Can you please help me correct my obvious misunderstanding? We'll perform a page copy of an anonymous page only if the page is not detected as exclusive to the process. So a better description could be: "In an private mapping, having the writable bit clear can indicate that next write access will result in copy-on-write during a page fault. Note that exclusive anonymous pages can be mapped read-only, and they might simply get remapped writable during the next write fault, avoiding a page copy."
On Thu, Mar 07, 2024 at 12:23:39AM +0100, Richard Weinberger wrote: > Bit 58 denotes that a PTE is writable. > The main use case is detecting CoW mappings. > > Signed-off-by: Richard Weinberger <richard@nod.at> > --- > Documentation/admin-guide/mm/pagemap.rst | 8 +++++++- > 1 file changed, 7 insertions(+), 1 deletion(-) > > diff --git a/Documentation/admin-guide/mm/pagemap.rst b/Documentation/admin-guide/mm/pagemap.rst > index f5f065c67615..81ffe3601b96 100644 > --- a/Documentation/admin-guide/mm/pagemap.rst > +++ b/Documentation/admin-guide/mm/pagemap.rst > @@ -21,7 +21,8 @@ There are four components to pagemap: > * Bit 56 page exclusively mapped (since 4.2) > * Bit 57 pte is uffd-wp write-protected (since 5.13) (see > Documentation/admin-guide/mm/userfaultfd.rst) > - * Bits 58-60 zero > + * Bit 58 pte is writable (since 6.10) I really think we need to be careful about talking about 'writable' again because people are easily confused about the difference between a writable _mapping_ and a writable _page table entry_. Of course you mention PTE here, but I think it might be better to say something like: * Bit 58 raw pte r/w flag (since 6.10) > + * Bits 59-60 zero > * Bit 61 page is file-page or shared-anon (since 3.5) > * Bit 62 page swapped > * Bit 63 page present > @@ -37,6 +38,11 @@ There are four components to pagemap: > precisely which pages are mapped (or in swap) and comparing mapped > pages between processes. > > + Bit 58 is useful to detect CoW mappings; however, it does not indicate > + whether the page mapping is writable or not. If an anonymous mapping is > + writable but the write bit is not set, it means that the next write access > + will cause a page fault, and copy-on-write will happen. > + David has addressed the copy vs. anon exclusive remap issue, but I also feel this needs some balking out. I would simply rephrase this in terms of whether a write fault occurs or not e.g.: Bit 58 indicates whether the PTE has the write flag set. If this flag is unset, then write accesses for this mapping will cause a fault for this page. If the mapping is private (whether anonymous or file-backed), this can result in a Copy-on-Write (though if anonymous-excusive the flag will simply be set). If file-backed, this being cleared may simply indicate that this file page is clean. > Efficient users of this interface will use ``/proc/pid/maps`` to > determine which areas of memory are actually mapped and llseek to > skip over unmapped regions. > -- > 2.35.3 >
diff --git a/Documentation/admin-guide/mm/pagemap.rst b/Documentation/admin-guide/mm/pagemap.rst index f5f065c67615..81ffe3601b96 100644 --- a/Documentation/admin-guide/mm/pagemap.rst +++ b/Documentation/admin-guide/mm/pagemap.rst @@ -21,7 +21,8 @@ There are four components to pagemap: * Bit 56 page exclusively mapped (since 4.2) * Bit 57 pte is uffd-wp write-protected (since 5.13) (see Documentation/admin-guide/mm/userfaultfd.rst) - * Bits 58-60 zero + * Bit 58 pte is writable (since 6.10) + * Bits 59-60 zero * Bit 61 page is file-page or shared-anon (since 3.5) * Bit 62 page swapped * Bit 63 page present @@ -37,6 +38,11 @@ There are four components to pagemap: precisely which pages are mapped (or in swap) and comparing mapped pages between processes. + Bit 58 is useful to detect CoW mappings; however, it does not indicate + whether the page mapping is writable or not. If an anonymous mapping is + writable but the write bit is not set, it means that the next write access + will cause a page fault, and copy-on-write will happen. + Efficient users of this interface will use ``/proc/pid/maps`` to determine which areas of memory are actually mapped and llseek to skip over unmapped regions.
Bit 58 denotes that a PTE is writable. The main use case is detecting CoW mappings. Signed-off-by: Richard Weinberger <richard@nod.at> --- Documentation/admin-guide/mm/pagemap.rst | 8 +++++++- 1 file changed, 7 insertions(+), 1 deletion(-)