| Message ID | 2929bfd-9893-a374-e463-4c3127ff9b9d@google.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | mm: allow pte_offset_map[_lock]() to fail | expand |
On 8 Jun 2023, at 21:10, Hugh Dickins wrote: > Make pte_offset_map() a wrapper for __pte_offset_map() (optionally > outputs pmdval), pte_offset_map_lock() a sparse __cond_lock wrapper for > __pte_offset_map_lock(): those __funcs added in mm/pgtable-generic.c. > > __pte_offset_map() do pmdval validation (including pmd_clear_bad() > when pmd_bad()), returning NULL if pmdval is not for a page table. > __pte_offset_map_lock() verify pmdval unchanged after getting the > lock, trying again if it changed. > > No #ifdef CONFIG_TRANSPARENT_HUGEPAGE around them: that could be done > to cover the imminent case, but we expect to generalize it later, and > it makes a mess of where to do the pmd_bad() clearing. > > Add pte_offset_map_nolock(): outputs ptl like pte_offset_map_lock(), > without actually taking the lock. This will be preferred to open uses of > pte_lockptr(), because (when split ptlock is in page table's struct page) > it points to the right lock for the returned pte pointer, even if *pmd > gets changed racily afterwards. > > Update corresponding Documentation. > > Do not add the anticipated rcu_read_lock() and rcu_read_unlock()s yet: > they have to wait until all architectures are balancing pte_offset_map()s > with pte_unmap()s (as in the arch series posted earlier). But comment > where they will go, so that it's easy to add them for experiments. And > only when those are in place can transient racy failure cases be enabled. > Add more safety for the PAE mismatched pmd_low pmd_high case at that time. > > Signed-off-by: Hugh Dickins <hughd@google.com> > --- > Documentation/mm/split_page_table_lock.rst | 17 ++++--- > include/linux/mm.h | 27 +++++++---- > include/linux/pgtable.h | 22 ++++++--- > mm/pgtable-generic.c | 56 ++++++++++++++++++++++ > 4 files changed, 101 insertions(+), 21 deletions(-) LGTM. Reviewed-by: Zi Yan <ziy@nvidia.com> -- Best Regards, Yan, Zi
Hi, Hugh
It seems this change makes pte_offset_map_lock not possible to be
called in out of tree modules,
otherwise it will report error like this:
ERROR: modpost: "__pte_offset_map_lock"
[../omap-modules/android-mainline/pvr/pvrsrvkm.ko] undefined!
Not sure if you have any idea about it, and any suggestions on how to
resolve it?
Thanks,
Yongqin Liu
On Fri, 9 Jun 2023 at 09:10, Hugh Dickins <hughd@google.com> wrote:
>
> Make pte_offset_map() a wrapper for __pte_offset_map() (optionally
> outputs pmdval), pte_offset_map_lock() a sparse __cond_lock wrapper for
> __pte_offset_map_lock(): those __funcs added in mm/pgtable-generic.c.
>
> __pte_offset_map() do pmdval validation (including pmd_clear_bad()
> when pmd_bad()), returning NULL if pmdval is not for a page table.
> __pte_offset_map_lock() verify pmdval unchanged after getting the
> lock, trying again if it changed.
>
> No #ifdef CONFIG_TRANSPARENT_HUGEPAGE around them: that could be done
> to cover the imminent case, but we expect to generalize it later, and
> it makes a mess of where to do the pmd_bad() clearing.
>
> Add pte_offset_map_nolock(): outputs ptl like pte_offset_map_lock(),
> without actually taking the lock. This will be preferred to open uses of
> pte_lockptr(), because (when split ptlock is in page table's struct page)
> it points to the right lock for the returned pte pointer, even if *pmd
> gets changed racily afterwards.
>
> Update corresponding Documentation.
>
> Do not add the anticipated rcu_read_lock() and rcu_read_unlock()s yet:
> they have to wait until all architectures are balancing pte_offset_map()s
> with pte_unmap()s (as in the arch series posted earlier). But comment
> where they will go, so that it's easy to add them for experiments. And
> only when those are in place can transient racy failure cases be enabled.
> Add more safety for the PAE mismatched pmd_low pmd_high case at that time.
>
> Signed-off-by: Hugh Dickins <hughd@google.com>
> ---
> Documentation/mm/split_page_table_lock.rst | 17 ++++---
> include/linux/mm.h | 27 +++++++----
> include/linux/pgtable.h | 22 ++++++---
> mm/pgtable-generic.c | 56 ++++++++++++++++++++++
> 4 files changed, 101 insertions(+), 21 deletions(-)
>
> diff --git a/Documentation/mm/split_page_table_lock.rst b/Documentation/mm/split_page_table_lock.rst
> index 50ee0dfc95be..a834fad9de12 100644
> --- a/Documentation/mm/split_page_table_lock.rst
> +++ b/Documentation/mm/split_page_table_lock.rst
> @@ -14,15 +14,20 @@ tables. Access to higher level tables protected by mm->page_table_lock.
> There are helpers to lock/unlock a table and other accessor functions:
>
> - pte_offset_map_lock()
> - maps pte and takes PTE table lock, returns pointer to the taken
> - lock;
> + maps PTE and takes PTE table lock, returns pointer to PTE with
> + pointer to its PTE table lock, or returns NULL if no PTE table;
> + - pte_offset_map_nolock()
> + maps PTE, returns pointer to PTE with pointer to its PTE table
> + lock (not taken), or returns NULL if no PTE table;
> + - pte_offset_map()
> + maps PTE, returns pointer to PTE, or returns NULL if no PTE table;
> + - pte_unmap()
> + unmaps PTE table;
> - pte_unmap_unlock()
> unlocks and unmaps PTE table;
> - pte_alloc_map_lock()
> - allocates PTE table if needed and take the lock, returns pointer
> - to taken lock or NULL if allocation failed;
> - - pte_lockptr()
> - returns pointer to PTE table lock;
> + allocates PTE table if needed and takes its lock, returns pointer to
> + PTE with pointer to its lock, or returns NULL if allocation failed;
> - pmd_lock()
> takes PMD table lock, returns pointer to taken lock;
> - pmd_lockptr()
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 27ce77080c79..3c2e56980853 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2787,14 +2787,25 @@ static inline void pgtable_pte_page_dtor(struct page *page)
> dec_lruvec_page_state(page, NR_PAGETABLE);
> }
>
> -#define pte_offset_map_lock(mm, pmd, address, ptlp) \
> -({ \
> - spinlock_t *__ptl = pte_lockptr(mm, pmd); \
> - pte_t *__pte = pte_offset_map(pmd, address); \
> - *(ptlp) = __ptl; \
> - spin_lock(__ptl); \
> - __pte; \
> -})
> +pte_t *__pte_offset_map(pmd_t *pmd, unsigned long addr, pmd_t *pmdvalp);
> +static inline pte_t *pte_offset_map(pmd_t *pmd, unsigned long addr)
> +{
> + return __pte_offset_map(pmd, addr, NULL);
> +}
> +
> +pte_t *__pte_offset_map_lock(struct mm_struct *mm, pmd_t *pmd,
> + unsigned long addr, spinlock_t **ptlp);
> +static inline pte_t *pte_offset_map_lock(struct mm_struct *mm, pmd_t *pmd,
> + unsigned long addr, spinlock_t **ptlp)
> +{
> + pte_t *pte;
> +
> + __cond_lock(*ptlp, pte = __pte_offset_map_lock(mm, pmd, addr, ptlp));
> + return pte;
> +}
> +
> +pte_t *pte_offset_map_nolock(struct mm_struct *mm, pmd_t *pmd,
> + unsigned long addr, spinlock_t **ptlp);
>
> #define pte_unmap_unlock(pte, ptl) do { \
> spin_unlock(ptl); \
> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> index 94235ff2706e..3fabbb018557 100644
> --- a/include/linux/pgtable.h
> +++ b/include/linux/pgtable.h
> @@ -94,14 +94,22 @@ static inline pte_t *pte_offset_kernel(pmd_t *pmd, unsigned long address)
> #define pte_offset_kernel pte_offset_kernel
> #endif
>
> -#if defined(CONFIG_HIGHPTE)
> -#define pte_offset_map(dir, address) \
> - ((pte_t *)kmap_local_page(pmd_page(*(dir))) + \
> - pte_index((address)))
> -#define pte_unmap(pte) kunmap_local((pte))
> +#ifdef CONFIG_HIGHPTE
> +#define __pte_map(pmd, address) \
> + ((pte_t *)kmap_local_page(pmd_page(*(pmd))) + pte_index((address)))
> +#define pte_unmap(pte) do { \
> + kunmap_local((pte)); \
> + /* rcu_read_unlock() to be added later */ \
> +} while (0)
> #else
> -#define pte_offset_map(dir, address) pte_offset_kernel((dir), (address))
> -#define pte_unmap(pte) ((void)(pte)) /* NOP */
> +static inline pte_t *__pte_map(pmd_t *pmd, unsigned long address)
> +{
> + return pte_offset_kernel(pmd, address);
> +}
> +static inline void pte_unmap(pte_t *pte)
> +{
> + /* rcu_read_unlock() to be added later */
> +}
> #endif
>
> /* Find an entry in the second-level page table.. */
> diff --git a/mm/pgtable-generic.c b/mm/pgtable-generic.c
> index d2fc52bffafc..c7ab18a5fb77 100644
> --- a/mm/pgtable-generic.c
> +++ b/mm/pgtable-generic.c
> @@ -10,6 +10,8 @@
> #include <linux/pagemap.h>
> #include <linux/hugetlb.h>
> #include <linux/pgtable.h>
> +#include <linux/swap.h>
> +#include <linux/swapops.h>
> #include <linux/mm_inline.h>
> #include <asm/tlb.h>
>
> @@ -229,3 +231,57 @@ pmd_t pmdp_collapse_flush(struct vm_area_struct *vma, unsigned long address,
> }
> #endif
> #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
> +
> +pte_t *__pte_offset_map(pmd_t *pmd, unsigned long addr, pmd_t *pmdvalp)
> +{
> + pmd_t pmdval;
> +
> + /* rcu_read_lock() to be added later */
> + pmdval = pmdp_get_lockless(pmd);
> + if (pmdvalp)
> + *pmdvalp = pmdval;
> + if (unlikely(pmd_none(pmdval) || is_pmd_migration_entry(pmdval)))
> + goto nomap;
> + if (unlikely(pmd_trans_huge(pmdval) || pmd_devmap(pmdval)))
> + goto nomap;
> + if (unlikely(pmd_bad(pmdval))) {
> + pmd_clear_bad(pmd);
> + goto nomap;
> + }
> + return __pte_map(&pmdval, addr);
> +nomap:
> + /* rcu_read_unlock() to be added later */
> + return NULL;
> +}
> +
> +pte_t *pte_offset_map_nolock(struct mm_struct *mm, pmd_t *pmd,
> + unsigned long addr, spinlock_t **ptlp)
> +{
> + pmd_t pmdval;
> + pte_t *pte;
> +
> + pte = __pte_offset_map(pmd, addr, &pmdval);
> + if (likely(pte))
> + *ptlp = pte_lockptr(mm, &pmdval);
> + return pte;
> +}
> +
> +pte_t *__pte_offset_map_lock(struct mm_struct *mm, pmd_t *pmd,
> + unsigned long addr, spinlock_t **ptlp)
> +{
> + spinlock_t *ptl;
> + pmd_t pmdval;
> + pte_t *pte;
> +again:
> + pte = __pte_offset_map(pmd, addr, &pmdval);
> + if (unlikely(!pte))
> + return pte;
> + ptl = pte_lockptr(mm, &pmdval);
> + spin_lock(ptl);
> + if (likely(pmd_same(pmdval, pmdp_get_lockless(pmd)))) {
> + *ptlp = ptl;
> + return pte;
> + }
> + pte_unmap_unlock(pte, ptl);
> + goto again;
> +}
> --
> 2.35.3
>
On Fri, Jul 28, 2023 at 09:53:29PM +0800, Yongqin Liu wrote: > Hi, Hugh > > It seems this change makes pte_offset_map_lock not possible to be > called in out of tree modules, > otherwise it will report error like this: > ERROR: modpost: "__pte_offset_map_lock" > [../omap-modules/android-mainline/pvr/pvrsrvkm.ko] undefined! > > Not sure if you have any idea about it, and any suggestions on how to > resolve it? Please explain why this module needs to map page tables
On Fri, 28 Jul 2023, Matthew Wilcox wrote: > On Fri, Jul 28, 2023 at 09:53:29PM +0800, Yongqin Liu wrote: > > Hi, Hugh > > > > It seems this change makes pte_offset_map_lock not possible to be > > called in out of tree modules, > > otherwise it will report error like this: > > ERROR: modpost: "__pte_offset_map_lock" > > [../omap-modules/android-mainline/pvr/pvrsrvkm.ko] undefined! > > > > Not sure if you have any idea about it, and any suggestions on how to > > resolve it? > > Please explain why this module needs to map page tables +1 Thank you for testing 6.5-rc, and I am sorry to have inconvenienced you. But there is not one example of an in-tree module needing that, which is a very strong hint that no module should be needing that. Sounds like pvrsrvkm.ko wants to muck around with page table entries, without the core mm knowing. Not something core mm can encourage! If what pvrsrvkm.ko is aiming to do there would be useful for others, maybe its owner can share that, and work with core mm developers to expose a generally useful interface - but that is not likely to be __pte_offset_map_lock itself. Hugh
On Sat, 29 Jul 2023 at 00:58, Hugh Dickins <hughd@google.com> wrote: > > On Fri, 28 Jul 2023, Matthew Wilcox wrote: > > On Fri, Jul 28, 2023 at 09:53:29PM +0800, Yongqin Liu wrote: > > > Hi, Hugh > > > > > > It seems this change makes pte_offset_map_lock not possible to be > > > called in out of tree modules, > > > otherwise it will report error like this: > > > ERROR: modpost: "__pte_offset_map_lock" > > > [../omap-modules/android-mainline/pvr/pvrsrvkm.ko] undefined! > > > > > > Not sure if you have any idea about it, and any suggestions on how to > > > resolve it? > > > > Please explain why this module needs to map page tables > > +1 Sorry, I am not able to give any explanation here, I am not familiar with the pvrsrvkm source, I just use it to have one working AOSP build. here is the source file where pte_offset_map_lock is called, https://android-git.linaro.org/kernel/omap-modules.git/tree/pvr/services4/srvkm/env/linux/osfunc.c?h=android-mainline#n3508 in case you could know something with a quick look. Otherwise, it has to wait for another one to report the problem again. > Thank you for testing 6.5-rc, and I am sorry to have inconvenienced you. > > But there is not one example of an in-tree module needing that, > which is a very strong hint that no module should be needing that. > > Sounds like pvrsrvkm.ko wants to muck around with page table entries, > without the core mm knowing. Not something core mm can encourage! > > If what pvrsrvkm.ko is aiming to do there would be useful for others, > maybe its owner can share that, and work with core mm developers to > expose a generally useful interface - but that is not likely to be > __pte_offset_map_lock itself. > Thanks for the explanation! Let's see if any other pvrsrvkm engineer or other out of tree modules could help give some explanations on this case or similar cases. Thanks, Yongqin Liu -- Best Regards, Yongqin Liu --------------------------------------------------------------- #mailing list linaro-android@lists.linaro.org http://lists.linaro.org/mailman/listinfo/linaro-android
On Sun, Aug 06, 2023 at 12:06:28AM +0800, Yongqin Liu wrote: > On Sat, 29 Jul 2023 at 00:58, Hugh Dickins <hughd@google.com> wrote: > > > > On Fri, 28 Jul 2023, Matthew Wilcox wrote: > > > On Fri, Jul 28, 2023 at 09:53:29PM +0800, Yongqin Liu wrote: > > > > Hi, Hugh > > > > > > > > It seems this change makes pte_offset_map_lock not possible to be > > > > called in out of tree modules, > > > > otherwise it will report error like this: > > > > ERROR: modpost: "__pte_offset_map_lock" > > > > [../omap-modules/android-mainline/pvr/pvrsrvkm.ko] undefined! > > > > > > > > Not sure if you have any idea about it, and any suggestions on how to > > > > resolve it? > > > > > > Please explain why this module needs to map page tables > > > > +1 > Sorry, I am not able to give any explanation here, > I am not familiar with the pvrsrvkm source, I just use it to have one > working AOSP build. > > here is the source file where pte_offset_map_lock is called, > https://android-git.linaro.org/kernel/omap-modules.git/tree/pvr/services4/srvkm/env/linux/osfunc.c?h=android-mainline#n3508 > in case you could know something with a quick look. Isn't this just get_user_pages()?
On 8/5/23 10:07, Matthew Wilcox wrote: ... >> Sorry, I am not able to give any explanation here, >> I am not familiar with the pvrsrvkm source, I just use it to have one >> working AOSP build. >> >> here is the source file where pte_offset_map_lock is called, >> https://android-git.linaro.org/kernel/omap-modules.git/tree/pvr/services4/srvkm/env/linux/osfunc.c?h=android-mainline#n3508 >> in case you could know something with a quick look. > > Isn't this just get_user_pages()? Or even just follow_page(), which looks like a nearly perfect drop-in replacement, especially since that android link also says, "The page in question must be present (i.e. no fault handling required)". thanks,
diff --git a/Documentation/mm/split_page_table_lock.rst b/Documentation/mm/split_page_table_lock.rst index 50ee0dfc95be..a834fad9de12 100644 --- a/Documentation/mm/split_page_table_lock.rst +++ b/Documentation/mm/split_page_table_lock.rst @@ -14,15 +14,20 @@ tables. Access to higher level tables protected by mm->page_table_lock. There are helpers to lock/unlock a table and other accessor functions: - pte_offset_map_lock() - maps pte and takes PTE table lock, returns pointer to the taken - lock; + maps PTE and takes PTE table lock, returns pointer to PTE with + pointer to its PTE table lock, or returns NULL if no PTE table; + - pte_offset_map_nolock() + maps PTE, returns pointer to PTE with pointer to its PTE table + lock (not taken), or returns NULL if no PTE table; + - pte_offset_map() + maps PTE, returns pointer to PTE, or returns NULL if no PTE table; + - pte_unmap() + unmaps PTE table; - pte_unmap_unlock() unlocks and unmaps PTE table; - pte_alloc_map_lock() - allocates PTE table if needed and take the lock, returns pointer - to taken lock or NULL if allocation failed; - - pte_lockptr() - returns pointer to PTE table lock; + allocates PTE table if needed and takes its lock, returns pointer to + PTE with pointer to its lock, or returns NULL if allocation failed; - pmd_lock() takes PMD table lock, returns pointer to taken lock; - pmd_lockptr() diff --git a/include/linux/mm.h b/include/linux/mm.h index 27ce77080c79..3c2e56980853 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -2787,14 +2787,25 @@ static inline void pgtable_pte_page_dtor(struct page *page) dec_lruvec_page_state(page, NR_PAGETABLE); } -#define pte_offset_map_lock(mm, pmd, address, ptlp) \ -({ \ - spinlock_t *__ptl = pte_lockptr(mm, pmd); \ - pte_t *__pte = pte_offset_map(pmd, address); \ - *(ptlp) = __ptl; \ - spin_lock(__ptl); \ - __pte; \ -}) +pte_t *__pte_offset_map(pmd_t *pmd, unsigned long addr, pmd_t *pmdvalp); +static inline pte_t *pte_offset_map(pmd_t *pmd, unsigned long addr) +{ + return __pte_offset_map(pmd, addr, NULL); +} + +pte_t *__pte_offset_map_lock(struct mm_struct *mm, pmd_t *pmd, + unsigned long addr, spinlock_t **ptlp); +static inline pte_t *pte_offset_map_lock(struct mm_struct *mm, pmd_t *pmd, + unsigned long addr, spinlock_t **ptlp) +{ + pte_t *pte; + + __cond_lock(*ptlp, pte = __pte_offset_map_lock(mm, pmd, addr, ptlp)); + return pte; +} + +pte_t *pte_offset_map_nolock(struct mm_struct *mm, pmd_t *pmd, + unsigned long addr, spinlock_t **ptlp); #define pte_unmap_unlock(pte, ptl) do { \ spin_unlock(ptl); \ diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h index 94235ff2706e..3fabbb018557 100644 --- a/include/linux/pgtable.h +++ b/include/linux/pgtable.h @@ -94,14 +94,22 @@ static inline pte_t *pte_offset_kernel(pmd_t *pmd, unsigned long address) #define pte_offset_kernel pte_offset_kernel #endif -#if defined(CONFIG_HIGHPTE) -#define pte_offset_map(dir, address) \ - ((pte_t *)kmap_local_page(pmd_page(*(dir))) + \ - pte_index((address))) -#define pte_unmap(pte) kunmap_local((pte)) +#ifdef CONFIG_HIGHPTE +#define __pte_map(pmd, address) \ + ((pte_t *)kmap_local_page(pmd_page(*(pmd))) + pte_index((address))) +#define pte_unmap(pte) do { \ + kunmap_local((pte)); \ + /* rcu_read_unlock() to be added later */ \ +} while (0) #else -#define pte_offset_map(dir, address) pte_offset_kernel((dir), (address)) -#define pte_unmap(pte) ((void)(pte)) /* NOP */ +static inline pte_t *__pte_map(pmd_t *pmd, unsigned long address) +{ + return pte_offset_kernel(pmd, address); +} +static inline void pte_unmap(pte_t *pte) +{ + /* rcu_read_unlock() to be added later */ +} #endif /* Find an entry in the second-level page table.. */ diff --git a/mm/pgtable-generic.c b/mm/pgtable-generic.c index d2fc52bffafc..c7ab18a5fb77 100644 --- a/mm/pgtable-generic.c +++ b/mm/pgtable-generic.c @@ -10,6 +10,8 @@ #include <linux/pagemap.h> #include <linux/hugetlb.h> #include <linux/pgtable.h> +#include <linux/swap.h> +#include <linux/swapops.h> #include <linux/mm_inline.h> #include <asm/tlb.h> @@ -229,3 +231,57 @@ pmd_t pmdp_collapse_flush(struct vm_area_struct *vma, unsigned long address, } #endif #endif /* CONFIG_TRANSPARENT_HUGEPAGE */ + +pte_t *__pte_offset_map(pmd_t *pmd, unsigned long addr, pmd_t *pmdvalp) +{ + pmd_t pmdval; + + /* rcu_read_lock() to be added later */ + pmdval = pmdp_get_lockless(pmd); + if (pmdvalp) + *pmdvalp = pmdval; + if (unlikely(pmd_none(pmdval) || is_pmd_migration_entry(pmdval))) + goto nomap; + if (unlikely(pmd_trans_huge(pmdval) || pmd_devmap(pmdval))) + goto nomap; + if (unlikely(pmd_bad(pmdval))) { + pmd_clear_bad(pmd); + goto nomap; + } + return __pte_map(&pmdval, addr); +nomap: + /* rcu_read_unlock() to be added later */ + return NULL; +} + +pte_t *pte_offset_map_nolock(struct mm_struct *mm, pmd_t *pmd, + unsigned long addr, spinlock_t **ptlp) +{ + pmd_t pmdval; + pte_t *pte; + + pte = __pte_offset_map(pmd, addr, &pmdval); + if (likely(pte)) + *ptlp = pte_lockptr(mm, &pmdval); + return pte; +} + +pte_t *__pte_offset_map_lock(struct mm_struct *mm, pmd_t *pmd, + unsigned long addr, spinlock_t **ptlp) +{ + spinlock_t *ptl; + pmd_t pmdval; + pte_t *pte; +again: + pte = __pte_offset_map(pmd, addr, &pmdval); + if (unlikely(!pte)) + return pte; + ptl = pte_lockptr(mm, &pmdval); + spin_lock(ptl); + if (likely(pmd_same(pmdval, pmdp_get_lockless(pmd)))) { + *ptlp = ptl; + return pte; + } + pte_unmap_unlock(pte, ptl); + goto again; +}
Make pte_offset_map() a wrapper for __pte_offset_map() (optionally outputs pmdval), pte_offset_map_lock() a sparse __cond_lock wrapper for __pte_offset_map_lock(): those __funcs added in mm/pgtable-generic.c. __pte_offset_map() do pmdval validation (including pmd_clear_bad() when pmd_bad()), returning NULL if pmdval is not for a page table. __pte_offset_map_lock() verify pmdval unchanged after getting the lock, trying again if it changed. No #ifdef CONFIG_TRANSPARENT_HUGEPAGE around them: that could be done to cover the imminent case, but we expect to generalize it later, and it makes a mess of where to do the pmd_bad() clearing. Add pte_offset_map_nolock(): outputs ptl like pte_offset_map_lock(), without actually taking the lock. This will be preferred to open uses of pte_lockptr(), because (when split ptlock is in page table's struct page) it points to the right lock for the returned pte pointer, even if *pmd gets changed racily afterwards. Update corresponding Documentation. Do not add the anticipated rcu_read_lock() and rcu_read_unlock()s yet: they have to wait until all architectures are balancing pte_offset_map()s with pte_unmap()s (as in the arch series posted earlier). But comment where they will go, so that it's easy to add them for experiments. And only when those are in place can transient racy failure cases be enabled. Add more safety for the PAE mismatched pmd_low pmd_high case at that time. Signed-off-by: Hugh Dickins <hughd@google.com> --- Documentation/mm/split_page_table_lock.rst | 17 ++++--- include/linux/mm.h | 27 +++++++---- include/linux/pgtable.h | 22 ++++++--- mm/pgtable-generic.c | 56 ++++++++++++++++++++++ 4 files changed, 101 insertions(+), 21 deletions(-)