| Message ID | 20211124235906.14437-1-surenb@google.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | [v2,1/2] mm: protect free_pgtables with mmap_lock write lock in exit_mmap | expand |
On Wed 24-11-21 15:59:05, Suren Baghdasaryan wrote: > oom-reaper and process_mrelease system call should protect against > races with exit_mmap which can destroy page tables while they > walk the VMA tree. oom-reaper protects from that race by setting > MMF_OOM_VICTIM and by relying on exit_mmap to set MMF_OOM_SKIP > before taking and releasing mmap_write_lock. process_mrelease has > to elevate mm->mm_users to prevent such race. Both oom-reaper and > process_mrelease hold mmap_read_lock when walking the VMA tree. > The locking rules and mechanisms could be simpler if exit_mmap takes > mmap_write_lock while executing destructive operations such as > free_pgtables. > Change exit_mmap to hold the mmap_write_lock when calling > free_pgtables. Operations like unmap_vmas() and unlock_range() are not > destructive and could run under mmap_read_lock but for simplicity we > take one mmap_write_lock during almost the entire operation. Note > also that because oom-reaper checks VM_LOCKED flag, unlock_range() > should not be allowed to race with it. > In most cases this lock should be uncontended. Previously, Kirill > reported ~4% regression caused by a similar change [1]. We reran the > same test and although the individual results are quite noisy, the > percentiles show lower regression with 1.6% being the worst case [2]. > The change allows oom-reaper and process_mrelease to execute safely > under mmap_read_lock without worries that exit_mmap might destroy page > tables from under them. > > [1] https://lore.kernel.org/all/20170725141723.ivukwhddk2voyhuc@node.shutemov.name/ > [2] https://lore.kernel.org/all/CAJuCfpGC9-c9P40x7oy=jy5SphMcd0o0G_6U1-+JAziGKG6dGA@mail.gmail.com/ > > Signed-off-by: Suren Baghdasaryan <surenb@google.com> > --- > changes in v2 > - Moved mmap_write_unlock to cover remove_vma loop as well, per Matthew Wilcox > > mm/mmap.c | 16 ++++++++-------- > 1 file changed, 8 insertions(+), 8 deletions(-) > > diff --git a/mm/mmap.c b/mm/mmap.c > index bfb0ea164a90..f4e09d390a07 100644 > --- a/mm/mmap.c > +++ b/mm/mmap.c > @@ -3142,25 +3142,27 @@ void exit_mmap(struct mm_struct *mm) > * to mmu_notifier_release(mm) ensures mmu notifier callbacks in > * __oom_reap_task_mm() will not block. > * > - * This needs to be done before calling munlock_vma_pages_all(), > + * This needs to be done before calling unlock_range(), > * which clears VM_LOCKED, otherwise the oom reaper cannot > * reliably test it. > */ > (void)__oom_reap_task_mm(mm); > > set_bit(MMF_OOM_SKIP, &mm->flags); Why do you keep this in place? Other than that looks OK to me. Maybe we want to add an explicit note that vm_ops::close cannot take mmap_sem in any form. The changelog should also mention that you have considered remove_vma and its previous no MM locking assumption. You can argue that fput is async and close callback shouldn't really need mmap_sem.
On Mon, Nov 29, 2021 at 3:23 AM Michal Hocko <mhocko@suse.com> wrote: > > On Wed 24-11-21 15:59:05, Suren Baghdasaryan wrote: > > oom-reaper and process_mrelease system call should protect against > > races with exit_mmap which can destroy page tables while they > > walk the VMA tree. oom-reaper protects from that race by setting > > MMF_OOM_VICTIM and by relying on exit_mmap to set MMF_OOM_SKIP > > before taking and releasing mmap_write_lock. process_mrelease has > > to elevate mm->mm_users to prevent such race. Both oom-reaper and > > process_mrelease hold mmap_read_lock when walking the VMA tree. > > The locking rules and mechanisms could be simpler if exit_mmap takes > > mmap_write_lock while executing destructive operations such as > > free_pgtables. > > Change exit_mmap to hold the mmap_write_lock when calling > > free_pgtables. Operations like unmap_vmas() and unlock_range() are not > > destructive and could run under mmap_read_lock but for simplicity we > > take one mmap_write_lock during almost the entire operation. Note > > also that because oom-reaper checks VM_LOCKED flag, unlock_range() > > should not be allowed to race with it. > > In most cases this lock should be uncontended. Previously, Kirill > > reported ~4% regression caused by a similar change [1]. We reran the > > same test and although the individual results are quite noisy, the > > percentiles show lower regression with 1.6% being the worst case [2]. > > The change allows oom-reaper and process_mrelease to execute safely > > under mmap_read_lock without worries that exit_mmap might destroy page > > tables from under them. > > > > [1] https://lore.kernel.org/all/20170725141723.ivukwhddk2voyhuc@node.shutemov.name/ > > [2] https://lore.kernel.org/all/CAJuCfpGC9-c9P40x7oy=jy5SphMcd0o0G_6U1-+JAziGKG6dGA@mail.gmail.com/ > > > > Signed-off-by: Suren Baghdasaryan <surenb@google.com> > > --- > > changes in v2 > > - Moved mmap_write_unlock to cover remove_vma loop as well, per Matthew Wilcox > > > > mm/mmap.c | 16 ++++++++-------- > > 1 file changed, 8 insertions(+), 8 deletions(-) > > > > diff --git a/mm/mmap.c b/mm/mmap.c > > index bfb0ea164a90..f4e09d390a07 100644 > > --- a/mm/mmap.c > > +++ b/mm/mmap.c > > @@ -3142,25 +3142,27 @@ void exit_mmap(struct mm_struct *mm) > > * to mmu_notifier_release(mm) ensures mmu notifier callbacks in > > * __oom_reap_task_mm() will not block. > > * > > - * This needs to be done before calling munlock_vma_pages_all(), > > + * This needs to be done before calling unlock_range(), > > * which clears VM_LOCKED, otherwise the oom reaper cannot > > * reliably test it. > > */ > > (void)__oom_reap_task_mm(mm); > > > > set_bit(MMF_OOM_SKIP, &mm->flags); > > Why do you keep this in place? Sorry for the delay, I was out last week. I missed your comment about removing MMF_OOM_SKIP at https://lore.kernel.org/all/YYrO%2FPwdsyaxJaNZ@dhcp22.suse.cz I'll look into removing it in a separate patch, which I think would be cleaner. > > Other than that looks OK to me. Maybe we want to add an explicit note > that vm_ops::close cannot take mmap_sem in any form. The changelog > should also mention that you have considered remove_vma and its previous > no MM locking assumption. You can argue that fput is async and close > callback shouldn't really need mmap_sem. Should I post another version of this patch with the patch description clarifying these points and additional comments as you suggested? > -- > Michal Hocko > SUSE Labs
On Mon, Dec 06, 2021 at 10:35:03AM -0800, Suren Baghdasaryan wrote: > > Other than that looks OK to me. Maybe we want to add an explicit note > > that vm_ops::close cannot take mmap_sem in any form. The changelog > > should also mention that you have considered remove_vma and its previous > > no MM locking assumption. You can argue that fput is async and close > > callback shouldn't really need mmap_sem. > > Should I post another version of this patch with the patch description > clarifying these points and additional comments as you suggested? fyi, vm_ops->close() is already called with the mmap_sem held for write in __split_vma(). If that needs to be documented, it's a separate patch because it's absolutely not a consequence of this patch.
On Mon 06-12-21 18:52:28, Matthew Wilcox wrote: > On Mon, Dec 06, 2021 at 10:35:03AM -0800, Suren Baghdasaryan wrote: > > > Other than that looks OK to me. Maybe we want to add an explicit note > > > that vm_ops::close cannot take mmap_sem in any form. The changelog > > > should also mention that you have considered remove_vma and its previous > > > no MM locking assumption. You can argue that fput is async and close > > > callback shouldn't really need mmap_sem. > > > > Should I post another version of this patch with the patch description > > clarifying these points and additional comments as you suggested? > > fyi, vm_ops->close() is already called with the mmap_sem held for write > in __split_vma(). If that needs to be documented, it's a separate patch > because it's absolutely not a consequence of this patch. Agreed! We definitely want to document that.
On Mon 06-12-21 10:35:03, Suren Baghdasaryan wrote: > On Mon, Nov 29, 2021 at 3:23 AM Michal Hocko <mhocko@suse.com> wrote: > > > > On Wed 24-11-21 15:59:05, Suren Baghdasaryan wrote: > > > oom-reaper and process_mrelease system call should protect against > > > races with exit_mmap which can destroy page tables while they > > > walk the VMA tree. oom-reaper protects from that race by setting > > > MMF_OOM_VICTIM and by relying on exit_mmap to set MMF_OOM_SKIP > > > before taking and releasing mmap_write_lock. process_mrelease has > > > to elevate mm->mm_users to prevent such race. Both oom-reaper and > > > process_mrelease hold mmap_read_lock when walking the VMA tree. > > > The locking rules and mechanisms could be simpler if exit_mmap takes > > > mmap_write_lock while executing destructive operations such as > > > free_pgtables. > > > Change exit_mmap to hold the mmap_write_lock when calling > > > free_pgtables. Operations like unmap_vmas() and unlock_range() are not > > > destructive and could run under mmap_read_lock but for simplicity we > > > take one mmap_write_lock during almost the entire operation. Note > > > also that because oom-reaper checks VM_LOCKED flag, unlock_range() > > > should not be allowed to race with it. > > > In most cases this lock should be uncontended. Previously, Kirill > > > reported ~4% regression caused by a similar change [1]. We reran the > > > same test and although the individual results are quite noisy, the > > > percentiles show lower regression with 1.6% being the worst case [2]. > > > The change allows oom-reaper and process_mrelease to execute safely > > > under mmap_read_lock without worries that exit_mmap might destroy page > > > tables from under them. > > > > > > [1] https://lore.kernel.org/all/20170725141723.ivukwhddk2voyhuc@node.shutemov.name/ > > > [2] https://lore.kernel.org/all/CAJuCfpGC9-c9P40x7oy=jy5SphMcd0o0G_6U1-+JAziGKG6dGA@mail.gmail.com/ > > > > > > Signed-off-by: Suren Baghdasaryan <surenb@google.com> > > > --- > > > changes in v2 > > > - Moved mmap_write_unlock to cover remove_vma loop as well, per Matthew Wilcox > > > > > > mm/mmap.c | 16 ++++++++-------- > > > 1 file changed, 8 insertions(+), 8 deletions(-) > > > > > > diff --git a/mm/mmap.c b/mm/mmap.c > > > index bfb0ea164a90..f4e09d390a07 100644 > > > --- a/mm/mmap.c > > > +++ b/mm/mmap.c > > > @@ -3142,25 +3142,27 @@ void exit_mmap(struct mm_struct *mm) > > > * to mmu_notifier_release(mm) ensures mmu notifier callbacks in > > > * __oom_reap_task_mm() will not block. > > > * > > > - * This needs to be done before calling munlock_vma_pages_all(), > > > + * This needs to be done before calling unlock_range(), > > > * which clears VM_LOCKED, otherwise the oom reaper cannot > > > * reliably test it. > > > */ > > > (void)__oom_reap_task_mm(mm); > > > > > > set_bit(MMF_OOM_SKIP, &mm->flags); > > > > Why do you keep this in place? > > Sorry for the delay, I was out last week. > I missed your comment about removing MMF_OOM_SKIP at > https://lore.kernel.org/all/YYrO%2FPwdsyaxJaNZ@dhcp22.suse.cz > I'll look into removing it in a separate patch, which I think would be cleaner. The point of this code was to sync up the oom_repaer and exit_mmap. Now that your patch uses proper locking for that to happen then MMF_OOM_SKIP is not really necessary. IIRC all you need to guarantee is that the vma tree is empty when exit_mmap does all its work - i.e set mm->mmap to NULL. You can do that after remove_vma loop but it would be equally safe at any time after vma = mm->mmap as the loop relies on the vma chain. Doing that after would be slightly nicer if you ask me. > > Other than that looks OK to me. Maybe we want to add an explicit note > > that vm_ops::close cannot take mmap_sem in any form. The changelog > > should also mention that you have considered remove_vma and its previous > > no MM locking assumption. You can argue that fput is async and close > > callback shouldn't really need mmap_sem. > > Should I post another version of this patch with the patch description > clarifying these points and additional comments as you suggested? Yes please.
On Tue, Dec 7, 2021 at 2:10 AM Michal Hocko <mhocko@suse.com> wrote: > > On Mon 06-12-21 10:35:03, Suren Baghdasaryan wrote: > > On Mon, Nov 29, 2021 at 3:23 AM Michal Hocko <mhocko@suse.com> wrote: > > > > > > On Wed 24-11-21 15:59:05, Suren Baghdasaryan wrote: > > > > oom-reaper and process_mrelease system call should protect against > > > > races with exit_mmap which can destroy page tables while they > > > > walk the VMA tree. oom-reaper protects from that race by setting > > > > MMF_OOM_VICTIM and by relying on exit_mmap to set MMF_OOM_SKIP > > > > before taking and releasing mmap_write_lock. process_mrelease has > > > > to elevate mm->mm_users to prevent such race. Both oom-reaper and > > > > process_mrelease hold mmap_read_lock when walking the VMA tree. > > > > The locking rules and mechanisms could be simpler if exit_mmap takes > > > > mmap_write_lock while executing destructive operations such as > > > > free_pgtables. > > > > Change exit_mmap to hold the mmap_write_lock when calling > > > > free_pgtables. Operations like unmap_vmas() and unlock_range() are not > > > > destructive and could run under mmap_read_lock but for simplicity we > > > > take one mmap_write_lock during almost the entire operation. Note > > > > also that because oom-reaper checks VM_LOCKED flag, unlock_range() > > > > should not be allowed to race with it. > > > > In most cases this lock should be uncontended. Previously, Kirill > > > > reported ~4% regression caused by a similar change [1]. We reran the > > > > same test and although the individual results are quite noisy, the > > > > percentiles show lower regression with 1.6% being the worst case [2]. > > > > The change allows oom-reaper and process_mrelease to execute safely > > > > under mmap_read_lock without worries that exit_mmap might destroy page > > > > tables from under them. > > > > > > > > [1] https://lore.kernel.org/all/20170725141723.ivukwhddk2voyhuc@node.shutemov.name/ > > > > [2] https://lore.kernel.org/all/CAJuCfpGC9-c9P40x7oy=jy5SphMcd0o0G_6U1-+JAziGKG6dGA@mail.gmail.com/ > > > > > > > > Signed-off-by: Suren Baghdasaryan <surenb@google.com> > > > > --- > > > > changes in v2 > > > > - Moved mmap_write_unlock to cover remove_vma loop as well, per Matthew Wilcox > > > > > > > > mm/mmap.c | 16 ++++++++-------- > > > > 1 file changed, 8 insertions(+), 8 deletions(-) > > > > > > > > diff --git a/mm/mmap.c b/mm/mmap.c > > > > index bfb0ea164a90..f4e09d390a07 100644 > > > > --- a/mm/mmap.c > > > > +++ b/mm/mmap.c > > > > @@ -3142,25 +3142,27 @@ void exit_mmap(struct mm_struct *mm) > > > > * to mmu_notifier_release(mm) ensures mmu notifier callbacks in > > > > * __oom_reap_task_mm() will not block. > > > > * > > > > - * This needs to be done before calling munlock_vma_pages_all(), > > > > + * This needs to be done before calling unlock_range(), > > > > * which clears VM_LOCKED, otherwise the oom reaper cannot > > > > * reliably test it. > > > > */ > > > > (void)__oom_reap_task_mm(mm); > > > > > > > > set_bit(MMF_OOM_SKIP, &mm->flags); > > > > > > Why do you keep this in place? > > > > Sorry for the delay, I was out last week. > > I missed your comment about removing MMF_OOM_SKIP at > > https://lore.kernel.org/all/YYrO%2FPwdsyaxJaNZ@dhcp22.suse.cz > > I'll look into removing it in a separate patch, which I think would be cleaner. > > The point of this code was to sync up the oom_repaer and exit_mmap. Now > that your patch uses proper locking for that to happen then MMF_OOM_SKIP > is not really necessary. IIRC all you need to guarantee is that the vma > tree is empty when exit_mmap does all its work - i.e set mm->mmap to > NULL. You can do that after remove_vma loop but it would be equally safe > at any time after vma = mm->mmap as the loop relies on the vma chain. > Doing that after would be slightly nicer if you ask me. Will do. But if you don't mind I'll post the removal of MMF_OOM_SKIP as a separate patch. This patchset has already been extensively tested and it will be easier for me to test MMF_OOM_SKIP removal separately. > > > > Other than that looks OK to me. Maybe we want to add an explicit note > > > that vm_ops::close cannot take mmap_sem in any form. The changelog > > > should also mention that you have considered remove_vma and its previous > > > no MM locking assumption. You can argue that fput is async and close > > > callback shouldn't really need mmap_sem. > > > > Should I post another version of this patch with the patch description > > clarifying these points and additional comments as you suggested? > > Yes please. Will re-post. So, to clarify, we want: - Patch description to include explanation that remove_vma is now being called under MM lock but this should not be a problem because fput and vm_ops->close do not and should not take mmap_sem. - Add a comment for vm_ops->close that the callback should not take mmap_sem, with explanation that __split_vma and exit_mmap use this callback with the mmap_sem write lock taken. Is that correct? > > -- > Michal Hocko > SUSE Labs
On Tue, Dec 7, 2021 at 8:50 AM Suren Baghdasaryan <surenb@google.com> wrote: > > On Tue, Dec 7, 2021 at 2:10 AM Michal Hocko <mhocko@suse.com> wrote: > > > > On Mon 06-12-21 10:35:03, Suren Baghdasaryan wrote: > > > On Mon, Nov 29, 2021 at 3:23 AM Michal Hocko <mhocko@suse.com> wrote: > > > > > > > > On Wed 24-11-21 15:59:05, Suren Baghdasaryan wrote: > > > > > oom-reaper and process_mrelease system call should protect against > > > > > races with exit_mmap which can destroy page tables while they > > > > > walk the VMA tree. oom-reaper protects from that race by setting > > > > > MMF_OOM_VICTIM and by relying on exit_mmap to set MMF_OOM_SKIP > > > > > before taking and releasing mmap_write_lock. process_mrelease has > > > > > to elevate mm->mm_users to prevent such race. Both oom-reaper and > > > > > process_mrelease hold mmap_read_lock when walking the VMA tree. > > > > > The locking rules and mechanisms could be simpler if exit_mmap takes > > > > > mmap_write_lock while executing destructive operations such as > > > > > free_pgtables. > > > > > Change exit_mmap to hold the mmap_write_lock when calling > > > > > free_pgtables. Operations like unmap_vmas() and unlock_range() are not > > > > > destructive and could run under mmap_read_lock but for simplicity we > > > > > take one mmap_write_lock during almost the entire operation. Note > > > > > also that because oom-reaper checks VM_LOCKED flag, unlock_range() > > > > > should not be allowed to race with it. > > > > > In most cases this lock should be uncontended. Previously, Kirill > > > > > reported ~4% regression caused by a similar change [1]. We reran the > > > > > same test and although the individual results are quite noisy, the > > > > > percentiles show lower regression with 1.6% being the worst case [2]. > > > > > The change allows oom-reaper and process_mrelease to execute safely > > > > > under mmap_read_lock without worries that exit_mmap might destroy page > > > > > tables from under them. > > > > > > > > > > [1] https://lore.kernel.org/all/20170725141723.ivukwhddk2voyhuc@node.shutemov.name/ > > > > > [2] https://lore.kernel.org/all/CAJuCfpGC9-c9P40x7oy=jy5SphMcd0o0G_6U1-+JAziGKG6dGA@mail.gmail.com/ > > > > > > > > > > Signed-off-by: Suren Baghdasaryan <surenb@google.com> > > > > > --- > > > > > changes in v2 > > > > > - Moved mmap_write_unlock to cover remove_vma loop as well, per Matthew Wilcox > > > > > > > > > > mm/mmap.c | 16 ++++++++-------- > > > > > 1 file changed, 8 insertions(+), 8 deletions(-) > > > > > > > > > > diff --git a/mm/mmap.c b/mm/mmap.c > > > > > index bfb0ea164a90..f4e09d390a07 100644 > > > > > --- a/mm/mmap.c > > > > > +++ b/mm/mmap.c > > > > > @@ -3142,25 +3142,27 @@ void exit_mmap(struct mm_struct *mm) > > > > > * to mmu_notifier_release(mm) ensures mmu notifier callbacks in > > > > > * __oom_reap_task_mm() will not block. > > > > > * > > > > > - * This needs to be done before calling munlock_vma_pages_all(), > > > > > + * This needs to be done before calling unlock_range(), > > > > > * which clears VM_LOCKED, otherwise the oom reaper cannot > > > > > * reliably test it. > > > > > */ > > > > > (void)__oom_reap_task_mm(mm); > > > > > > > > > > set_bit(MMF_OOM_SKIP, &mm->flags); > > > > > > > > Why do you keep this in place? > > > > > > Sorry for the delay, I was out last week. > > > I missed your comment about removing MMF_OOM_SKIP at > > > https://lore.kernel.org/all/YYrO%2FPwdsyaxJaNZ@dhcp22.suse.cz > > > I'll look into removing it in a separate patch, which I think would be cleaner. > > > > The point of this code was to sync up the oom_repaer and exit_mmap. Now > > that your patch uses proper locking for that to happen then MMF_OOM_SKIP > > is not really necessary. IIRC all you need to guarantee is that the vma > > tree is empty when exit_mmap does all its work - i.e set mm->mmap to > > NULL. You can do that after remove_vma loop but it would be equally safe > > at any time after vma = mm->mmap as the loop relies on the vma chain. > > Doing that after would be slightly nicer if you ask me. > > Will do. But if you don't mind I'll post the removal of MMF_OOM_SKIP > as a separate patch. This patchset has already been extensively tested > and it will be easier for me to test MMF_OOM_SKIP removal separately. > > > > > > > Other than that looks OK to me. Maybe we want to add an explicit note > > > > that vm_ops::close cannot take mmap_sem in any form. The changelog > > > > should also mention that you have considered remove_vma and its previous > > > > no MM locking assumption. You can argue that fput is async and close > > > > callback shouldn't really need mmap_sem. > > > > > > Should I post another version of this patch with the patch description > > > clarifying these points and additional comments as you suggested? > > > > Yes please. > > Will re-post. So, to clarify, we want: > - Patch description to include explanation that remove_vma is now > being called under MM lock but this should not be a problem because > fput and vm_ops->close do not and should not take mmap_sem. > - Add a comment for vm_ops->close that the callback should not take > mmap_sem, with explanation that __split_vma and exit_mmap use this > callback with the mmap_sem write lock taken. > Is that correct? Assuming my understanding was correct, posted v3 at https://lore.kernel.org/all/20211207215031.2251719-1-surenb@google.com/ > > > > > -- > > Michal Hocko > > SUSE Labs
diff --git a/mm/mmap.c b/mm/mmap.c index bfb0ea164a90..f4e09d390a07 100644 --- a/mm/mmap.c +++ b/mm/mmap.c @@ -3142,25 +3142,27 @@ void exit_mmap(struct mm_struct *mm) * to mmu_notifier_release(mm) ensures mmu notifier callbacks in * __oom_reap_task_mm() will not block. * - * This needs to be done before calling munlock_vma_pages_all(), + * This needs to be done before calling unlock_range(), * which clears VM_LOCKED, otherwise the oom reaper cannot * reliably test it. */ (void)__oom_reap_task_mm(mm); set_bit(MMF_OOM_SKIP, &mm->flags); - mmap_write_lock(mm); - mmap_write_unlock(mm); } + mmap_write_lock(mm); if (mm->locked_vm) unlock_range(mm->mmap, ULONG_MAX); arch_exit_mmap(mm); vma = mm->mmap; - if (!vma) /* Can happen if dup_mmap() received an OOM */ + if (!vma) { + /* Can happen if dup_mmap() received an OOM */ + mmap_write_unlock(mm); return; + } lru_add_drain(); flush_cache_mm(mm); @@ -3171,16 +3173,14 @@ void exit_mmap(struct mm_struct *mm) free_pgtables(&tlb, vma, FIRST_USER_ADDRESS, USER_PGTABLES_CEILING); tlb_finish_mmu(&tlb); - /* - * Walk the list again, actually closing and freeing it, - * with preemption enabled, without holding any MM locks. - */ + /* Walk the list again, actually closing and freeing it. */ while (vma) { if (vma->vm_flags & VM_ACCOUNT) nr_accounted += vma_pages(vma); vma = remove_vma(vma); cond_resched(); } + mmap_write_unlock(mm); vm_unacct_memory(nr_accounted); }
oom-reaper and process_mrelease system call should protect against races with exit_mmap which can destroy page tables while they walk the VMA tree. oom-reaper protects from that race by setting MMF_OOM_VICTIM and by relying on exit_mmap to set MMF_OOM_SKIP before taking and releasing mmap_write_lock. process_mrelease has to elevate mm->mm_users to prevent such race. Both oom-reaper and process_mrelease hold mmap_read_lock when walking the VMA tree. The locking rules and mechanisms could be simpler if exit_mmap takes mmap_write_lock while executing destructive operations such as free_pgtables. Change exit_mmap to hold the mmap_write_lock when calling free_pgtables. Operations like unmap_vmas() and unlock_range() are not destructive and could run under mmap_read_lock but for simplicity we take one mmap_write_lock during almost the entire operation. Note also that because oom-reaper checks VM_LOCKED flag, unlock_range() should not be allowed to race with it. In most cases this lock should be uncontended. Previously, Kirill reported ~4% regression caused by a similar change [1]. We reran the same test and although the individual results are quite noisy, the percentiles show lower regression with 1.6% being the worst case [2]. The change allows oom-reaper and process_mrelease to execute safely under mmap_read_lock without worries that exit_mmap might destroy page tables from under them. [1] https://lore.kernel.org/all/20170725141723.ivukwhddk2voyhuc@node.shutemov.name/ [2] https://lore.kernel.org/all/CAJuCfpGC9-c9P40x7oy=jy5SphMcd0o0G_6U1-+JAziGKG6dGA@mail.gmail.com/ Signed-off-by: Suren Baghdasaryan <surenb@google.com> --- changes in v2 - Moved mmap_write_unlock to cover remove_vma loop as well, per Matthew Wilcox mm/mmap.c | 16 ++++++++-------- 1 file changed, 8 insertions(+), 8 deletions(-)