[PATCH v2 1/2] mm: protect free_pgtables with mmap_lock write lock in exit_mmap

Suren Baghdasaryan
2021-11-24T23:59:14Z

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);
- 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);
}

--
2.34.0.rc2.393.gf8c9666880-goog