On Wed, 18 Apr 2018, Michal Hocko wrote: > > Since exit_mmap() is done without the protection of mm->mmap_sem, it is > > possible for the oom reaper to concurrently operate on an mm until > > MMF_OOM_SKIP is set. > > > > This allows munlock_vma_pages_all() to concurrently run while the oom > > reaper is operating on a vma. Since munlock_vma_pages_range() depends on > > clearing VM_LOCKED from vm_flags before actually doing the munlock to > > determine if any other vmas are locking the same memory, the check for > > VM_LOCKED in the oom reaper is racy. > > > > This is especially noticeable on architectures such as powerpc where > > clearing a huge pmd requires serialize_against_pte_lookup(). If the pmd > > is zapped by the oom reaper during follow_page_mask() after the check for > > pmd_none() is bypassed, this ends up deferencing a NULL ptl. > > > > Fix this by reusing MMF_UNSTABLE to specify that an mm should not be > > reaped. This prevents the concurrent munlock_vma_pages_range() and > > unmap_page_range(). The oom reaper will simply not operate on an mm that > > has the bit set and leave the unmapping to exit_mmap(). > > This will further complicate the protocol and actually theoretically > restores the oom lockup issues because the oom reaper doesn't set > MMF_OOM_SKIP when racing with exit_mmap so we fully rely that nothing > blocks there... So the resulting code is more fragile and tricky. > exit_mmap() does not block before set_bit(MMF_OOM_SKIP) once it is entered. > Can we try a simpler way and get back to what I was suggesting before > [1] and simply not play tricks with > down_write(&mm->mmap_sem); > up_write(&mm->mmap_sem); > > and use the write lock in exit_mmap for oom_victims? > > Andrea wanted to make this more clever but this is the second fallout > which could have been prevented. The patch would be smaller and the > locking protocol easier > > [1] http://lkml.kernel.org/r/20170727065023.GB20970@xxxxxxxxxxxxxx > exit_mmap() doesn't need to protect munlock, unmap, or freeing pgtables with mm->mmap_sem; the issue is that you need to start holding it in this case before munlock and then until at least the end of free_pgtables(). Anything in between also needlessly holds it so could introduce weird lockdep issues that only trigger for oom victims, i.e. they could be very rare on some configs. I don't necessarily like holding a mutex over functions where it's actually not needed, not only as a general principle but also because the oom reaper can now infer that reaping isn't possible just because it can't do down_read() and isn't aware the thread is actually in exit_mmap() needlessly holding it. I like how the oom reaper currently retries on failing to grab mm->mmap_sem and then backs out because it's assumed it can't make forward progress. Adding additional complication for situations where mm->mmap_sem is contended (and munlock to free_pgtables() can take a long time for certain processes) to check if it's actually already in exit_mmap() would seem more complicated than this. The patch is simply using MMF_UNSTABLE rather than MMF_OOM_SKIP to serialize exit_mmap() with the oom reaper and doing it before anything interesting in exit_mmap() because without it the munlock can trivially race with unmap_page_range() and cause a NULL pointer or #GP on a pmd or pte. The way Andrea implemented it is fine, we simply have revealed a race between munlock_vma_pages_all() and unmap_page_range() that needs it to do set_bit(); down_write(); up_write(); earlier.
