On Thu, Feb 23, 2023 at 6:51 AM Hyeonggon Yoo [off-list ref] wrote:

On Wed, Feb 15, 2023 at 09:17:31PM -0800, Suren Baghdasaryan wrote:

quoted

Decisions about whether VMAs can be merged, split or expanded must be
made while VMAs are protected from the changes which can affect that
decision. For example, merge_vma uses vma->anon_vma in its decision

Did you mean vma_merge()?

Correct.

quoted

whether the VMA can be merged. Meanwhile, page fault handler changes
vma->anon_vma during COW operation.
Write-lock all VMAs which might be affected by a merge or split operation
before making decision how such operations should be performed.

It doesn't make sense (to me) to update vma->anon_vma during COW fault.

AFAIK children's vma->anon_vma is allocated during fork() and
page->anon_vma is updated on COW to reduce rmap walking because it's now
unshared, no?

As patch 26 just falls back to mmap_lock if no anon_vma is set,
I think we can assume nothing updates vma->anon_vma (and its interval
tree) if we are holding mmap_lock in write mode.

Or am I missing something?

No, I think you are right. Patch 26 was added in the later versions of
this patchset and at the time this patch was written vma->anon_vma
could change during page fault handling under only per-VMA lock
protection.
So, this patch can be simplified. We still want to prevent concurrent
page faults while VMA is being merged or split (simply because par-VMA
lock that page fault handler holds might become the wrong one if the
VMA is split or merged from under it) but the timing of taking per-VMA
lock does not have to be *before* can_vma_merge_{before|after}. Does
that make sense?

--
Regards,
Hyeonggon

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