[PATCH v3 26/35] mm: fall back to mmap_lock if vma->anon_vma is not yet set

[Matthew Wilcox]

On Wed, Feb 15, 2023 at 09:17:41PM -0800, Suren Baghdasaryan wrote:
> When vma->anon_vma is not set, page fault handler will set it by either
> reusing anon_vma of an adjacent VMA if VMAs are compatible or by
> allocating a new one. find_mergeable_anon_vma() walks VMA tree to find
> a compatible adjacent VMA and that requires not only the faulting VMA
> to be stable but also the tree structure and other VMAs inside that tree.
> Therefore locking just the faulting VMA is not enough for this search.
> Fall back to taking mmap_lock when vma->anon_vma is not set. This
> situation happens only on the first page fault and should not affect
> overall performance.

I think I asked this before, but don't remember getting an aswer.
Why do we defer setting anon_vma to the first fault?  Why don't we
set it up at mmap time?