On Thu, Feb 16, 2023 at 7:44 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: > > 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? Yeah, I remember that conversation Matthew and I could not find the definitive answer at the time. I'll look into that again or maybe someone can answer it here. In the end rather than changing that logic I decided to skip vma->anon_vma==NULL cases because I measured them being less than 0.01% of all page faults, so ROI from changing that would be quite low. But I agree that the logic is weird and maybe we can improve that. I will have to review that again when I'm working on eliminating all these special cases we skip, like swap/userfaults/etc.
