On Fri, Feb 17, 2023 at 2:21 AM Hyeonggon Yoo <42.hyeyoo@xxxxxxxxx> wrote: > > On Fri, Feb 17, 2023 at 11:15 AM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote: > > > > On Thu, Feb 16, 2023 at 11:43 AM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote: > > > > > > 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. > > > > After looking into it again I'm still under the impression that > > vma->anon_vma is populated lazily (during the first page fault rather > > than at mmap time) to avoid doing extra work for areas which are never > > faulted. Though I might be missing some important detail here. > > I think this is because the kernel cannot merge VMAs that have > different anon_vmas? > > Enabling lazy population of anon_vma could potentially increase the > chances of merging VMAs. Hmm. Do you have a clear explanation why merging chances increase this way? A couple of possibilities I can think of would be: 1. If after mmap'ing a VMA and before faulting the first page into it we often change something that affects anon_vma_compatible() decision, like vm_policy; 2. When mmap'ing VMAs we do not map them consecutively but the final arrangement is actually contiguous. Don't think either of those cases would be very representative of a usual case but maybe I'm wrong or there is another reason? > > > > 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. > > -- > To unsubscribe from this group and stop receiving emails from it, send an email to kernel-team+unsubscribe@xxxxxxxxxxx. >
