On Fri, Feb 17, 2023 at 08:10:35AM -0800, Suren Baghdasaryan wrote: > On Fri, Feb 17, 2023 at 8:05 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: > > > > On Thu, Feb 16, 2023 at 06:14:59PM -0800, Suren Baghdasaryan 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. > > > > How often does userspace call mmap() and then _never_ fault on it? > > I appreciate that userspace might mmap() gigabytes of address space and > > then only end up using a small amount of it, so populating it lazily > > makes sense. But creating a region and never faulting on it? The only > > use-case I can think of is loading shared libraries: > > > > openat(AT_FDCWD, "/lib/x86_64-linux-gnu/libc.so.6", O_RDONLY|O_CLOEXEC) = 3 > > (...) > > mmap(NULL, 1970000, PROT_READ, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7f0ce612e000 > > mmap(0x7f0ce6154000, 1396736, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x26000) = 0x7f0ce6154000 > > mmap(0x7f0ce62a9000, 339968, PROT_READ, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x17b000) = 0x7f0ce62a9000 > > mmap(0x7f0ce62fc000, 24576, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x1ce000) = 0x7f0ce62fc000 > > mmap(0x7f0ce6302000, 53072, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x7f0ce6302000 > > > > but that's a file-backed VMA, not an anon VMA. > > Might the case of dup_mmap() while forking be the reason why a VMA in > the child process might be never used while parent uses it (or visa > versa)? Again, I'm not sure this is the reason but I can find no other > good explanation. I found an explanation! Well, a partial one. If we MAP_PRIVATE a file mapping (like, er those ones up there) and only take read faults on it, we can postpone allocation of the anon_vma indefinitely. But once we take a write fault in that VMA, we need to allocate an anon_vma for it so that we can track the anonymous pages that have been allocated to satisfy the copy-on-write (see do_cow_fault()). However, I think in that caase, we could probably skip the find_mergeable_anon_vma() step. We don't today; we check whether a->vm_file == b->vm_file in anon_vma_compatible, but I wonder if that triggers often.
