On Wed, May 20, 2026 at 05:51:23AM +0000, Suren Baghdasaryan wrote: > On Tue, May 19, 2026 at 12:53 PM Lorenzo Stoakes <[email protected]> wrote: > > > > On Mon, May 18, 2026 at 12:56:59PM -0700, Suren Baghdasaryan wrote: > > > > > > > > > > I think we either need to fix `fork()`, or keep the current > > > > behavior of dropping the VMA lock before performing I/O. > > > > > > I see. So, this problem arises from the fact that we are changing the > > > pagefaults requiring I/O operation to hold VMA lock... > > > And you want to lock VMA on fork only if vma_is_anonymous(vma) || > > > is_cow_mapping(vma->vm_flags). So, we will be blocking page faults for > > > anonymous and COW VMAs only while holding mmap_write_lock, preventing > > > any VMA modification. On the surface, that looks ok to me but I might > > > be missing some corner cases. If nobody sees any obvious issues, I > > > think it's worth a try. > > > > Not sure if you noticed but I did raise concerns ;) > > Sorry, I didn't realize your first comment was a conceptual objection > to this approach of allowing page faults to race with the fork.
Ah yeah it's understandable I think there's been so many threads in this conversation that it's easy to get lost :) > > > > > > I wonder if you've confused the fault path and fork here, as I think Barry > > has > > been a little unclear on that. > > > > What's being suggested in this thread is to fundamentally change fork > > behaviour > > so it's different from the entire history of the kernel (or - presumably - > > at > > least recent history :) and permit concurrent page faults to occur on a > > forking > > process. > > > > I absolutely object to this for being pretty crazy. I mean I'm not sure we > > really want to be simultaneously modifying page tables while invoking > > copy_page_range()? No? > > > > OK you cover anon and MAP_PRIVATE file-backed but hang on there's > > VM_COPY_ON_FORK too.. so PFN mapped, mixed map and (the accursed) UFFD W/P > > as > > well as possibly-guard region containing VMAs now can have page tables > > raced. > > Ugh, yeah, I realize now this is a minefield. Resolving all possible > races there would not be trivial and might introduce other performance > issues. Yeah, it's dangerous waters :) > > > > > That's not to mention anything else that relies on serialisation here (this > > would be changing how forking has been done in general) that we may or may > > not > > know about. > > > > The risk level is high, for what amounts to a hack to work around the fault > > issue. > > > > I suggest that if we have a problem with the fault path, let's look at the > > fault > > path :) > > > > So yeah I'm very opposed to this unless I'm somehow horribly mistaken here > > or a > > very convincing argument is made. > > So, current approach of dropping locks during I/O sounds like still > the best solution. Yeah _of those proposed_ I think importantly. This doesn't mean there aren't other potential solutions. Thanks, Lorenzo > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > I'd also like to get Suren's input, however. > > > > > > > > Yes. of course. > > > > > > > > > > > > > > Thanks, Lorenzo > > > > > > > > Best Regards > > > > Barry > > > > Cheers, Lorenzo
