On 10/30/25 20:47, Lorenzo Stoakes wrote: > On Thu, Oct 30, 2025 at 07:47:34PM +0100, Vlastimil Babka wrote: >> > >> > Could we use MADVISE_VMA_READ_LOCK mode (would be actually an improvement >> > over the current MADVISE_MMAP_READ_LOCK), together with the atomic flag >> > setting? I think the places that could race with us to cause RMW use vma >> > write lock so that would be excluded. Fork AFAICS unfortunately doesn't >> > (for >> > the oldmm) and it probably would't make sense to start doing it. Maybe we >> > could think of something to deal with this special case... >> >> During discussion with Pedro off-list I realized fork takes mmap lock for >> write on the old mm, so if we kept taking mmap sem for read, then vma lock >> for read in addition (which should be cheap enough, also we'd only need it >> in case VM_MAYBE_GUARD is not yet set), and set the flag atomicaly, perhaps >> that would cover all non-bening races? >> >> > > We take VMA write lock in dup_mmap() on each mpnt (old VMA).
Ah yes I thought it was the new one. > We take the VMA write lock (vma_start_write()) for each mpnt. > > We then vm_area_dup() the mpnt to the new VMA before calling: > > copy_page_range() > -> vma_needs_copy() > > Which is where the check is done. > > So we are holding the VMA write lock, so a VMA read lock should suffice no? Yeah, even better! > For belts + braces we could atomically read the flag in vma_needs_copy(), > though note it's intended VM_COPY_ON_FORK could have more than one flag. > > We could drop that for now and be explicit. Great!
