On Thu, Sep 01, 2022 at 10:34:48AM -0700, Suren Baghdasaryan wrote: > Resending to fix the issue with the In-Reply-To tag in the original > submission at [4]. > > This is a proof of concept for per-vma locks idea that was discussed > during SPF [1] discussion at LSF/MM this year [2], which concluded with > suggestion that “a reader/writer semaphore could be put into the VMA > itself; that would have the effect of using the VMA as a sort of range > lock. There would still be contention at the VMA level, but it would be an > improvement.” This patchset implements this suggested approach. > > When handling page faults we lookup the VMA that contains the faulting > page under RCU protection and try to acquire its lock. If that fails we > fall back to using mmap_lock, similar to how SPF handled this situation. > > One notable way the implementation deviates from the proposal is the way > VMAs are marked as locked. Because during some of mm updates multiple > VMAs need to be locked until the end of the update (e.g. vma_merge, > split_vma, etc). Tracking all the locked VMAs, avoiding recursive locks > and other complications would make the code more complex. Therefore we > provide a way to "mark" VMAs as locked and then unmark all locked VMAs > all at once. This is done using two sequence numbers - one in the > vm_area_struct and one in the mm_struct. VMA is considered locked when > these sequence numbers are equal. To mark a VMA as locked we set the > sequence number in vm_area_struct to be equal to the sequence number > in mm_struct. To unlock all VMAs we increment mm_struct's seq number. > This allows for an efficient way to track locked VMAs and to drop the > locks on all VMAs at the end of the update. I like it - the sequence numbers are a stroke of genuius. For what it's doing the patchset seems almost small. Two complaints so far: - I don't like the vma_mark_locked() name. To me it says that the caller already took or is taking the lock and this function is just marking that we're holding the lock, but it's really taking a different type of lock. But this function can block, it really is taking a lock, so it should say that. This is AFAIK a new concept, not sure I'm going to have anything good either, but perhaps vma_lock_multiple()? - I don't like the #ifdef and the separate fallback path in the fault handlers. Can we make find_and_lock_anon_vma() do the right thing, and not fail unless e.g. there isn't a vma at that address? Just have it wait for vm_lock_seq to change and then retry if needed.
