On Tue 17-01-23 16:04:26, Michal Hocko wrote: > On Mon 09-01-23 12:53:07, Suren Baghdasaryan wrote: > > Introduce a per-VMA rw_semaphore to be used during page fault handling > > instead of mmap_lock. Because there are cases when multiple VMAs need > > to be exclusively locked during VMA tree modifications, instead of the > > usual lock/unlock patter we mark a VMA as locked by taking per-VMA lock > > exclusively and setting vma->lock_seq to the current mm->lock_seq. When > > mmap_write_lock holder is done with all modifications and drops mmap_lock, > > it will increment mm->lock_seq, effectively unlocking all VMAs marked as > > locked. > > I have to say I was struggling a bit with the above and only understood > what you mean by reading the patch several times. I would phrase it like > this (feel free to use if you consider this to be an improvement). > > Introduce a per-VMA rw_semaphore. The lock implementation relies on a > per-vma and per-mm sequence counters to note exclusive locking: > - read lock - (implemented by vma_read_trylock) requires the the > vma (vm_lock_seq) and mm (mm_lock_seq) sequence counters to > differ. If they match then there must be a vma exclusive lock > held somewhere. > - read unlock - (implemented by vma_read_unlock) is a trivial > vma->lock unlock. > - write lock - (vma_write_lock) requires the mmap_lock to be > held exclusively and the current mm counter is noted to the vma > side. This will allow multiple vmas to be locked under a single > mmap_lock write lock (e.g. during vma merging). The vma counter > is modified under exclusive vma lock. Didn't realize one more thing. Unlike standard write lock this implementation allows to be called multiple times under a single mmap_lock. In a sense it is more of mark_vma_potentially_modified than a lock. > - write unlock - (vma_write_unlock_mm) is a batch release of all > vma locks held. It doesn't pair with a specific > vma_write_lock! It is done before exclusive mmap_lock is > released by incrementing mm sequence counter (mm_lock_seq). > - write downgrade - if the mmap_lock is downgraded to the read > lock all vma write locks are released as well (effectivelly > same as write unlock). -- Michal Hocko SUSE Labs
