On Sat, Jan 11, 2025 at 6:59 PM Wei Yang <richard.weiyang@xxxxxxxxx> wrote: > > On Fri, Jan 10, 2025 at 08:25:58PM -0800, Suren Baghdasaryan wrote: > >rw_semaphore is a sizable structure of 40 bytes and consumes > >considerable space for each vm_area_struct. However vma_lock has > >two important specifics which can be used to replace rw_semaphore > >with a simpler structure: > >1. Readers never wait. They try to take the vma_lock and fall back to > >mmap_lock if that fails. > >2. Only one writer at a time will ever try to write-lock a vma_lock > >because writers first take mmap_lock in write mode. > >Because of these requirements, full rw_semaphore functionality is not > >needed and we can replace rw_semaphore and the vma->detached flag with > >a refcount (vm_refcnt). > > This paragraph is merged into the above one in the commit log, which may not > what you expect. > > Just a format issue, not sure why they are not separated. I'll double-check the formatting. Thanks! > > >When vma is in detached state, vm_refcnt is 0 and only a call to > >vma_mark_attached() can take it out of this state. Note that unlike > >before, now we enforce both vma_mark_attached() and vma_mark_detached() > >to be done only after vma has been write-locked. vma_mark_attached() > >changes vm_refcnt to 1 to indicate that it has been attached to the vma > >tree. When a reader takes read lock, it increments vm_refcnt, unless the > >top usable bit of vm_refcnt (0x40000000) is set, indicating presence of > >a writer. When writer takes write lock, it sets the top usable bit to > >indicate its presence. If there are readers, writer will wait using newly > >introduced mm->vma_writer_wait. Since all writers take mmap_lock in write > >mode first, there can be only one writer at a time. The last reader to > >release the lock will signal the writer to wake up. > >refcount might overflow if there are many competing readers, in which case > >read-locking will fail. Readers are expected to handle such failures. > >In summary: > >1. all readers increment the vm_refcnt; > >2. writer sets top usable (writer) bit of vm_refcnt; > >3. readers cannot increment the vm_refcnt if the writer bit is set; > >4. in the presence of readers, writer must wait for the vm_refcnt to drop > >to 1 (ignoring the writer bit), indicating an attached vma with no readers; > > It waits until to (VMA_LOCK_OFFSET + 1) as indicates in __vma_start_write(), > if I am right. Yeah, that's why I mentioned "(ignoring the writer bit)" but maybe that's too confusing. How about "drop to 1 (plus the VMA_LOCK_OFFSET writer bit)? > > >5. vm_refcnt overflow is handled by the readers. > > > >While this vm_lock replacement does not yet result in a smaller > >vm_area_struct (it stays at 256 bytes due to cacheline alignment), it > >allows for further size optimization by structure member regrouping > >to bring the size of vm_area_struct below 192 bytes. > > > -- > Wei Yang > Help you, Help me
