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. >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. >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
