Re: [PATCH v10 12/18] mm: replace vm_lock and detached flag with a reference count

[Vlastimil Babka <vbabka@suse.cz>]

On 2/13/25 23:46, 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).
> 
> 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 (plus the VMA_LOCK_OFFSET writer bit), indicating an attached vma
> with no readers;
> 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.
> 
> Suggested-by: Peter Zijlstra <peterz@infradead.org>
> Suggested-by: Matthew Wilcox <willy@infradead.org>
> Signed-off-by: Suren Baghdasaryan <surenb@google.com>

with the fix,

Reviewed-by: Vlastimil Babka <vbabka@suse.cz>