Re: [PATCH v9 11/17] mm: replace vm_lock and detached flag with a reference count

[Wei Yang <richard.weiyang@gmail.com>]

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
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