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

[Suren Baghdasaryan <surenb@google.com>]

On Sat, Jan 11, 2025 at 6:59 PM Wei Yang <richard.weiyang@gmail.com> 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