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

Wei Yang
2025-01-13T00:59:43Z

On Sun, Jan 12, 2025 at 09:35:25AM -0800, Suren Baghdasaryan wrote:
>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)?
>

Hmm.. hard to say. It is a little confusing, but I don't have a better one :-(

>>
>> >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

--
Wei Yang
Help you, Help me