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From:   Suren Baghdasaryan <surenb@google.com>
Date:   Tue, 17 Jan 2023 13:21:47 -0800
Message-ID: <CAJuCfpF2ciMbMFgqa4GqYqg8zjpnmBoNZDG23V0+w017LXeU3w@mail.gmail.com>
Subject: Re: [PATCH 12/41] mm: add per-VMA lock and helper functions to
 control it
To:     Michal Hocko <mhocko@suse.com>
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On Tue, Jan 17, 2023 at 7:12 AM Michal Hocko <mhocko@suse.com> wrote:
>
> On Tue 17-01-23 16:04:26, Michal Hocko wrote:
> > On Mon 09-01-23 12:53:07, Suren Baghdasaryan wrote:
> > > Introduce a per-VMA rw_semaphore to be used during page fault handling
> > > instead of mmap_lock. Because there are cases when multiple VMAs need
> > > to be exclusively locked during VMA tree modifications, instead of the
> > > usual lock/unlock patter we mark a VMA as locked by taking per-VMA lock
> > > exclusively and setting vma->lock_seq to the current mm->lock_seq. When
> > > mmap_write_lock holder is done with all modifications and drops mmap_lock,
> > > it will increment mm->lock_seq, effectively unlocking all VMAs marked as
> > > locked.
> >
> > I have to say I was struggling a bit with the above and only understood
> > what you mean by reading the patch several times. I would phrase it like
> > this (feel free to use if you consider this to be an improvement).
> >
> > Introduce a per-VMA rw_semaphore. The lock implementation relies on a
> > per-vma and per-mm sequence counters to note exclusive locking:
> >         - read lock - (implemented by vma_read_trylock) requires the the
> >           vma (vm_lock_seq) and mm (mm_lock_seq) sequence counters to
> >           differ. If they match then there must be a vma exclusive lock
> >           held somewhere.
> >         - read unlock - (implemented by vma_read_unlock) is a trivial
> >           vma->lock unlock.
> >         - write lock - (vma_write_lock) requires the mmap_lock to be
> >           held exclusively and the current mm counter is noted to the vma
> >           side. This will allow multiple vmas to be locked under a single
> >           mmap_lock write lock (e.g. during vma merging). The vma counter
> >           is modified under exclusive vma lock.
>
> Didn't realize one more thing.
>             Unlike standard write lock this implementation allows to be
>             called multiple times under a single mmap_lock. In a sense
>             it is more of mark_vma_potentially_modified than a lock.

In the RFC it was called vma_mark_locked() originally and renames were
discussed in the email thread ending here:
https://lore.kernel.org/all/621612d7-c537-3971-9520-a3dec7b43cb4@suse.cz/.
If other names are preferable I'm open to changing them.

>
> >         - write unlock - (vma_write_unlock_mm) is a batch release of all
> >           vma locks held. It doesn't pair with a specific
> >           vma_write_lock! It is done before exclusive mmap_lock is
> >           released by incrementing mm sequence counter (mm_lock_seq).
> >       - write downgrade - if the mmap_lock is downgraded to the read
> >         lock all vma write locks are released as well (effectivelly
> >         same as write unlock).
> --
> Michal Hocko
> SUSE Labs