Re: [PATCH 12/41] mm: add per-VMA lock and helper functions to control it

[Suren Baghdasaryan <surenb@google.com>]

On Tue, Jan 17, 2023 at 1:54 PM Matthew Wilcox <willy@infradead.org> wrote:
>
> On Tue, Jan 17, 2023 at 01:21:47PM -0800, Suren Baghdasaryan wrote:
> > 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.
>
> I don't want to bikeshed this, but rather than locking it seems to be
> more:
>
>         vma_start_read()
>         vma_end_read()
>         vma_start_write()
>         vma_end_write()
>         vma_downgrade_write()

Couple corrections, we would have to have vma_start_tryread() and
vma_end_write_all(). Also there is no vma_downgrade_write().
mmap_write_downgrade() simply does vma_end_write_all().

>
> ... and that these are _implemented_ with locks (in part) is an
> implementation detail?
>
> Would that reduce people's confusion?
>
> > >
> > > >         - 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