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

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Date:   Tue, 17 Jan 2023 16:12:42 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Suren Baghdasaryan <surenb@...gle.com>
Cc:     akpm@...ux-foundation.org, michel@...pinasse.org,
        jglisse@...gle.com, vbabka@...e.cz, hannes@...xchg.org,
        mgorman@...hsingularity.net, dave@...olabs.net,
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        ldufour@...ux.ibm.com, laurent.dufour@...ibm.com,
        paulmck@...nel.org, luto@...nel.org, songliubraving@...com,
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        hughd@...gle.com, bigeasy@...utronix.de, kent.overstreet@...ux.dev,
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        arjunroy@...gle.com, soheil@...gle.com, hughlynch@...gle.com,
        leewalsh@...gle.com, posk@...gle.com, linux-mm@...ck.org,
        linux-arm-kernel@...ts.infradead.org,
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Subject: Re: [PATCH 12/41] mm: add per-VMA lock and helper functions to
 control it

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.

>         - 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