Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Tue, 17 Jan 2023 16:12:42 +0100
From:   Michal Hocko <mhocko@suse.com>
To:     Suren Baghdasaryan <surenb@google.com>
Cc:     akpm@linux-foundation.org, michel@lespinasse.org,
        jglisse@google.com, vbabka@suse.cz, hannes@cmpxchg.org,
        mgorman@techsingularity.net, dave@stgolabs.net,
        willy@infradead.org, liam.howlett@oracle.com, peterz@infradead.org,
        ldufour@linux.ibm.com, laurent.dufour@fr.ibm.com,
        paulmck@kernel.org, luto@kernel.org, songliubraving@fb.com,
        peterx@redhat.com, david@redhat.com, dhowells@redhat.com,
        hughd@google.com, bigeasy@linutronix.de, kent.overstreet@linux.dev,
        punit.agrawal@bytedance.com, lstoakes@gmail.com,
        peterjung1337@gmail.com, rientjes@google.com,
        axelrasmussen@google.com, joelaf@google.com, minchan@google.com,
        jannh@google.com, shakeelb@google.com, tatashin@google.com,
        edumazet@google.com, gthelen@google.com, gurua@google.com,
        arjunroy@google.com, soheil@google.com, hughlynch@google.com,
        leewalsh@google.com, posk@google.com, linux-mm@kvack.org,
        linux-arm-kernel@lists.infradead.org,
        linuxppc-dev@lists.ozlabs.org, x86@kernel.org,
        linux-kernel@vger.kernel.org, kernel-team@android.com
Subject: Re: [PATCH 12/41] mm: add per-VMA lock and helper functions to
 control it
Message-ID: <Y8a66gshQkkhC1cT@dhcp22.suse.cz>
References: <20230109205336.3665937-1-surenb@google.com>
 <20230109205336.3665937-13-surenb@google.com>
 <Y8a4+bV1dYNAiUkD@dhcp22.suse.cz>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <Y8a4+bV1dYNAiUkD@dhcp22.suse.cz>
Precedence: bulk

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