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Date: Thu, 5 Sep 2024 15:35:55 -0700
From: Eric Biggers <ebiggers@kernel.org>
To: Mikulas Patocka <mpatocka@redhat.com>
Cc: dm-devel@lists.linux.dev, Alasdair Kergon <agk@redhat.com>,
	Mike Snitzer <snitzer@kernel.org>, Tejun Heo <tj@kernel.org>,
	Lai Jiangshan <jiangshanlai@gmail.com>,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	Sami Tolvanen <samitolvanen@google.com>
Subject: Re: [PATCH] dm verity: don't use WQ_MEM_RECLAIM
Message-ID: <20240905223555.GA1512@sol.localdomain>
References: <20240904040444.56070-1-ebiggers@kernel.org>
 <086a76c4-98da-d9d1-9f2f-6249c3d55fe9@redhat.com>
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In-Reply-To: <086a76c4-98da-d9d1-9f2f-6249c3d55fe9@redhat.com>

On Thu, Sep 05, 2024 at 08:21:46PM +0200, Mikulas Patocka wrote:
> 
> 
> On Tue, 3 Sep 2024, Eric Biggers wrote:
> 
> > From: Eric Biggers <ebiggers@google.com>
> > 
> > Since dm-verity doesn't support writes, the kernel's memory reclaim code
> > will never wait on dm-verity work.  That makes the use of WQ_MEM_RECLAIM
> > in dm-verity unnecessary.  WQ_MEM_RECLAIM has been present from the
> > beginning of dm-verity, but I could not find a justification for it;
> > I suspect it was just copied from dm-crypt which does support writes.
> > 
> > Therefore, remove WQ_MEM_RECLAIM from dm-verity.  This eliminates the
> > creation of an unnecessary rescuer thread per dm-verity device.
> > 
> > Signed-off-by: Eric Biggers <ebiggers@google.com>
> 
> Hmm. I can think about a case where you have read-only dm-verity device, 
> on the top of that you have dm-snapshot device and on the top of that you 
> have a writable filesystem.
> 
> When the filesystem needs to write data, it submits some write bios. When 
> dm-snapshot receives these write bios, it will read from the dm-verity 
> device and write to the snapshot's exception store device. So, dm-verity 
> needs WQ_MEM_RECLAIM in this case.
> 
> Mikulas
> 

Yes, unfortunately that sounds correct.

This means that any workqueue involved in fulfilling block device I/O,
regardless of whether that I/O is read or write, has to use WQ_MEM_RECLAIM.

I wonder if there's any way to safely share the rescuer threads.

- Eric