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	Mon, 05 Oct 2026 08:23:23 +0000
Date: Mon, 5 Oct 2026 01:23:22 -0700
From: Christoph Hellwig <hch@infradead.org>
To: Alexandre Ghiti <alexghiti@meta.com>
Cc: Christoph Hellwig <hch@infradead.org>, Alexandre Ghiti <alex@ghiti.fr>,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org,
	linux-block@vger.kernel.org, cgroups@vger.kernel.org,
	Andrew Morton <akpm@linux-foundation.org>, david@kernel.org,
	Johannes Weiner <hannes@cmpxchg.org>,
	Yosry Ahmed <yosry@kernel.org>, Nhat Pham <nphamcs@gmail.com>,
	Chengming Zhou <chengming.zhou@linux.dev>,
	Jens Axboe <axboe@kernel.dk>, Tejun Heo <tj@kernel.org>,
	Josef Bacik <josef@toxicpanda.com>, Chris Li <chrisl@kernel.org>,
	Kairui Song <kasong@tencent.com>,
	Kemeng Shi <shikemeng@huaweicloud.com>,
	Baoquan He <baoquan.he@linux.dev>, Barry Song <baohua@kernel.org>,
	Youngjun Park <youngjun.park@lge.com>
Subject: Re: [RFC PATCH 1/4] block: do not issue background swap bios as root
Message-ID: <asNeenTiIme0kzI6@infradead.org>
References: <20260928081900.4187482-1-alex@ghiti.fr>
 <20260928081900.4187482-2-alex@ghiti.fr>
 <aro28ZGuYgmNg_n5@infradead.org>
 <CAEmasaX-6v_EpwAxHtnhA5SXk=jGW8+Lm4m8zAC=+f0yXmiCxw@mail.gmail.com>
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On Tue, Sep 29, 2026 at 11:16:03AM +0200, Alexandre Ghiti wrote:
> > >
> > > Such a bio cannot simply drop REQ_SWAP: dm-crypt and dm-thin rely on it
> > > for limit_swap_bios, which caps in-flight swap bios to avoid a memory
> > > deadlock, see commit a666e5c05e7c ("dm: fix deadlock when swapping to
> > > encrypted device").
> >
> > Is that true?  REQ_BACKGROUND very much implies the I/O can the throttled
> > and thus better not be foreground paging, and thus the caller should be
> > able to tolerate a failure.
> >
> 
> Yes, I agree, zswap writebacks are only best effort and in case of
> failure, the compressed entry still lives. I don't see how
> REQ_BACKGROUND could be an issue there?

The issue is that by definition it doesn't need REQ_SWAP then.

Then again the whole concept of REQ_SWAP looks sketchy to me, non-swap
paging can be just as important for forward progress, so if a driver
needs it, it has other issues anyway.