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	Mon, 28 Sep 2026 09:44:17 +0000
Date: Mon, 28 Sep 2026 02:44:17 -0700
From: Christoph Hellwig <hch@infradead.org>
To: Alexandre Ghiti <alex@ghiti.fr>
Cc: 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: <aro28ZGuYgmNg_n5@infradead.org>
References: <20260928081900.4187482-1-alex@ghiti.fr>
 <20260928081900.4187482-2-alex@ghiti.fr>
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On Mon, Sep 28, 2026 at 10:18:36AM +0200, Alexandre Ghiti wrote:
> bio_issue_as_root_blkg() issues every REQ_SWAP bio as root, bypassing
> the cgroup IO controllers, because reclaim cannot stall indefinitely on
> swap writes. Some swap writes are not urgent though: zswap writeback is
> best effort.
> 
> 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.