linux-kernel - Re: [RFC PATCHv5 0/6] zram: introduce writeback bio batching

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Message-ID: <nx6o4gwpetxjeyfbu4xyibulvldr3xz6lyfjrar62cidy5gxum@xmx4ojyq3mbf>
Date: Sun, 23 Nov 2025 09:08:25 +0900
From: Sergey Senozhatsky <senozhatsky@...omium.org>
To: Gao Xiang <hsiangkao@...ux.alibaba.com>
Cc: Sergey Senozhatsky <senozhatsky@...omium.org>, 
	Yuwen Chen <ywen.chen@...mail.com>, akpm@...ux-foundation.org, bgeffon@...gle.com, 
	licayy@...look.com, linux-block@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-mm@...ck.org, minchan@...nel.org, richardycc@...gle.com
Subject: Re: [RFC PATCHv5 0/6] zram: introduce writeback bio batching

On (25/11/22 22:09), Gao Xiang wrote:
> > I thought you were talking about the backing device being
> > ext4/btrfs.  Sorry, I don't have enough context/knowledge
> > to understand what you're getting at.  zram has been doing
> > writeback for ages, I really don't know what you mean by
> > "to act like this".
> 
> I mean, if zram is formatted as ext4, and then mount it;
> and then there is a backing file which is also in another
> ext4, you'd need a workqueue to do writeback I/Os (or needs
> a loop device to transit), was that the original question
> raised by Yuwen?

We take pages of data from zram0 and write them straight to
the backing device.  Those writes don't go through vfs/fs so
fs on the backing device will simply be corrupted, as far as
I can tell.  This is not intendant use case for zram writeback.