linux-kernel - Re: [PATCH] mm/page-writeback: Consolidate wb_thresh bumping logic into __wb_calc

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Message-ID: <mqe2boksd5ztaz7xyabyp4sbtufxthcnrbwrjayghe4hpfbp4w@wjqsm467sjp5>
Date: Wed, 15 Jan 2025 17:07:36 +0100
From: Jan Kara <jack@...e.cz>
To: Guenter Roeck <linux@...ck-us.net>
Cc: Jan Kara <jack@...e.cz>, Jim Zhao <jimzhao.ai@...il.com>, 
	akpm@...ux-foundation.org, willy@...radead.org, linux-fsdevel@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] mm/page-writeback: Consolidate wb_thresh bumping logic
 into __wb_calc_thresh

On Tue 14-01-25 07:01:08, Guenter Roeck wrote:
> On 1/14/25 05:19, Jan Kara wrote:
> > On Mon 13-01-25 15:05:25, Guenter Roeck wrote:
> > > On Thu, Nov 21, 2024 at 06:05:39PM +0800, Jim Zhao wrote:
> > > > Address the feedback from "mm/page-writeback: raise wb_thresh to prevent
> > > > write blocking with strictlimit"(39ac99852fca98ca44d52716d792dfaf24981f53).
> > > > The wb_thresh bumping logic is scattered across wb_position_ratio,
> > > > __wb_calc_thresh, and wb_update_dirty_ratelimit. For consistency,
> > > > consolidate all wb_thresh bumping logic into __wb_calc_thresh.
> > > > 
> > > > Reviewed-by: Jan Kara <jack@...e.cz>
> > > > Signed-off-by: Jim Zhao <jimzhao.ai@...il.com>
> > > 
> > > This patch triggers a boot failure with one of my 'sheb' boot tests.
> > > It is seen when trying to boot from flash (mtd). The log says
> > > 
> > > ...
> > > Starting network: 8139cp 0000:00:02.0 eth0: link down
> > > udhcpc: started, v1.33.0
> > > EXT2-fs (mtdblock3): error: ext2_check_folio: bad entry in directory #363: : directory entry across blocks - offset=0, inode=27393, rec_len=3072, name_len=2
> > > udhcpc: sending discover
> > > udhcpc: sending discover
> > > udhcpc: sending discover
> > > EXT2-fs (mtdblock3): error: ext2_check_folio: bad entry in directory #363: : directory entry across blocks - offset=0, inode=27393, rec_len=3072, name_len=2
> > 
> > Thanks for report! Uh, I have to say I'm very confused by this. It is clear
> > than when ext2 detects the directory corruption (we fail checking directory
> > inode 363 which is likely /etc/init.d/), the boot fails in interesting
> > ways. What is unclear is how the commit can possibly cause ext2 directory
> > corruption.  If you didn't verify reverting the commit fixes the issue, I'd
> > be suspecting bad bisection but that obviously isn't the case :-)
> > 
> > Ext2 is storing directory data in the page cache so at least it uses the
> > subsystem which the patch impacts but how writeback throttling can cause
> > ext2 directory corruption is beyond me. BTW, do you recreate the root
> > filesystem before each boot? How exactly?
> 
> I use pre-built root file systems. For sheb, they are at
> https://github.com/groeck/linux-build-test/tree/master/rootfs/sheb

Thanks. So the problematic directory is /usr/share/udhcpc/ where we
read apparently bogus metadata at the beginning of that directory.

> I don't think this is related to ext2 itself. Booting an ext2 image from
> ata/ide drive works.

Interesting this is specific to mtd. I'll read the patch carefully again if
something rings a bell.

								Honza

-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR