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

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Message-ID: <b4m3w6wuw3h6ke7qlvimly7nok4ymjvnej2vx3lnds3vysyopr@6b5bnifyst24>
Date: Tue, 14 Jan 2025 14:19:01 +0100
From: Jan Kara <jack@...e.cz>
To: Guenter Roeck <linux@...ck-us.net>
Cc: Jim Zhao <jimzhao.ai@...il.com>, akpm@...ux-foundation.org, 
	jack@...e.cz, 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 Mon 13-01-25 15:05:25, Guenter Roeck wrote:
> Hi,
> 
> 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?

								Honza

> udhcpc: no lease, failing
> FAIL
> /etc/init.d/S55runtest: line 29: awk: Permission denied
> Found console ttySC1
> /etc/init.d/S55runtest: line 45: awk: Permission denied
> can't run '/sbin/getty': No such device or address
> 
> and boot stalls from there. There is no obvious kernel log message
> associated with the problem. Reverting this patch fixes the problem.
> 
> Bisect log is attached for reference.
> 
> Guenter
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR