linux-ext4 - Re: Ext4 iomap warning during btrfs/136 (yes, it's from btrfs test cases)

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Message-ID: <20250811154935.GD7942@frogsfrogsfrogs>
Date: Mon, 11 Aug 2025 08:49:35 -0700
From: "Darrick J. Wong" <djwong@...nel.org>
To: Qu Wenruo <quwenruo.btrfs@....com>
Cc: Zhang Yi <yi.zhang@...weicloud.com>, Qu Wenruo <wqu@...e.com>,
	Theodore Ts'o <tytso@....edu>,
	linux-ext4 <linux-ext4@...r.kernel.org>,
	linux-btrfs <linux-btrfs@...r.kernel.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>
Subject: Re: Ext4 iomap warning during btrfs/136 (yes, it's from btrfs test
 cases)

On Sun, Aug 10, 2025 at 07:36:48AM +0930, Qu Wenruo wrote:
> 
> 
> 在 2025/8/9 18:39, Zhang Yi 写道:
> > On 2025/8/9 6:11, Qu Wenruo wrote:
> > > 在 2025/8/8 21:46, Theodore Ts'o 写道:
> > > > On Fri, Aug 08, 2025 at 06:20:56PM +0930, Qu Wenruo wrote:
> > > > > 
> > > > > 在 2025/8/8 17:22, Qu Wenruo 写道:
> > > > > > Hi,
> > > > > > 
> > > > > > [BACKGROUND]
> > > > > > Recently I'm testing btrfs with 16KiB block size.
> > > > > > 
> > > > > > Currently btrfs is artificially limiting subpage block size to 4K.
> > > > > > But there is a simple patch to change it to support all block sizes <=
> > > > > > page size in my branch:
> > > > > > 
> > > > > > https://github.com/adam900710/linux/tree/larger_bs_support
> > > > > > 
> > > > > > [IOMAP WARNING]
> > > > > > And I'm running into a very weird kernel warning at btrfs/136, with 16K
> > > > > > block size and 64K page size.
> > > > > > 
> > > > > > The problem is, the problem happens with ext3 (using ext4 modeule) with
> > > > > > 16K block size, and no btrfs is involved yet.
> > > > 
> > > > 
> > > > Thanks for the bug report!  This looks like it's an issue with using
> > > > indirect block-mapped file with a 16k block size.  I tried your
> > > > reproducer using a 1k block size on an x86_64 system, which is how I
> > > > test problem caused by the block size < page size.  It didn't
> > > > reproduce there, so it looks like it really needs a 16k block size.
> > > > 
> > > > Can you say something about what system were you running your testing
> > > > on --- was it an arm64 system, or a powerpc 64 system (the two most
> > > > common systems with page size > 4k)?  (I assume you're not trying to
> > > > do this on an Itanic.  :-)   And was the page size 16k or 64k?
> > > 
> > > The architecture is aarch64, the host board is Rock5B (cheap and fast enough), the test machine is a VM on that board, with ovmf as the UEFI firmware.
> > > 
> > > The kernel is configured to use 64K page size, the *ext3* system is using 16K block size.
> > > 
> > > Currently I tried the following combination with 64K page size and ext3, the result looks like the following
> > > 
> > > - 2K block size
> > > - 4K block size
> > >    All fine
> > > 
> > > - 8K block size
> > > - 16K block size
> > >    All the same kernel warning and never ending fsstress
> > > 
> > > - 32K block size
> > > - 64K block size
> > >    All fine
> > > 
> > > I am surprised as you that, not all subpage block size are having problems, just 2 of the less common combinations failed.
> > > 
> > > And the most common ones (4K, page size) are all fine.
> > > 
> > > Finally, if using ext4 not ext3, all combinations above are fine again.
> > > 
> > > So I ran out of ideas why only 2 block sizes fail here...
> > > 
> > 
> > This issue is caused by an overflow in the calculation of the hole's
> > length on the forth-level depth for non-extent inodes. For a file system
> > with a 4KB block size, the calculation will not overflow. For a 64KB
> > block size, the queried position will not reach the fourth level, so this
> > issue only occur on the filesystem with a 8KB and 16KB block size.
> > 
> > Hi, Wenruo, could you try the following fix?
> > 
> > diff --git a/fs/ext4/indirect.c b/fs/ext4/indirect.c
> > index 7de327fa7b1c..d45124318200 100644
> > --- a/fs/ext4/indirect.c
> > +++ b/fs/ext4/indirect.c
> > @@ -539,7 +539,7 @@ int ext4_ind_map_blocks(handle_t *handle, struct inode *inode,
> >   	int indirect_blks;
> >   	int blocks_to_boundary = 0;
> >   	int depth;
> > -	int count = 0;
> > +	u64 count = 0;
> >   	ext4_fsblk_t first_block = 0;
> > 
> >   	trace_ext4_ind_map_blocks_enter(inode, map->m_lblk, map->m_len, flags);
> > @@ -588,7 +588,7 @@ int ext4_ind_map_blocks(handle_t *handle, struct inode *inode,
> >   		count++;
> >   		/* Fill in size of a hole we found */
> >   		map->m_pblk = 0;
> > -		map->m_len = min_t(unsigned int, map->m_len, count);
> > +		map->m_len = umin(map->m_len, count);
> >   		goto cleanup;
> >   	}
> 
> It indeed solves the problem.
> 
> Tested-by: Qu Wenruo <wqu@...e.com>

Can we get the relevant chunks of this test turned into a tests/ext4/
fstest so that the ext4 developers have a regression test that doesn't
require setting up btrfs, please?

--D

> Thanks,
> Qu
> 
> > Thanks,
> > Yi.
> > 
> 
>