linux-kernel - Re: [PATCH 22/25] fs/buffer: prevent WARN_ON in __alloc_pages

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Message-ID: <adccaa99-ffbc-4fbf-9210-47932724c184@huaweicloud.com>
Date: Sat, 25 Oct 2025 14:32:45 +0800
From: Baokun Li <libaokun@...weicloud.com>
To: Matthew Wilcox <willy@...radead.org>, "Darrick J. Wong"
 <djwong@...nel.org>
Cc: linux-ext4@...r.kernel.org, tytso@....edu, adilger.kernel@...ger.ca,
 jack@...e.cz, linux-kernel@...r.kernel.org, kernel@...kajraghav.com,
 mcgrof@...nel.org, linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
 yi.zhang@...wei.com, yangerkun@...wei.com, chengzhihao1@...wei.com,
 libaokun1@...wei.com
Subject: Re: [PATCH 22/25] fs/buffer: prevent WARN_ON in
 __alloc_pages_slowpath() when BS > PS

On 2025-10-25 12:45, Matthew Wilcox wrote:
> On Sat, Oct 25, 2025 at 11:22:18AM +0800, libaokun@...weicloud.com wrote:
>> +	while (1) {
>> +		folio = __filemap_get_folio(mapping, index, fgp_flags,
>> +					    gfp & ~__GFP_NOFAIL);
>> +		if (!IS_ERR(folio) || !(gfp & __GFP_NOFAIL))
>> +			return folio;
>> +
>> +		if (PTR_ERR(folio) != -ENOMEM && PTR_ERR(folio) != -EAGAIN)
>> +			return folio;
>> +
>> +		memalloc_retry_wait(gfp);
>> +	}
> No, absolutely not.  We're not having open-coded GFP_NOFAIL semantics.
> The right way forward is for ext4 to use iomap, not for buffer heads
> to support large block sizes.

ext4 only calls getblk_unmovable or __getblk when reading critical
metadata. Both of these functions set __GFP_NOFAIL to ensure that
metadata reads do not fail due to memory pressure.

Both functions eventually call grow_dev_folio(), which is why we
handle the __GFP_NOFAIL logic there. xfs_buf_alloc_backing_mem()
has similar logic, but XFS manages its own metadata, allowing it
to use vmalloc for memory allocation.

ext4 Direct I/O has already switched to iomap, and patches to
support iomap for Buffered I/O are currently under iteration.

But as far as I know, iomap does not support metadata, and XFS does not
use iomap to read metadata either.

Am I missing something here?

-- 
With Best Regards,
Baokun Li