linux-ext4 - Re: [inline_data] ext4: Stale flags before sync when convert to non-inline

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Message-ID: <20231130040651.GC510020@mit.edu>
Date: Wed, 29 Nov 2023 23:06:51 -0500
From: "Theodore Ts'o" <tytso@....edu>
To: Daniel Dawson <danielcdawson@...il.com>
Cc: Andreas Dilger <adilger.kernel@...ger.ca>, linux-ext4@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [inline_data] ext4: Stale flags before sync when convert to
 non-inline

On Tue, Nov 28, 2023 at 10:15:09PM -0800, Daniel Dawson wrote:
> When a file is converted from inline to non-inline, it has stale flags until
> sync.
> 
> If a file with inline data is written to such that it must become
> non-inline, it temporarily appears to have the inline data flag and not (if
> applicable) the extent flag. This is corrected on sync, but can cause
> problems in certain situations.

The issue here is delayed allocation.  When you write to a file with
delayed allocation enabled, the file system doesn't decide where the
data will be written on the storage device until the last possible
minute, when writeback occurs.  This can be triggered by a fsync(2) or
sync(2) system call,

> Why is this a problem? Because some code will fail under such a condition,
> for example, lseek(..., SEEK_HOLE) will result in ENOENT. I ran into this
> with Gentoo's Portage, which uses the call to handle sparse files when
> copying. Sometimes, an ebuild creates a temporary file that is quickly
> copied, and apparently the temporary is written in small increments,
> triggering this.

This is caused by missing logic in ext4_iomap_begin_report().  For the
non-inline case, this function does this:

	ret = ext4_map_blocks(NULL, inode, &map, 0);
	if (ret < 0)
		return ret;
	if (ret == 0)
		delalloc = ext4_iomap_is_delalloc(inode, &map);

For a non-inline file, if you write some data blocks that hasn't been
written back due to delayed allocation, ext4_map_blocks() won't be
able to map the logical block to a physical block.  This logic is
missing in the inline_data case:

	if (ext4_has_inline_data(inode)) {
		ret = ext4_inline_data_iomap(inode, iomap);
		if (ret != -EAGAIN) {
			if (ret == 0 && offset >= iomap->length)
				ret = -ENOENT;
			return ret;
		}
	}

What's missing is a call to ext4_iomap_is_delalloc() in the case where
ret == 0, and then setting the delayed allocation flag:

	if (delalloc && iomap->type == IOMAP_HOLE)
		iomap->type = IOMAP_DELALLOC;

This will deal with the combination of inline_data and delayed
allocation for SEEK_HOLE and for FIEMAP.

I'll need to turn this into an actual patch and then create a test to
validate the patch but I'm pretty sure this should deal with the
problem you've come across.

Cheers,

						- Ted