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

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Date: Wed, 24 Jan 2024 17:13:18 +0000
From: Luis Henriques <lhenriques@...e.de>
To: Daniel Dawson <danielcdawson@...il.com>
Cc: Theodore Ts'o <tytso@....edu>,  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

Daniel Dawson <danielcdawson@...il.com> writes:

> On 11/28/23 10:15 PM, Daniel Dawson wrote:
>> When a file is converted from inline to non-inline, it has stale flags until
>> sync.
>
>> Why is this a problem? Because some code will fail under such a condition, for
>> example, lseek(..., SEEK_HOLE) will result in ENOENT.
>
>
> Just tested. Still happening on 6.8-rc1.

FWIW, I've been looking into a similar issue related with inline-data and
delayed allocation.  It may however be quite different because it seems to
add small block sizes into the mix:

  https://bugzilla.kernel.org/show_bug.cgi?id=200681

Unfortunately, I'm still trying to find my way around all this code and I
can't say I fully understand the whole flow using the reproducer provided
in that bugzilla.

Bellow, I'm inlining a patch that started as debug patch that I've used to
try to understand what was going on.  It seems to workaround that bug, but
I know it's not a real fix -- I don't yet understand what's going on.

Regarding your specific usecase, I can reproduce it and, unfortunately, I
don't thing Ted's suggestion will fix it as I don't even see
ext4_iomap_begin_report() being executed at all.  Anyway, just my 2
cents... let's see if I can come up with something.

Cheers,
-- 
Luís

diff --git a/fs/ext4/inline.c b/fs/ext4/inline.c
index d5bd1e3a5d36..d0c3d6fd48de 100644
--- a/fs/ext4/inline.c
+++ b/fs/ext4/inline.c
@@ -528,7 +528,19 @@ int ext4_readpage_inline(struct inode *inode, struct folio *folio)
 	if (!folio->index)
 		ret = ext4_read_inline_folio(inode, folio);
 	else if (!folio_test_uptodate(folio)) {
-		folio_zero_segment(folio, 0, folio_size(folio));
+		struct buffer_head *bh, *head;
+		size_t start = 0;
+
+		head = folio_buffers(folio);
+		if (head) {
+			bh = head;
+			do {
+				if (!buffer_uptodate(bh))
+					break;
+				start += inode->i_sb->s_blocksize;
+			} while ((bh = bh->b_this_page) != head);
+		}
+		folio_zero_segment(folio, start, folio_size(folio));
 		folio_mark_uptodate(folio);
 	}