linux-ext4 - [PATCH v2] mmp: do not use O_DIRECT when working with regular file

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Message-Id: <20210218095146.265302-1-lczerner@redhat.com>
Date:   Thu, 18 Feb 2021 10:51:46 +0100
From:   Lukas Czerner <lczerner@...hat.com>
To:     linux-ext4@...r.kernel.org
Cc:     Eric Sandeen <sandeen@...hat.com>
Subject: [PATCH v2] mmp: do not use O_DIRECT when working with regular file

Currently the mmp block is read using O_DIRECT to avoid any caching that
may be done by the VM. However when working with regular files this
creates alignment issues when the device of the host file system has
sector size larger than the blocksize of the file system in the file
we're working with.

This can be reproduced with t_mmp_fail test when run on the device with
4k sector size because the mke2fs fails when trying to read the mmp
block.

Fix it by disabling O_DIRECT when working with regular files. I don't
think there is any risk of doing so since the file system layer, unlike
shared block device, should guarantee cache consistency.

Signed-off-by: Lukas Czerner <lczerner@...hat.com>
Reviewed-by: Eric Sandeen <sandeen@...hat.com>
---
v2: Fix comment - it avoids problems when the sector size is larger not
    smaller than blocksize

 lib/ext2fs/mmp.c | 22 +++++++++++-----------
 1 file changed, 11 insertions(+), 11 deletions(-)

diff --git a/lib/ext2fs/mmp.c b/lib/ext2fs/mmp.c
index c21ae272..cca2873b 100644
--- a/lib/ext2fs/mmp.c
+++ b/lib/ext2fs/mmp.c
@@ -57,21 +57,21 @@ errcode_t ext2fs_mmp_read(ext2_filsys fs, blk64_t mmp_blk, void *buf)
 	 * regardless of how the io_manager is doing reads, to avoid caching of
 	 * the MMP block by the io_manager or the VM.  It needs to be fresh. */
 	if (fs->mmp_fd <= 0) {
+		struct stat st;
 		int flags = O_RDWR | O_DIRECT;
 
-retry:
+		/*
+		 * There is no reason for using O_DIRECT if we're working with
+		 * regular file. Disabling it also avoids problems with
+		 * alignment when the device of the host file system has sector
+		 * size larger than blocksize of the fs we're working with.
+		 */
+		if (stat(fs->device_name, &st) == 0 &&
+		    S_ISREG(st.st_mode))
+			flags &= ~O_DIRECT;
+
 		fs->mmp_fd = open(fs->device_name, flags);
 		if (fs->mmp_fd < 0) {
-			struct stat st;
-
-			/* Avoid O_DIRECT for filesystem image files if open
-			 * fails, since it breaks when running on tmpfs. */
-			if (errno == EINVAL && (flags & O_DIRECT) &&
-			    stat(fs->device_name, &st) == 0 &&
-			    S_ISREG(st.st_mode)) {
-				flags &= ~O_DIRECT;
-				goto retry;
-			}
 			retval = EXT2_ET_MMP_OPEN_DIRECT;
 			goto out;
 		}
-- 
2.26.2