Re: [PATCH v2] mmp: do not use O_DIRECT when working with regular file

On Fri, Feb 19, 2021 at 01:08:17PM +0300, Alexey Lyashkov wrote:

Andreas,

What about to disable a O_DIRECT global on any block devices in the e2fsprogs library as this don’t work on 4k disk drives at all ?
Instead of fixing an O_DIRECT access with patches sends early.

Why would it not work at all ? This is a fix for a specific problem and
I am not currently aware of ony other problems e2fsprogs should have
with 4k sector size drives. Do you have a specific problem in mind ?

Thanks!
-Lukas

Alex

quoted

19 февр. 2021 г., в 1:20, Andreas Dilger [off-list ref] написал(а):

On Feb 18, 2021, at 2:51 AM, Lukas Czerner [off-list ref] wrote:

quoted

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 <redacted>
Reviewed-by: Eric Sandeen <redacted>

Reviewed-by: Andreas Dilger <redacted>

quoted

---
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

Cheers, Andreas