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

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Date:   Thu, 18 Feb 2021 15:20:20 -0700
From:   Andreas Dilger <adilger@...ger.ca>
To:     Lukas Czerner <lczerner@...hat.com>
Cc:     linux-ext4@...r.kernel.org, Eric Sandeen <sandeen@...hat.com>
Subject: Re: [PATCH v2] mmp: do not use O_DIRECT when working with regular
 file

On Feb 18, 2021, at 2:51 AM, Lukas Czerner <lczerner@...hat.com> wrote:
> 
> 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>

Reviewed-by: Andreas Dilger <adilger@...ger.ca>

> ---
> 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






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