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

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Date:   Tue, 16 Feb 2021 15:24:00 -0600
From:   Eric Sandeen <sandeen@...deen.net>
To:     Lukas Czerner <lczerner@...hat.com>, linux-ext4@...r.kernel.org
Subject: Re: [PATCH] mmp: do not use O_DIRECT when working with regular file

On 2/12/21 3:37 AM, Lukas Czerner wrote:
> Currently the mmp block is read using O_DIRECT to avoid any caching tha
> 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 smaller 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 file. 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>
> ---
>  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..1ac22194 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 smaller than blocksize of the fs we're working with.

I think the problem is when the host filesystem that contains the image is on
a device with a logical sector size which is /larger/ than the image filesystem's
block size, right? Not smaller?

Because then you might not be able to do an image-filesystem-block-aligned direct
IO on it, if it's sub-logical-block-size for the host filesystem/device, and lands
within the larger host sector at an offset?

otherwise, this seems at least as reasonable to me as the previous tmpfs work
around, so other than the question about the comment,

Reviewed-by: Eric Sandeen <sandeen@...hat.com>


> +		 */
> +		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;
>  		}
>