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

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Date:   Fri, 19 Feb 2021 13:08:17 +0300
From:   Alexey Lyashkov <alexey.lyashkov@...il.com>
To:     Andreas Dilger <adilger@...ger.ca>,
        Artem Blagodarenko <artem.blagodarenko@...il.com>
Cc:     Lukas Czerner <lczerner@...hat.com>, 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

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.


Alex

> 19 февр. 2021 г., в 1:20, Andreas Dilger <adilger@...ger.ca> написал(а):
> 
> 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
> 
> 
> 
> 
>