linux-kernel - Re: [PATCH 2/6] fs: Add FS_XFLAG

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Date: Mon, 5 Feb 2024 12:58:30 +0000
From: John Garry <john.g.garry@...cle.com>
To: "Darrick J. Wong" <djwong@...nel.org>
Cc: hch@....de, viro@...iv.linux.org.uk, brauner@...nel.org,
        dchinner@...hat.com, jack@...e.cz, chandan.babu@...cle.com,
        martin.petersen@...cle.com, linux-kernel@...r.kernel.org,
        linux-xfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        tytso@....edu, jbongio@...gle.com, ojaswin@...ux.ibm.com
Subject: Re: [PATCH 2/6] fs: Add FS_XFLAG_ATOMICWRITES flag

On 02/02/2024 17:57, Darrick J. Wong wrote:
> On Wed, Jan 24, 2024 at 02:26:41PM +0000, John Garry wrote:
>> Add a flag indicating that a regular file is enabled for atomic writes.
> 
> This is a file attribute that mirrors an ondisk inode flag.  Actual
> support for untorn file writes (for now) depends on both the iflag and
> the underlying storage devices, which we can only really check at statx
> and pwrite time.  This is the same story as FS_XFLAG_DAX, which signals
> to the fs that we should try to enable the fsdax IO path on the file
> (instead of the regular page cache), but applications have to query
> STAT_ATTR_DAX to find out if they really got that IO path.

To be clear, are you suggesting to add this info to the commit message?

> 
> "try to enable atomic writes", perhaps? >
> (and the comment for FS_XFLAG_DAX ought to read "try to use DAX for IO")

To me that sounds like "try to use DAX for IO, and, if not possible, 
fall back on some other method" - is that reality of what that flag does?

Thanks,
John

> 
> --D
> 
>>
>> Signed-off-by: John Garry <john.g.garry@...cle.com>
>> ---
>>   include/uapi/linux/fs.h | 1 +
>>   1 file changed, 1 insertion(+)
>>
>> diff --git a/include/uapi/linux/fs.h b/include/uapi/linux/fs.h
>> index a0975ae81e64..b5b4e1db9576 100644
>> --- a/include/uapi/linux/fs.h
>> +++ b/include/uapi/linux/fs.h
>> @@ -140,6 +140,7 @@ struct fsxattr {
>>   #define FS_XFLAG_FILESTREAM	0x00004000	/* use filestream allocator */
>>   #define FS_XFLAG_DAX		0x00008000	/* use DAX for IO */
>>   #define FS_XFLAG_COWEXTSIZE	0x00010000	/* CoW extent size allocator hint */
>> +#define FS_XFLAG_ATOMICWRITES	0x00020000	/* atomic writes enabled */
>>   #define FS_XFLAG_HASATTR	0x80000000	/* no DIFLAG for this	*/
>>   
>>   /* the read-only stuff doesn't really belong here, but any other place is
>> -- 
>> 2.31.1
>>
>>