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Subject: Re: [PATCH] ext4: remove unnecessary ext4_inode_datasync_dirty in
 read path
To:     Dave Chinner <david@fromorbit.com>
Cc:     tytso@mit.edu, adilger.kernel@dilger.ca,
        linux-ext4@vger.kernel.org, mingkaidong@gmail.com
References: <20211102024258.210439-1-sunrise_l@sjtu.edu.cn>
 <20211103002843.GC418105@dread.disaster.area>
From:   Zhongwei Cai <sunrise_l@sjtu.edu.cn>
Message-ID: <ffb199dc-f7ae-ba03-db57-bf7acc3d0636@sjtu.edu.cn>
Date:   Thu, 4 Nov 2021 17:29:59 +0800
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On 11/3/21 8:28 AM, Dave Chinner wrote:
 > IOWs, we expect the IOMAP_F_DIRTY flag to be set on all types of
 > iomap mapping calls if the inode is dirty, not just IOMAP_WRITE
 > calls.

Thanks for correction!

 > /*
 >  * Flags reported by the file system from iomap_begin:
 >  *
 >  * IOMAP_F_NEW indicates that the blocks have been newly allocated
 >  * and need zeroing for areas that no data is copied to.
 >  *
 >  * IOMAP_F_DIRTY indicates the inode has uncommitted metadata needed
 >  * to access written data and requires fdatasync to commit them to
 >  * persistent  storage. This needs to take into account metadata
 >  * changes that  *may* be made at IO completion, such as file size
 >  * updates from direct IO.
 >  *
 >  * IOMAP_F_SHARED indicates that the blocks are shared, and will
 >  * need to be unshared as part a write.
 >  *
 >  * IOMAP_F_MERGED indicates that the iomap contains the merge of
 >  * multiple block mappings.
 >  *
 >  * IOMAP_F_BUFFER_HEAD indicates that the file system requires the
 >  * use of buffer heads for this mapping.
 >  */

According to the comments in include/linux/iomap.h, it seems other
flags in the iomap indicates the iomap-related status, but the
IOMAP_F_DIRTY flag only indicates the status of the inode. So can
I_DIRTY_INODE or I_DIRTY_PAGES flags in the inode replace it?

And for ext4 at least we can do

    /*
     * Writes that span EOF might trigger an I/O size update on completion,
     * so consider them to be dirty for the purpose of O_DSYNC, even if
     * there is no other metadata changes being made or are pending.
     */
     if (ext4_inode_datasync_dirty(inode) ||
-       offset + length > i_size_read(inode))
+       ((flags & IOMAP_WRITE) && offset + length > i_size_read(inode)))
         iomap->flags |= IOMAP_F_DIRTY;

, since only writes that span EOF might trigger an update. How
do you feel about it?

Best,

Zhongwei.