Re: [PATCH] ext4: remove unnecessary ext4_inode_datasync_dirty in read path

[Dave Chinner <david@fromorbit.com>]

On Thu, Nov 04, 2021 at 05:29:59PM +0800, Zhongwei Cai wrote:
> 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?

No. Some filesystems don't track inode metadata dirty status using
the VFS inode; instead they track it more efficiently in internal
inode and/or journal based structures. Hence the only way to get
"inode needs journal flush for data stability" information to
generic IO code is to have a specific per-IO mapping flag for 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?

ext4 can do what it likes here, but given that the problem that was
being addressed was avoiding lock contention in
ext4_inode_datasync_dirty(), this appears to be a completely
unnecessary change as it doesn't address the problem being reported.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com