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

Zhongwei Cai
2021-11-04T09:30:20Z

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.