Re: [PATCH 1/2] iomap: make buffered writes work with RWF_DONTCACHE

["Darrick J. Wong" <djwong@kernel.org>]

On Mon, Feb 03, 2025 at 09:32:38AM -0700, Jens Axboe wrote:
> Add iomap buffered write support for RWF_DONTCACHE. If RWF_DONTCACHE is
> set for a write, mark the folios being written as uncached. Then
> writeback completion will drop the pages. The write_iter handler simply
> kicks off writeback for the pages, and writeback completion will take
> care of the rest.
> 
> This still needs the user of the iomap buffered write helpers to call
> folio_end_dropbehind_write() upon successful issue of the writes.

I thought iomap calls folio_end_writeback, which cares of that?  So xfs
doesn't itself have to call folio_end_dropbehind_write?

> Signed-off-by: Jens Axboe <axboe@kernel.dk>
> ---
>  fs/iomap/buffered-io.c | 4 ++++
>  include/linux/iomap.h  | 1 +
>  2 files changed, 5 insertions(+)
> 
> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> index d303e6c8900c..ea863c3cf510 100644
> --- a/fs/iomap/buffered-io.c
> +++ b/fs/iomap/buffered-io.c
> @@ -603,6 +603,8 @@ struct folio *iomap_get_folio(struct iomap_iter *iter, loff_t pos, size_t len)
>  
>  	if (iter->flags & IOMAP_NOWAIT)
>  		fgp |= FGP_NOWAIT;
> +	if (iter->flags & IOMAP_DONTCACHE)
> +		fgp |= FGP_DONTCACHE;
>  	fgp |= fgf_set_order(len);
>  
>  	return __filemap_get_folio(iter->inode->i_mapping, pos >> PAGE_SHIFT,
> @@ -1034,6 +1036,8 @@ iomap_file_buffered_write(struct kiocb *iocb, struct iov_iter *i,
>  
>  	if (iocb->ki_flags & IOCB_NOWAIT)
>  		iter.flags |= IOMAP_NOWAIT;
> +	if (iocb->ki_flags & IOCB_DONTCACHE)
> +		iter.flags |= IOMAP_DONTCACHE;
>  
>  	while ((ret = iomap_iter(&iter, ops)) > 0)
>  		iter.processed = iomap_write_iter(&iter, i);
> diff --git a/include/linux/iomap.h b/include/linux/iomap.h
> index 75bf54e76f3b..26b0dbe23e62 100644
> --- a/include/linux/iomap.h
> +++ b/include/linux/iomap.h
> @@ -183,6 +183,7 @@ struct iomap_folio_ops {
>  #define IOMAP_DAX		0
>  #endif /* CONFIG_FS_DAX */
>  #define IOMAP_ATOMIC		(1 << 9)
> +#define IOMAP_DONTCACHE		(1 << 10)

This needs a mention in the iomap documentation.  If the patch below
accurately summarizes what it does nowadays, then you can add it to the
series with a:

Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>

--D

diff --git a/Documentation/filesystems/iomap/design.rst b/Documentation/filesystems/iomap/design.rst
index b0d0188a095e55..7b91546750f59e 100644
--- a/Documentation/filesystems/iomap/design.rst
+++ b/Documentation/filesystems/iomap/design.rst
@@ -352,6 +352,11 @@ operations:
    ``IOMAP_NOWAIT`` is often set on behalf of ``IOCB_NOWAIT`` or
    ``RWF_NOWAIT``.
 
+ * ``IOMAP_DONTCACHE`` is set when the caller wishes to perform a
+   buffered file I/O and would like the kernel to drop the pagecache
+   after the I/O completes, if it isn't already being used by another
+   thread.
+
 If it is necessary to read existing file contents from a `different
 <https://lore.kernel.org/all/20191008071527.29304-9-hch@lst.de/>`_
 device or address range on a device, the filesystem should return that
diff --git a/Documentation/filesystems/iomap/operations.rst b/Documentation/filesystems/iomap/operations.rst
index 2c7f5df9d8b037..584ff549f9a659 100644
--- a/Documentation/filesystems/iomap/operations.rst
+++ b/Documentation/filesystems/iomap/operations.rst
@@ -131,6 +131,8 @@ These ``struct kiocb`` flags are significant for buffered I/O with iomap:
 
  * ``IOCB_NOWAIT``: Turns on ``IOMAP_NOWAIT``.
 
+ * ``IOCB_DONTCACHE``: Turns on ``IOMAP_DONTCACHE``.
+
 Internal per-Folio State
 ------------------------