[PATCH v2] iomap: don't lose folio dropbehind state for overwrites

[PATCH v2] iomap: don't lose folio dropbehind state for overwrites

[Jens Axboe]

DONTCACHE I/O must have the completion punted to a workqueue, just like
what is done for unwritten extents, as the completion needs task context
to perform the invalidation of the folio(s). However, if writeback is
started off filemap_fdatawrite_range() off generic_sync() and it's an
overwrite, then the DONTCACHE marking gets lost as iomap_add_to_ioend()
don't look at the folio being added and no further state is passed down
to help it know that this is a dropbehind/DONTCACHE write.

Check if the folio being added is marked as dropbehind, and set
IOMAP_IOEND_DONTCACHE if that is the case. Then XFS can factor this into
the decision making of completion context in xfs_submit_ioend().
Additionally include this ioend flag in the NOMERGE flags, to avoid
mixing it with unrelated IO.

Since this is the 3rd flag that will cause XFS to punt the completion to
a workqueue, add a helper so that each one of them can get appropriately
commented.

This fixes extra page cache being instantiated when the write performed
is an overwrite, rather than newly instantiated blocks.

Fixes: b2cd5ae693a3 ("iomap: make buffered writes work with RWF_DONTCACHE")
Signed-off-by: Jens Axboe <axboe@kernel.dk>

---

Since v1:
- Add xfs_ioend_needs_wq_completion() helper as per Dave's suggestion

diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
index 233abf598f65..3729391a18f3 100644
--- a/fs/iomap/buffered-io.c
+++ b/fs/iomap/buffered-io.c
@@ -1691,6 +1691,8 @@ static int iomap_add_to_ioend(struct iomap_writepage_ctx *wpc,
 		ioend_flags |= IOMAP_IOEND_UNWRITTEN;
 	if (wpc->iomap.flags & IOMAP_F_SHARED)
 		ioend_flags |= IOMAP_IOEND_SHARED;
+	if (folio_test_dropbehind(folio))
+		ioend_flags |= IOMAP_IOEND_DONTCACHE;
 	if (pos == wpc->iomap.offset && (wpc->iomap.flags & IOMAP_F_BOUNDARY))
 		ioend_flags |= IOMAP_IOEND_BOUNDARY;
 
diff --git a/fs/xfs/xfs_aops.c b/fs/xfs/xfs_aops.c
index 26a04a783489..63151feb9c3f 100644
--- a/fs/xfs/xfs_aops.c
+++ b/fs/xfs/xfs_aops.c
@@ -436,6 +436,25 @@ xfs_map_blocks(
 	return 0;
 }
 
+static bool
+xfs_ioend_needs_wq_completion(
+	struct iomap_ioend	*ioend)
+{
+	/* Changing inode size requires a transaction. */
+	if (xfs_ioend_is_append(ioend))
+		return true;
+
+	/* Extent manipulation requires a transaction. */
+	if (ioend->io_flags & (IOMAP_IOEND_UNWRITTEN | IOMAP_IOEND_SHARED))
+		return true;
+
+	/* Page cache invalidation cannot be done in irq context. */
+	if (ioend->io_flags & IOMAP_IOEND_DONTCACHE)
+		return true;
+
+	return false;
+}
+
 static int
 xfs_submit_ioend(
 	struct iomap_writepage_ctx *wpc,
@@ -460,8 +479,7 @@ xfs_submit_ioend(
 	memalloc_nofs_restore(nofs_flag);
 
 	/* send ioends that might require a transaction to the completion wq */
-	if (xfs_ioend_is_append(ioend) ||
-	    (ioend->io_flags & (IOMAP_IOEND_UNWRITTEN | IOMAP_IOEND_SHARED)))
+	if (xfs_ioend_needs_wq_completion(ioend))
 		ioend->io_bio.bi_end_io = xfs_end_bio;
 
 	if (status)
diff --git a/include/linux/iomap.h b/include/linux/iomap.h
index 68416b135151..522644d62f30 100644
--- a/include/linux/iomap.h
+++ b/include/linux/iomap.h
@@ -377,13 +377,16 @@ sector_t iomap_bmap(struct address_space *mapping, sector_t bno,
 #define IOMAP_IOEND_BOUNDARY		(1U << 2)
 /* is direct I/O */
 #define IOMAP_IOEND_DIRECT		(1U << 3)
+/* is DONTCACHE I/O */
+#define IOMAP_IOEND_DONTCACHE		(1U << 4)
 
 /*
  * Flags that if set on either ioend prevent the merge of two ioends.
  * (IOMAP_IOEND_BOUNDARY also prevents merges, but only one-way)
  */
 #define IOMAP_IOEND_NOMERGE_FLAGS \
-	(IOMAP_IOEND_SHARED | IOMAP_IOEND_UNWRITTEN | IOMAP_IOEND_DIRECT)
+	(IOMAP_IOEND_SHARED | IOMAP_IOEND_UNWRITTEN | IOMAP_IOEND_DIRECT | \
+	 IOMAP_IOEND_DONTCACHE)
 
 /*
  * Structure for writeback I/O completions.

-- 
Jens Axboe

Re: [PATCH v2] iomap: don't lose folio dropbehind state for overwrites

[Dave Chinner]

On Tue, May 27, 2025 at 05:01:31PM -0600, Jens Axboe wrote:
> DONTCACHE I/O must have the completion punted to a workqueue, just like
> what is done for unwritten extents, as the completion needs task context
> to perform the invalidation of the folio(s). However, if writeback is
> started off filemap_fdatawrite_range() off generic_sync() and it's an
> overwrite, then the DONTCACHE marking gets lost as iomap_add_to_ioend()
> don't look at the folio being added and no further state is passed down
> to help it know that this is a dropbehind/DONTCACHE write.
> 
> Check if the folio being added is marked as dropbehind, and set
> IOMAP_IOEND_DONTCACHE if that is the case. Then XFS can factor this into
> the decision making of completion context in xfs_submit_ioend().
> Additionally include this ioend flag in the NOMERGE flags, to avoid
> mixing it with unrelated IO.
> 
> Since this is the 3rd flag that will cause XFS to punt the completion to
> a workqueue, add a helper so that each one of them can get appropriately
> commented.
> 
> This fixes extra page cache being instantiated when the write performed
> is an overwrite, rather than newly instantiated blocks.
> 
> Fixes: b2cd5ae693a3 ("iomap: make buffered writes work with RWF_DONTCACHE")
> Signed-off-by: Jens Axboe <axboe@kernel.dk>

LGTM.

Reviewed-by: Dave Chinner <dchinner@redhat.com>
-- 
Dave Chinner
david@fromorbit.com

Re: [PATCH v2] iomap: don't lose folio dropbehind state for overwrites

[Christoph Hellwig]

Looks good:

Reviewed-by: Christoph Hellwig <hch@lst.de>

Re: [PATCH v2] iomap: don't lose folio dropbehind state for overwrites

[Christian Brauner]

On Tue, 27 May 2025 17:01:31 -0600, Jens Axboe wrote:
> DONTCACHE I/O must have the completion punted to a workqueue, just like
> what is done for unwritten extents, as the completion needs task context
> to perform the invalidation of the folio(s). However, if writeback is
> started off filemap_fdatawrite_range() off generic_sync() and it's an
> overwrite, then the DONTCACHE marking gets lost as iomap_add_to_ioend()
> don't look at the folio being added and no further state is passed down
> to help it know that this is a dropbehind/DONTCACHE write.
> 
> [...]

Applied to the vfs.fixes branch of the vfs/vfs.git tree.
Patches in the vfs.fixes branch should appear in linux-next soon.

Please report any outstanding bugs that were missed during review in a
new review to the original patch series allowing us to drop it.

It's encouraged to provide Acked-bys and Reviewed-bys even though the
patch has now been applied. If possible patch trailers will be updated.

Note that commit hashes shown below are subject to change due to rebase,
trailer updates or similar. If in doubt, please check the listed branch.

tree:   https://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs.git
branch: vfs.fixes

[1/1] iomap: don't lose folio dropbehind state for overwrites
      https://git.kernel.org/vfs/vfs/c/34ecde3c5606