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From:   Andreas Gruenbacher <agruenba@redhat.com>
To:     Christoph Hellwig <hch@infradead.org>,
        "Darrick J . Wong" <djwong@kernel.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Matthew Wilcox <willy@infradead.org>
Cc:     Andreas Gruenbacher <agruenba@redhat.com>,
        linux-xfs@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-ext4@vger.kernel.org, cluster-devel@redhat.com
Subject: [RFC v3 5/7] iomap: Get page in page_prepare handler
Date:   Fri, 16 Dec 2022 16:06:24 +0100
Message-Id: <20221216150626.670312-6-agruenba@redhat.com>
In-Reply-To: <20221216150626.670312-1-agruenba@redhat.com>
References: <20221216150626.670312-1-agruenba@redhat.com>
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Precedence: bulk

Change the iomap ->page_prepare() handler to get and return a locked
folio instead of doing that in iomap_write_begin().  This allows to
recover from out-of-memory situations in ->page_prepare(), which
eliminates the corresponding error handling code in iomap_write_begin().
The ->page_done() handler is now not called with a NULL folio anymore.

Filesystems are expected to use the iomap_folio_prepare() helper for
getting locked folios in their ->page_prepare() handlers.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
---
 fs/gfs2/bmap.c         | 16 +++++++++++++---
 fs/iomap/buffered-io.c | 21 +++++++++------------
 include/linux/iomap.h  |  9 +++++----
 3 files changed, 27 insertions(+), 19 deletions(-)

diff --git a/fs/gfs2/bmap.c b/fs/gfs2/bmap.c
index 11115fce68cb..cd5984d3ba50 100644
--- a/fs/gfs2/bmap.c
+++ b/fs/gfs2/bmap.c
@@ -959,15 +959,25 @@ static int __gfs2_iomap_get(struct inode *inode, loff_t pos, loff_t length,
 	goto out;
 }
 
-static int gfs2_iomap_page_prepare(struct inode *inode, loff_t pos,
-				   unsigned len)
+static struct folio *
+gfs2_iomap_page_prepare(struct iomap_iter *iter, loff_t pos, unsigned len)
 {
+	struct inode *inode = iter->inode;
 	unsigned int blockmask = i_blocksize(inode) - 1;
 	struct gfs2_sbd *sdp = GFS2_SB(inode);
 	unsigned int blocks;
+	struct folio *folio;
+	int status;
 
 	blocks = ((pos & blockmask) + len + blockmask) >> inode->i_blkbits;
-	return gfs2_trans_begin(sdp, RES_DINODE + blocks, 0);
+	status = gfs2_trans_begin(sdp, RES_DINODE + blocks, 0);
+	if (status)
+		return ERR_PTR(status);
+
+	folio = iomap_folio_prepare(iter, pos);
+	if (!folio)
+		gfs2_trans_end(sdp);
+	return folio;
 }
 
 static void gfs2_iomap_page_done(struct inode *inode, loff_t pos,
diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
index f0f167ca1b2e..6b7c1a10b8ec 100644
--- a/fs/iomap/buffered-io.c
+++ b/fs/iomap/buffered-io.c
@@ -602,7 +602,7 @@ static void iomap_folio_done(struct iomap_iter *iter, loff_t pos, size_t ret,
 
 	if (page_ops && page_ops->page_done) {
 		page_ops->page_done(iter->inode, pos, ret, folio);
-	} else if (folio) {
+	} else {
 		folio_unlock(folio);
 		folio_put(folio);
 	}
@@ -635,17 +635,14 @@ static int iomap_write_begin(struct iomap_iter *iter, loff_t pos,
 	if (!mapping_large_folio_support(iter->inode->i_mapping))
 		len = min_t(size_t, len, PAGE_SIZE - offset_in_page(pos));
 
-	if (page_ops && page_ops->page_prepare) {
-		status = page_ops->page_prepare(iter->inode, pos, len);
-		if (status)
-			return status;
-	}
-
-	folio = iomap_folio_prepare(iter, pos);
-	if (!folio) {
-		status = (iter->flags & IOMAP_NOWAIT) ? -EAGAIN : -ENOMEM;
-		iomap_folio_done(iter, pos, 0, NULL);
-		return status;
+	if (page_ops && page_ops->page_prepare)
+		folio = page_ops->page_prepare(iter, pos, len);
+	else
+		folio = iomap_folio_prepare(iter, pos);
+	if (IS_ERR_OR_NULL(folio)) {
+		if (!folio)
+			return (iter->flags & IOMAP_NOWAIT) ? -EAGAIN : -ENOMEM;
+		return PTR_ERR(folio);
 	}
 
 	/*
diff --git a/include/linux/iomap.h b/include/linux/iomap.h
index 0bf16ef22d81..c74ab8c53b47 100644
--- a/include/linux/iomap.h
+++ b/include/linux/iomap.h
@@ -13,6 +13,7 @@
 struct address_space;
 struct fiemap_extent_info;
 struct inode;
+struct iomap_iter;
 struct iomap_dio;
 struct iomap_writepage_ctx;
 struct iov_iter;
@@ -131,12 +132,12 @@ static inline bool iomap_inline_data_valid(const struct iomap *iomap)
  * associated with them.
  *
  * When page_prepare succeeds, page_done will always be called to do any
- * cleanup work necessary.  In that page_done call, @folio will be NULL if the
- * associated folio could not be obtained.  When folio is not NULL, page_done
- * is responsible for unlocking and putting the folio.
+ * cleanup work necessary.  page_done is responsible for unlocking and putting
+ * @folio.
  */
 struct iomap_page_ops {
-	int (*page_prepare)(struct inode *inode, loff_t pos, unsigned len);
+	struct folio *(*page_prepare)(struct iomap_iter *iter, loff_t pos,
+			unsigned len);
 	void (*page_done)(struct inode *inode, loff_t pos, unsigned copied,
 			struct folio *folio);
 
-- 
2.38.1