linux-kernel - [PATCH v2 1/3] afs: Handle len being extending over page end in write_begin/write

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Message-ID: <162391824293.1173366.15452474691364794223.stgit@warthog.procyon.org.uk>
Date:   Thu, 17 Jun 2021 09:24:02 +0100
From:   David Howells <dhowells@...hat.com>
To:     linux-cachefs@...hat.com, linux-afs@...ts.infradead.org
Cc:     "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Jeff Layton <jlayton@...nel.org>, dhowells@...hat.com,
        Jeff Layton <jlayton@...nel.org>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH v2 1/3] afs: Handle len being extending over page end in
 write_begin/write_end

With transparent huge pages, in the future, write_begin() and write_end()
may be passed a length parameter that, in combination with the offset into
the page, exceeds the length of that page.  This allows
grab_cache_page_write_begin() to better choose the size of THP to allocate.

Fix afs's functions to handle this by trimming the length as needed after
the page has been allocated.

[Removed the now-unnecessary index var; spotted by kernel test robot]

Fixes: e1b1240c1ff5 ("netfs: Add write_begin helper")
Reported-by: Matthew Wilcox (Oracle) <willy@...radead.org>
Signed-off-by: David Howells <dhowells@...hat.com>
Acked-by: Jeff Layton <jlayton@...nel.org>
cc: linux-afs@...ts.infradead.org
Link: https://lore.kernel.org/r/162367681795.460125.11729955608839747375.stgit@warthog.procyon.org.uk/ # v1
---

 fs/afs/write.c |   13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/fs/afs/write.c b/fs/afs/write.c
index a523bb86915d..56e2cff2cb87 100644
--- a/fs/afs/write.c
+++ b/fs/afs/write.c
@@ -25,7 +25,8 @@ int afs_set_page_dirty(struct page *page)
 }
 
 /*
- * prepare to perform part of a write to a page
+ * Prepare to perform part of a write to a page.  Note that len may extend
+ * beyond the end of the page.
  */
 int afs_write_begin(struct file *file, struct address_space *mapping,
 		    loff_t pos, unsigned len, unsigned flags,
@@ -36,7 +37,6 @@ int afs_write_begin(struct file *file, struct address_space *mapping,
 	unsigned long priv;
 	unsigned f, from;
 	unsigned t, to;
-	pgoff_t index;
 	int ret;
 
 	_enter("{%llx:%llu},%llx,%x",
@@ -51,8 +51,8 @@ int afs_write_begin(struct file *file, struct address_space *mapping,
 	if (ret < 0)
 		return ret;
 
-	index = page->index;
-	from = pos - index * PAGE_SIZE;
+	from = offset_in_thp(page, pos);
+	len = min_t(size_t, len, thp_size(page) - from);
 	to = from + len;
 
 try_again:
@@ -103,7 +103,8 @@ int afs_write_begin(struct file *file, struct address_space *mapping,
 }
 
 /*
- * finalise part of a write to a page
+ * Finalise part of a write to a page.  Note that len may extend beyond the end
+ * of the page.
  */
 int afs_write_end(struct file *file, struct address_space *mapping,
 		  loff_t pos, unsigned len, unsigned copied,
@@ -111,7 +112,7 @@ int afs_write_end(struct file *file, struct address_space *mapping,
 {
 	struct afs_vnode *vnode = AFS_FS_I(file_inode(file));
 	unsigned long priv;
-	unsigned int f, from = pos & (thp_size(page) - 1);
+	unsigned int f, from = offset_in_thp(page, pos);
 	unsigned int t, to = from + copied;
 	loff_t i_size, maybe_i_size;