linux-kernel - [PATCH] block: fallocate: avoid false positive on collision detection

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Message-Id: <20210107124022.900172-1-mlevitsk@redhat.com>
Date:   Thu,  7 Jan 2021 14:40:22 +0200
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     linux-kernel@...r.kernel.org
Cc:     "Darrick J . Wong" <darrick.wong@...cle.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel@...r.kernel.org (open list:FILESYSTEMS (VFS and
        infrastructure)), Jan Kara <jack@...e.cz>,
        Maxim Levitsky <mlevitsk@...hat.com>
Subject: [PATCH] block: fallocate: avoid false positive on collision detection

Align start and end on page boundaries before calling
invalidate_inode_pages2_range.

This might allow us to miss a collision if the write and the discard were done
to the same page and do overlap but it is still better than returning -EBUSY
if those writes didn't overlap.

Signed-off-by: Maxim Levitsky <mlevitsk@...hat.com>
---
 fs/block_dev.c | 20 ++++++++++++++++----
 1 file changed, 16 insertions(+), 4 deletions(-)

diff --git a/fs/block_dev.c b/fs/block_dev.c
index 9e84b1928b94..97f0d16661b5 100644
--- a/fs/block_dev.c
+++ b/fs/block_dev.c
@@ -1970,6 +1970,7 @@ static long blkdev_fallocate(struct file *file, int mode, loff_t start,
 	loff_t end = start + len - 1;
 	loff_t isize;
 	int error;
+	pgoff_t invalidate_first_page, invalidate_last_page;
 
 	/* Fail if we don't recognize the flags. */
 	if (mode & ~BLKDEV_FALLOC_FL_SUPPORTED)
@@ -2020,12 +2021,23 @@ static long blkdev_fallocate(struct file *file, int mode, loff_t start,
 
 	/*
 	 * Invalidate again; if someone wandered in and dirtied a page,
-	 * the caller will be given -EBUSY.  The third argument is
-	 * inclusive, so the rounding here is safe.
+	 * the caller will be given -EBUSY.
+	 *
+	 * If the start/end of the range is not page aligned, exclude the
+	 * non aligned regions to avoid false positives.
 	 */
+	invalidate_first_page = DIV_ROUND_UP(start, PAGE_SIZE);
+	invalidate_last_page = end >> PAGE_SHIFT;
+
+	if ((end + 1) & PAGE_MASK)
+		invalidate_last_page--;
+
+	if (invalidate_last_page < invalidate_first_page)
+		return 0;
+
 	return invalidate_inode_pages2_range(bdev->bd_inode->i_mapping,
-					     start >> PAGE_SHIFT,
-					     end >> PAGE_SHIFT);
+					     invalidate_first_page,
+					     invalidate_last_page);
 }
 
 const struct file_operations def_blk_fops = {
-- 
2.26.2