linux-ext4 - [PATCH] ext4: Fix SEEK

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Date:   Thu, 11 May 2017 18:49:08 +0200
From:   Jan Kara <jack@...e.cz>
To:     Ted Tso <tytso@....edu>
Cc:     <linux-ext4@...r.kernel.org>, Jan Kara <jack@...e.cz>,
        stable@...r.kernel.org, Zheng Liu <wenqing.lz@...bao.com>
Subject: [PATCH] ext4: Fix SEEK_HOLE

Currently, SEEK_HOLE implementation in ext4 may both return that there's
a hole at some offset although that offset already has data and skip
some holes during a search for the next hole. The first problem is
demostrated by:

xfs_io -c "falloc 0 256k" -c "pwrite 0 56k" -c "seek -h 0" file
wrote 57344/57344 bytes at offset 0
56 KiB, 14 ops; 0.0000 sec (2.054 GiB/sec and 538461.5385 ops/sec)
Whence	Result
HOLE	0

Where we can see that SEEK_HOLE wrongly returned offset 0 as containing
a hole although we have written data there. The second problem can be
demonstrated by:

xfs_io -c "falloc 0 256k" -c "pwrite 0 56k" -c "pwrite 128k 8k"
       -c "seek -h 0" file

wrote 57344/57344 bytes at offset 0
56 KiB, 14 ops; 0.0000 sec (1.978 GiB/sec and 518518.5185 ops/sec)
wrote 8192/8192 bytes at offset 131072
8 KiB, 2 ops; 0.0000 sec (2 GiB/sec and 500000.0000 ops/sec)
Whence	Result
HOLE	139264

Where we can see that hole at offsets 56k..128k has been ignored by the
SEEK_HOLE call.

The underlying problem is in the ext4_find_unwritten_pgoff()
implementation which in some cases fails to update returned offset when it
finds a hole and in some cases the condition for detecting a hole is
just wrong.

Fix ext4_find_unwritten_pgoff() to always properly update returned
offset and detect a hole.

CC: stable@...r.kernel.org
Fixes: c8c0df241cc2719b1262e627f999638411934f60
CC: Zheng Liu <wenqing.lz@...bao.com>
Signed-off-by: Jan Kara <jack@...e.cz>
---
 fs/ext4/file.c | 25 +++++++++----------------
 1 file changed, 9 insertions(+), 16 deletions(-)

diff --git a/fs/ext4/file.c b/fs/ext4/file.c
index 831fd6beebf0..e4de6769376b 100644
--- a/fs/ext4/file.c
+++ b/fs/ext4/file.c
@@ -486,29 +486,18 @@ static int ext4_find_unwritten_pgoff(struct inode *inode,
 					  (pgoff_t)num);
 		if (nr_pages == 0) {
 			if (whence == SEEK_DATA)
-				break;
+				goto out;
 
 			BUG_ON(whence != SEEK_HOLE);
-			/*
-			 * If this is the first time to go into the loop and
-			 * offset is not beyond the end offset, it will be a
-			 * hole at this offset
-			 */
-			if (lastoff == startoff || lastoff < endoff)
-				found = 1;
 			break;
 		}
 
 		/*
-		 * If this is the first time to go into the loop and
-		 * offset is smaller than the first page offset, it will be a
-		 * hole at this offset.
+		 * If current offset is smaller than the first page offset,
+		 * there is a hole at this offset.
 		 */
-		if (lastoff == startoff && whence == SEEK_HOLE &&
-		    lastoff < page_offset(pvec.pages[0])) {
-			found = 1;
+		if (whence == SEEK_HOLE && lastoff < page_offset(pvec.pages[0]))
 			break;
-		}
 
 		for (i = 0; i < nr_pages; i++) {
 			struct page *page = pvec.pages[i];
@@ -571,13 +560,17 @@ static int ext4_find_unwritten_pgoff(struct inode *inode,
 		if (nr_pages < num && whence == SEEK_HOLE) {
 			found = 1;
 			*offset = lastoff;
-			break;
+			goto out;
 		}
 
 		index = pvec.pages[i - 1]->index + 1;
 		pagevec_release(&pvec);
 	} while (index <= end);
 
+	if (whence == SEEK_HOLE && lastoff < endoff) {
+		found = 1;
+		*offset = lastoff;
+	}
 out:
 	pagevec_release(&pvec);
 	return found;
-- 
2.12.0