[Devel] [PATCH vz7 3/3] ext4: fix off-by-one on max nr_pages in ext4_find_unwritten_pgoff()

[Maxim Patlasov]

Backport 624327f8794704c5066b11a52f9da6a09dce7f9a from ml:

    ext4: fix off-by-one on max nr_pages in ext4_find_unwritten_pgoff()

    ext4_find_unwritten_pgoff() is used to search for offset of hole or
    data in page range [index, end] (both inclusive), and the max number
    of pages to search should be at least one, if end == index.
    Otherwise the only page is missed and no hole or data is found,
    which is not correct.

    When block size is smaller than page size, this can be demonstrated
    by preallocating a file with size smaller than page size and writing
    data to the last block. E.g. run this xfs_io command on a 1k block
    size ext4 on x86_64 host.

      # xfs_io -fc "falloc 0 3k" -c "pwrite 2k 1k" \
                -c "seek -d 0" /mnt/ext4/testfile
      wrote 1024/1024 bytes at offset 2048
      1 KiB, 1 ops; 0.0000 sec (42.459 MiB/sec and 43478.2609 ops/sec)
      Whence  Result
      DATA    EOF

    Data at offset 2k was missed, and lseek(2) returned ENXIO.

    This is unconvered by generic/285 subtest 07 and 08 on ppc64 host,
    where pagesize is 64k. Because a recent change to generic/285
    reduced the preallocated file size to smaller than 64k.

    Signed-off-by: Eryu Guan <eguan at redhat.com>
    Signed-off-by: Theodore Ts'o <tytso at mit.edu>
    Reviewed-by: Jan Kara <jack at suse.cz>

Signed-off-by: Maxim Patlasov <mpatlasov at virtuozzo.com>

https://jira.sw.ru/browse/PSBM-68292
---
 fs/ext4/file.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/ext4/file.c b/fs/ext4/file.c
index be8da78..771f9be 100644
--- a/fs/ext4/file.c
+++ b/fs/ext4/file.c
@@ -450,7 +450,7 @@ static int ext4_find_unwritten_pgoff(struct inode *inode,
 		int i, num;
 		unsigned long nr_pages;
 
-		num = min_t(pgoff_t, end - index, PAGEVEC_SIZE);
+		num = min_t(pgoff_t, end - index, PAGEVEC_SIZE - 1) + 1;
 		nr_pages = pagevec_lookup(&pvec, inode->i_mapping, index,
 					  (pgoff_t)num);
 		if (nr_pages == 0)