[Devel] [PATCH RHEL7 COMMIT] ms/ext4: fix off-by-one on max nr_pages in ext4_find_unwritten_pgoff()

[Konstantin Khorenko]

The commit is pushed to "branch-rh7-3.10.0-514.26.1.vz7.33.x-ovz" and will appear at https://src.openvz.org/scm/ovz/vzkernel.git
after rh7-3.10.0-514.26.1.vz7.33.17
------>
commit f894b17b65ae268a91f18c590f12497201b74131
Author: Maxim Patlasov <mpatlasov at virtuozzo.com>
Date:   Wed Jul 26 12:01:38 2017 +0400

    ms/ext4: fix off-by-one on max nr_pages in ext4_find_unwritten_pgoff()
    
    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>
    
    https://jira.sw.ru/browse/PSBM-68292
    
    Signed-off-by: Maxim Patlasov <mpatlasov at virtuozzo.com>
---
 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)