[Devel] [PATCH RHEL7 COMMIT] direct-io: fix uninitialized warning in do_direct_IO()

[Konstantin Khorenko]

The commit is pushed to "branch-rh7-3.10.0-1127.8.2.vz7.161.x-ovz" and will appear at https://src.openvz.org/scm/ovz/vzkernel.git
after rh7-3.10.0-1127.8.2.vz7.161.1
------>
commit 81818ac3761eaa3c5d7d7272dfe45de00538e0ff
Author: Boaz Harrosh <boaz at plexistor.com>
Date:   Mon May 25 17:52:24 2020 +0300

    direct-io: fix uninitialized warning in do_direct_IO()
    
    ms commit 6fcc5420bfb9
    
    The following warnings:
    
      fs/direct-io.c: In function ‘__blockdev_direct_IO’:
      fs/direct-io.c:1011:12: warning: ‘to’ may be used uninitialized in this function [-Wmaybe-uninitialized]
      fs/direct-io.c:913:16: note: ‘to’ was declared here
      fs/direct-io.c:1011:12: warning: ‘from’ may be used uninitialized in this function [-Wmaybe-uninitialized]
      fs/direct-io.c:913:10: note: ‘from’ was declared here
    
    are false positive because dio_get_page() either fails, or sets both
    'from' and 'to'.
    
    Paul Bolle said ...
    Maybe it's better to move initializing "to" and "from" out of
    dio_get_page(). That _might_ make it easier for both the the reader and
    the compiler to understand what's going on. Something like this:
    
    Christoph Hellwig said ...
    The fix of moving the code definitively looks nicer, while I think
    uninitialized_var is horrible wart that won't get anywhere near my code.
    
    Boaz Harrosh: I agree with Christoph and Paul
    
    Signed-off-by: Boaz Harrosh <boaz at plexistor.com>
    Signed-off-by: Christoph Hellwig <hch at lst.de>
    Signed-off-by: Kirill Tkhai <ktkhai at virtuozzo.com>
    
    =====================
    Patchset description:
    [00/30] fs,direct_IO: Switch to iov_iter and allow bio_vec for ext4
    
    This patchset transforms direct_IO callbacks, blockdev_direct_IO
    and its underlining functions to iov_iter, and introduces complete
    support of iov_iter for ext4.
    
    Supported iov_iter subtypes for ext4 is iovec and bio_vec. The first
    is for traditional user-submitted aio, while bio_vec is the type,
    which is important for us, since we use it in ploop.
    
    bio_vec operates with pages instead of user addresses (like iovec
    does), so it requires specific callbacks in do_blockdev_direct_IO()
    and in the functions it calls.
    
    The patchset reworks do_blockdev_direct_IO() in the same manner
    as in mainstrean. The most of rest patches are prepared manually,
    since we have significant differences to ms (RHEL7 patches, our
    direct IO patches for FUSE; all they have changed many functions).
    At the end, kaio engine (resulting in direct_IO) became possible
    to be enabled for ext4.
    
    https://jira.sw.ru/browse/PSBM-99793
---
 fs/direct-io.c | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/fs/direct-io.c b/fs/direct-io.c
index 17e5938a4511a..6f0678619939a 100644
--- a/fs/direct-io.c
+++ b/fs/direct-io.c
@@ -206,9 +206,8 @@ static inline int dio_refill_pages(struct dio *dio, struct dio_submit *sdio)
  * L1 cache.
  */
 static inline struct page *dio_get_page(struct dio *dio,
-		struct dio_submit *sdio, size_t *from, size_t *to)
+					struct dio_submit *sdio)
 {
-	int n;
 	if (dio_pages_present(sdio) == 0) {
 		int ret;
 
@@ -217,10 +216,7 @@ static inline struct page *dio_get_page(struct dio *dio,
 			return ERR_PTR(ret);
 		BUG_ON(dio_pages_present(sdio) == 0);
 	}
-	n = sdio->head++;
-	*from = n ? 0 : sdio->from;
-	*to = (n == sdio->tail - 1) ? sdio->to : PAGE_SIZE;
-	return dio->pages[n];
+	return dio->pages[sdio->head];
 }
 
 static void dio_iodone_helper(struct dio *dio, loff_t offset,
@@ -997,11 +993,15 @@ static int do_direct_IO(struct dio *dio, struct dio_submit *sdio,
 	while (sdio->block_in_file < sdio->final_block_in_request) {
 		struct page *page;
 		size_t from, to;
-		page = dio_get_page(dio, sdio, &from, &to);
+
+		page = dio_get_page(dio, sdio);
 		if (IS_ERR(page)) {
 			ret = PTR_ERR(page);
 			goto out;
 		}
+		from = sdio->head ? 0 : sdio->from;
+		to = (sdio->head == sdio->tail - 1) ? sdio->to : PAGE_SIZE;
+		sdio->head++;
 
 		while (from < to) {
 			unsigned this_chunk_bytes;	/* # of bytes mapped */