linux-kernel - Re: [f2fs-dev] [PATCH 04/28] f2fs: replace a build-time warning with runtime WARN

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Date:   Wed, 26 Oct 2016 16:57:05 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Chao Yu <chao@...nel.org>
Cc:     Jaegeuk Kim <jaegeuk@...nel.org>, Chao Yu <yuchao0@...wei.com>,
        linux-kernel@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net,
        Weichao Guo <guoweichao@...wei.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [f2fs-dev] [PATCH 04/28] f2fs: replace a build-time warning with runtime WARN_ON

On Wednesday, October 26, 2016 10:05:00 PM CEST Chao Yu wrote:
> On 2016/10/18 6:05, Arnd Bergmann wrote:
> > gcc is unsure about the use of last_ofs_in_node, which might happen
> > without a prior initialization:
> > 
> > fs/f2fs//git/arm-soc/fs/f2fs/data.c: In function ‘f2fs_map_blocks’:
> > fs/f2fs/data.c:799:54: warning: ‘last_ofs_in_node’ may be used uninitialized in this function [-Wmaybe-uninitialized]
> >    if (prealloc && dn.ofs_in_node != last_ofs_in_node + 1) {
> 
> In each round of dnode block traverse, we will init 'prealloc' and then update
> 'prealloc' and 'last_ofs_in_node' together in below lines of f2fs_map_blocks:
>                         if (flag == F2FS_GET_BLOCK_PRE_AIO) {
>                                 if (blkaddr == NULL_ADDR) {
>                                         prealloc++;
>                                         last_ofs_in_node = dn.ofs_in_node;
>                                 }
>                         }
> 
> Then in below codes, it is safe to use 'last_ofs_in_node' since we will check
> 'prealloc' firstly, so if 'prealloc' is non-zero, 'last_ofs_in_node' must be valid.
>         if (prealloc && dn.ofs_in_node != last_ofs_in_node + 1) {
> 
> So I think we should not add WARN_ON there.

Ok, that make sense. Thanks for taking a closer look!

Should we just set last_ofs_in_node to the same value as ofs_in_node
before the loop?

diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
index 9ae194f..14db4b7 100644
--- a/fs/f2fs/data.c
+++ b/fs/f2fs/data.c
@@ -716,7 +716,7 @@ int f2fs_map_blocks(struct inode *inode, struct f2fs_map_blocks *map,
 	}
 
 	prealloc = 0;
-	ofs_in_node = dn.ofs_in_node;
+	last_ofs_in_node = ofs_in_node = dn.ofs_in_node;
 	end_offset = ADDRS_PER_PAGE(dn.node_page, inode);
 
 next_block:

	Arnd