linux-kernel - Re: [f2fs-dev] [PATCH] f2fs: fix retry logic in f2fs_write_cache

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Message-ID: <20200528191839.GA180586@google.com>
Date:   Thu, 28 May 2020 12:18:39 -0700
From:   Jaegeuk Kim <jaegeuk@...nel.org>
To:     Chao Yu <yuchao0@...wei.com>
Cc:     Sahitya Tummala <stummala@...eaurora.org>,
        linux-f2fs-devel@...ts.sourceforge.net,
        linux-kernel@...r.kernel.org
Subject: Re: [f2fs-dev] [PATCH] f2fs: fix retry logic in
 f2fs_write_cache_pages()

On 05/28, Chao Yu wrote:
> On 2020/5/28 10:45, Chao Yu wrote:
> > On 2020/5/27 10:20, Sahitya Tummala wrote:
> >> In case a compressed file is getting overwritten, the current retry
> >> logic doesn't include the current page to be retried now as it sets
> >> the new start index as 0 and new end index as writeback_index - 1.
> >> This causes the corresponding cluster to be uncompressed and written
> >> as normal pages without compression. Fix this by allowing writeback to
> >> be retried for the current page as well (in case of compressed page
> >> getting retried due to index mismatch with cluster index). So that
> >> this cluster can be written compressed in case of overwrite.
> >>
> >> Signed-off-by: Sahitya Tummala <stummala@...eaurora.org>
> >> ---
> >>  fs/f2fs/data.c | 2 +-
> >>  1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >> diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
> >> index 4af5fcd..bfd1df4 100644
> >> --- a/fs/f2fs/data.c
> >> +++ b/fs/f2fs/data.c
> >> @@ -3024,7 +3024,7 @@ static int f2fs_write_cache_pages(struct address_space *mapping,
> >>  	if ((!cycled && !done) || retry) {
> > 
> > IMO, we add retry logic in wrong place, you can see that cycled value is
> > zero only if wbc->range_cyclic is true, in that case writeback_index is valid.
> > 
> > However if retry is true and wbc->range_cyclic is false, then writeback_index
> > would be uninitialized variable.
> > 
> > Thoughts?
> > 
> > Thanks,
> > 
> >>  		cycled = 1;
> >>  		index = 0;
> >> -		end = writeback_index - 1;
> 
> BTW, I notice that range_cyclic writeback flow was refactored in below commit,
> and skeleton of f2fs.writepages was copied from mm/page-writeback.c::write_cache_pages(),
> I guess we need follow that change.
> 
> 64081362e8ff ("mm/page-writeback.c: fix range_cyclic writeback vs writepages deadlock")

Is that something like this?

---
 fs/f2fs/data.c | 11 ++---------
 1 file changed, 2 insertions(+), 9 deletions(-)

diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
index 48a622b95b76e..28fcdf0d4dcb9 100644
--- a/fs/f2fs/data.c
+++ b/fs/f2fs/data.c
@@ -2861,7 +2861,6 @@ static int f2fs_write_cache_pages(struct address_space *mapping,
 	pgoff_t index;
 	pgoff_t end;		/* Inclusive */
 	pgoff_t done_index;
-	int cycled;
 	int range_whole = 0;
 	xa_mark_t tag;
 	int nwritten = 0;
@@ -2879,17 +2878,12 @@ static int f2fs_write_cache_pages(struct address_space *mapping,
 	if (wbc->range_cyclic) {
 		writeback_index = mapping->writeback_index; /* prev offset */
 		index = writeback_index;
-		if (index == 0)
-			cycled = 1;
-		else
-			cycled = 0;
 		end = -1;
 	} else {
 		index = wbc->range_start >> PAGE_SHIFT;
 		end = wbc->range_end >> PAGE_SHIFT;
 		if (wbc->range_start == 0 && wbc->range_end == LLONG_MAX)
 			range_whole = 1;
-		cycled = 1; /* ignore range_cyclic tests */
 	}
 	if (wbc->sync_mode == WB_SYNC_ALL || wbc->tagged_writepages)
 		tag = PAGECACHE_TAG_TOWRITE;
@@ -3054,10 +3048,9 @@ static int f2fs_write_cache_pages(struct address_space *mapping,
 		}
 	}
 #endif
-	if ((!cycled && !done) || retry) {
-		cycled = 1;
+	if (retry) {
 		index = 0;
-		end = writeback_index - 1;
+		end = -1;
 		goto retry;
 	}
 	if (wbc->range_cyclic || (range_whole && wbc->nr_to_write > 0))
-- 
2.27.0.rc0.183.gde8f92d652-goog