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

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Date:   Mon, 1 Jun 2020 14:45:50 +0530
From:   Sahitya Tummala <stummala@...eaurora.org>
To:     Jaegeuk Kim <jaegeuk@...nel.org>
Cc:     Chao Yu <yuchao0@...wei.com>,
        linux-f2fs-devel@...ts.sourceforge.net,
        linux-kernel@...r.kernel.org, stummala@...eaurora.org
Subject: Re: [f2fs-dev] [PATCH] f2fs: fix retry logic in
 f2fs_write_cache_pages()

Hi Chao,

Can you please help review below diff given by Jaegeuk?
If it looks good, I can send a v2.

Thanks,

On Thu, May 28, 2020 at 12:18:39PM -0700, Jaegeuk Kim wrote:
> 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
> 

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