linux-kernel - Re: [PATCH] f2fs: fix small discards when se->valid

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Date:   Tue, 3 Jan 2017 17:55:10 -0800
From:   Jaegeuk Kim <jaegeuk@...nel.org>
To:     Yunlong Song <yunlong.song@...wei.com>
Cc:     cm224.lee@...sung.com, yuchao0@...wei.com, chao@...nel.org,
        sylinux@....com, bintian.wang@...wei.com,
        linux-fsdevel@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] f2fs: fix small discards when se->valid_blocks is zero

Hi Yunlong,

On 01/03, Yunlong Song wrote:
> In the small discard case, when se->valid_blocks is zero, the add_discard_addrs
> will directly return without __add_discard_entry. This will cause the file
> delete have no small discard. The case is like this:
> 
> 1. Allocate free 2M segment
> 2. Write a file (size n blocks < 512) in that 2M segment, se->valid_blocks = n
> 3. Delete that file, se->valid_blocks = 0, add_discard_addrs will return without
> sending any discard of that file, and forever due to cur_map[i] ^ ckpt_map[i] =
> 0 after that checkpoint

During this checkpoint, that'll be discarded as a prefree segment, no?
Note that, if this is a current segment, f2fs won't discard it until it is
fully allocated.

Thanks,

> 
> Signed-off-by: Yunlong Song <yunlong.song@...wei.com>
> ---
>  fs/f2fs/segment.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
> index 0738f48..8610f14 100644
> --- a/fs/f2fs/segment.c
> +++ b/fs/f2fs/segment.c
> @@ -838,7 +838,7 @@ static void add_discard_addrs(struct f2fs_sb_info *sbi, struct cp_control *cpc)
>  		return;
>  
>  	if (!force) {
> -		if (!test_opt(sbi, DISCARD) || !se->valid_blocks ||
> +		if (!test_opt(sbi, DISCARD) ||
>  		    SM_I(sbi)->nr_discards >= SM_I(sbi)->max_discards)
>  			return;
>  	}
> -- 
> 1.8.5.2