linux-kernel - Re: [PATCH] f2fs: fix indefinite loop scanning for free nid

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Date:   Tue, 18 Aug 2020 16:29:05 +0800
From:   Chao Yu <yuchao0@...wei.com>
To:     Sahitya Tummala <stummala@...eaurora.org>,
        Jaegeuk Kim <jaegeuk@...nel.org>,
        <linux-f2fs-devel@...ts.sourceforge.net>
CC:     <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] f2fs: fix indefinite loop scanning for free nid

On 2020/8/14 16:05, Sahitya Tummala wrote:
> If the sbi->ckpt->next_free_nid is not NAT block aligned and if there
> are free nids in that NAT block between the start of the block and
> next_free_nid, then those free nids will not be scanned in scan_nat_page().
> This results into mismatch between nm_i->available_nids and the sum of
> nm_i->free_nid_count of all NAT blocks scanned. And nm_i->available_nids
> will always be greater than the sum of free nids in all the blocks.
> Under this condition, if we use all the currently scanned free nids,
> then it will loop forever in f2fs_alloc_nid() as nm_i->available_nids
> is still not zero but nm_i->free_nid_count of that partially scanned
> NAT block is zero.
> 
> Fix this to align the nm_i->next_scan_nid to the first nid of the
> corresponding NAT block.
> 
> Signed-off-by: Sahitya Tummala <stummala@...eaurora.org>
> ---
>   fs/f2fs/node.c | 2 ++
>   1 file changed, 2 insertions(+)
> 
> diff --git a/fs/f2fs/node.c b/fs/f2fs/node.c
> index 9bbaa26..d615e59 100644
> --- a/fs/f2fs/node.c
> +++ b/fs/f2fs/node.c
> @@ -2402,6 +2402,8 @@ static int __f2fs_build_free_nids(struct f2fs_sb_info *sbi,
>   			if (IS_ERR(page)) {
>   				ret = PTR_ERR(page);
>   			} else {
> +				if (nid % NAT_ENTRY_PER_BLOCK)
> +					nid = NAT_BLOCK_OFFSET(nid) * NAT_ENTRY_PER_BLOCK;

How about moving this logic to the beginning of __f2fs_build_free_nids(),
after nid reset?

BTW, it looks we can add unlikely in this judgment condition?

Thanks,

>   				ret = scan_nat_page(sbi, page, nid);
>   				f2fs_put_page(page, 1);
>   			}
>