Re: [PATCH v2] f2fs: Fix indefinite loop in f2fs_gc()

[Chao Yu]

Hi Sahitya,

On 2019/8/6 19:19, Sahitya Tummala wrote:
> Policy - Foreground GC, LFS and greedy GC mode.
> 
> Under this policy, f2fs_gc() loops forever to GC as it doesn't have
> enough free segements to proceed and thus it keeps calling gc_more
> for the same victim segment.  This can happen if the selected victim
> segment could not be GC'd due to failed blkaddr validity check i.e.
> is_alive() returns false for the blocks set in current validity map.
> 
> Fix this by keeping track of such invalid segments and skip those
> segments for selection in get_victim_by_default() to avoid endless
> GC loop under such error scenarios.
> 
> Signed-off-by: Sahitya Tummala <stummala@xxxxxxxxxxxxxx>
> ---
> v2: fix as per Chao's suggestion to handle this error case
> 
>  fs/f2fs/gc.c      | 15 ++++++++++++++-
>  fs/f2fs/segment.c |  5 +++++
>  fs/f2fs/segment.h |  3 +++
>  3 files changed, 22 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/f2fs/gc.c b/fs/f2fs/gc.c
> index 8974672..321a78a 100644
> --- a/fs/f2fs/gc.c
> +++ b/fs/f2fs/gc.c
> @@ -382,6 +382,14 @@ static int get_victim_by_default(struct f2fs_sb_info *sbi,
>  			nsearched++;
>  		}
>  
> +		/*
> +		 * skip selecting the invalid segno (that is failed due to block
> +		 * validity check failed during GC) to avoid endless GC loop in
> +		 * such cases.
> +		 */
> +		if (test_bit(segno, sm->invalid_segmap))
> +			goto next;
> +
>  		secno = GET_SEC_FROM_SEG(sbi, segno);
>  
>  		if (sec_usage_check(sbi, secno))
> @@ -975,6 +983,7 @@ static int gc_data_segment(struct f2fs_sb_info *sbi, struct f2fs_summary *sum,
>  	int off;
>  	int phase = 0;
>  	int submitted = 0;
> +	struct sit_info *sit_i = SIT_I(sbi);
>  
>  	start_addr = START_BLOCK(sbi, segno);
>  
> @@ -1008,8 +1017,12 @@ static int gc_data_segment(struct f2fs_sb_info *sbi, struct f2fs_summary *sum,
>  		}
>  
>  		/* Get an inode by ino with checking validity */
> -		if (!is_alive(sbi, entry, &dni, start_addr + off, &nofs))
> +		if (!is_alive(sbi, entry, &dni, start_addr + off, &nofs)) {
> +			if (!test_and_set_bit(segno, sit_i->invalid_segmap))
> +				f2fs_err(sbi, "invalid blkaddr %u in seg %u is found\n",
> +						start_addr + off, segno);

Oh, there is some normal cases in is_alive(), such as f2fs_get_node_page() or
f2fs_get_node_info() failure due to no memory, we should bypass such cases. I
guess something like this:

if (source_blkaddr != blkaddr) {
	if (unlikely(check_valid_map(sbi, segno, off))) {
		if (!test_and_set_bit(segno, sit_i->invalid_segmap)) {
			f2fs_err(sbi, "invalid blkaddr %u in seg %u is found\n",
				start_addr + off, segno);
			set_sbi_flag(sbi, SBI_NEED_FSCK);
		}
	}
	return false;
}

I think this will be safe to call check_valid_map(), because there should be no
race in between is_alive() and update_sit_entry() from all paths due to node
page lock dependence.

One more concern is should we use this under CONFIG_F2FS_CHECK_FS? If there is
actually such a bug can cause data inconsistency, we'd better find the root
cause in debug version.

Thanks,

>  			continue;
> +		}
>  
>  		if (phase == 2) {
>  			f2fs_ra_node_page(sbi, dni.ino);
> diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
> index a661ac3..d45a1d3 100644
> --- a/fs/f2fs/segment.c
> +++ b/fs/f2fs/segment.c
> @@ -4017,6 +4017,10 @@ static int build_sit_info(struct f2fs_sb_info *sbi)
>  		return -ENOMEM;
>  #endif
>  
> +	sit_i->invalid_segmap = f2fs_kvzalloc(sbi, bitmap_size, GFP_KERNEL);
> +	if (!sit_i->invalid_segmap)
> +		return -ENOMEM;
> +
>  	/* init SIT information */
>  	sit_i->s_ops = &default_salloc_ops;
>  
> @@ -4518,6 +4522,7 @@ static void destroy_sit_info(struct f2fs_sb_info *sbi)
>  #ifdef CONFIG_F2FS_CHECK_FS
>  	kvfree(sit_i->sit_bitmap_mir);
>  #endif
> +	kvfree(sit_i->invalid_segmap);
>  	kvfree(sit_i);
>  }
>  
> diff --git a/fs/f2fs/segment.h b/fs/f2fs/segment.h
> index b746028..bc5dbe8 100644
> --- a/fs/f2fs/segment.h
> +++ b/fs/f2fs/segment.h
> @@ -246,6 +246,9 @@ struct sit_info {
>  	unsigned long long min_mtime;		/* min. modification time */
>  	unsigned long long max_mtime;		/* max. modification time */
>  
> +	/* list of segments to be ignored by GC in case of errors */
> +	unsigned long *invalid_segmap;
> +
>  	unsigned int last_victim[MAX_GC_POLICY]; /* last victim segment # */
>  };
>  
>