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

[Sahitya Tummala]

Hi Chao,

On Wed, Aug 07, 2019 at 10:04:16AM +0800, Chao Yu wrote:
> 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

Oh, yes, I have missed this point.

> guess something like this:
> 
> if (source_blkaddr != blkaddr) {
> 	if (unlikely(check_valid_map(sbi, segno, off))) {

check_valid_map() is validated before is_alive(). So I think this check again
may not be needed. What do you think?

> 		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.
> 

Yes, I agree with you. I will include this under CONFIG_F2FS_CHECK_FS.

Thanks,

> 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 # */
> >  };
> >  
> > 

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