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

[Chao Yu]

On 2020/8/18 17:55, Sahitya Tummala wrote:
> On Tue, Aug 18, 2020 at 04:29:05PM +0800, Chao Yu wrote:
> > 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@xxxxxxxxxxxxxx>
> > > ---
> > >   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?
>
> Sure, I will move it.
>
> > BTW, it looks we can add unlikely in this judgment condition?
>
> But it may not be an unlikely as it can happen whenever checkpoint is done,
> based on the next available free nid in function next_free_nid(), which can happen
> quite a few times, right?

Oh, yes, I missed that place, please ignore that suggestion.. :)

Thanks,

> Hitting the loop forever issue condition due to this could be a rare/difficult to
> reproduce but this check itself may not be unlikely in my opinion.
>
> Thanks,
>
> > Thanks,
> >
> > >   				ret = scan_nat_page(sbi, page, nid);
> > >   				f2fs_put_page(page, 1);
> > >   			}