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

[Sahitya Tummala]

On Tue, Aug 18, 2020 at 03:25:47PM +0530, 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?
> 
> 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.
> 

Sorry, I was wrong above. During CP we update only ckpt->next_free_nid but not
the nm_i->next_free_nid, which is done only once during boot up.

So yes, I will mark it as unlikely conditiona.

Thanks,

> Thanks,
> 
> > 
> > Thanks,
> > 
> > >  				ret = scan_nat_page(sbi, page, nid);
> > >  				f2fs_put_page(page, 1);
> > >  			}
> > >
> 
> -- 
> --
> Sent by a consultant of the Qualcomm Innovation Center, Inc.
> The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum.

-- 
--
Sent by a consultant of the Qualcomm Innovation Center, Inc.
The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum.