RE: [PATCH 2/3] f2fs: fix get stale meta pages when build free nids

[Chao Yu]

Hi Wanpeng,

> -----Original Message-----
> From: Wanpeng Li [mailto:wanpeng.li@xxxxxxxxxxxxxxx]
> Sent: Monday, March 09, 2015 12:25 PM
> To: Jaegeuk Kim
> Cc: Changman Lee; Chao Yu; linux-f2fs-devel@xxxxxxxxxxxxxxxxxxxxx;
> linux-fsdevel@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; Wanpeng Li
> Subject: Re: [PATCH 2/3] f2fs: fix get stale meta pages when build free nids
> 
> Hi Jaegeuk,
> On Sun, Mar 08, 2015 at 09:18:47PM -0700, Jaegeuk Kim wrote:
> >Hi Wanpeng,
> >
> >On Mon, Mar 09, 2015 at 11:00:54AM +0800, Wanpeng Li wrote:
> >> The blocks of nat pages to be scanned will be readahead when build free
> >> nids. The start blkno will be adjusted to 0 when the intended range over
> >> max_nids, then function ra_meta_pages readahead the right blkno, however,
> >> function get_current_nat_page which is called in build_free_nids still
> >> get the meta pages w/ stale blkno, this patch fix it by adjusting the nid
> >> and the blkno which will be readahead in avdance in order to readahead and
> >> get the right meta pages.
> >>
> >> Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxxxxxx>
> >> ---
> >>  fs/f2fs/checkpoint.c | 3 ---
> >>  fs/f2fs/node.c       | 9 ++++++++-
> >>  2 files changed, 8 insertions(+), 4 deletions(-)
> >>
> >> diff --git a/fs/f2fs/checkpoint.c b/fs/f2fs/checkpoint.c
> >> index 53bc328..aedb441 100644
> >> --- a/fs/f2fs/checkpoint.c
> >> +++ b/fs/f2fs/checkpoint.c
> >> @@ -129,9 +129,6 @@ int ra_meta_pages(struct f2fs_sb_info *sbi, block_t start, int nrpages,
> int type
> >>
> >>  		switch (type) {
> >>  		case META_NAT:
> >> -			if (unlikely(blkno >=
> >> -					NAT_BLOCK_OFFSET(NM_I(sbi)->max_nid)))
> >> -				blkno = 0;
> >>  			/* get nat block addr */
> >>  			fio.blk_addr = current_nat_addr(sbi,
> >>  					blkno * NAT_ENTRY_PER_BLOCK);
> >> diff --git a/fs/f2fs/node.c b/fs/f2fs/node.c
> >> index 1a7e925..8751375 100644
> >> --- a/fs/f2fs/node.c
> >> +++ b/fs/f2fs/node.c
> >> @@ -1509,13 +1509,20 @@ static void build_free_nids(struct f2fs_sb_info *sbi)
> >>  	struct f2fs_summary_block *sum = curseg->sum_blk;
> >>  	int i = 0;
> >>  	nid_t nid = nm_i->next_scan_nid;
> >> +	block_t blkno = NAT_BLOCK_OFFSET(nid);
> >>
> >>  	/* Enough entries */
> >>  	if (nm_i->fcnt > NAT_ENTRY_PER_BLOCK)
> >>  		return;
> >>
> >> +	if (unlikely(blkno >=
> >> +			NAT_BLOCK_OFFSET(NM_I(sbi)->max_nid))) {
> >> +		blkno = 0;
> >> +		nid = 0;
> >> +	}
> >
> >The assumption here is that the nid does not exceeed max_nid, since the
> >previous call already set next_scan_nid to 0, if it exceeds max_nid.
> >So, we don't need to do like this.
> 
> So get_current_nat_page in while(1) still get the wrong meta page for
> one time, right? The nid will be set to 0 in the second round.

When we break while (1), our nid will not exceed max_nid because:
	while (1)
		...
		if (unlikely(nid >= nm_i->max_nid))
			nid = 0;

then next_scan_nid is assigned nid, for next round in build_free_nids
our nid which is assigned next_scan_nid will also not exceed max_nid.

So get_current_nat_page will get the right meta page.

Thanks,

> 
> Regards,
> Wanpeng Li
> 
> >
> >BTW, even if ra_meta_pages doesn't get the below condition,
> >		if (unlikely(blkno >=
> >				NAT_BLOCK_OFFSET(NM_I(sbi)->max_nid)))
> >			blkno = 0;
> >IMO, it is still worth to remain that to avoid any bugs due to further code
> >changes. It makes sense that unlikely was used for this too.
> >
> >Thanks,
> >
> >> +
> >>  	/* readahead nat pages to be scanned */
> >> -	ra_meta_pages(sbi, NAT_BLOCK_OFFSET(nid), FREE_NID_PAGES, META_NAT);
> >> +	ra_meta_pages(sbi, blkno, FREE_NID_PAGES, META_NAT);
> >>
> >>  	while (1) {
> >>  		struct page *page = get_current_nat_page(sbi, nid);
> >> --
> >> 1.9.1

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