linux-kernel - Re: [f2fs-dev] [PATCH 5/5] f2fs: check the node block address of newly allocated nid

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Date:	Thu, 20 Aug 2015 08:35:23 -0700
From:	Jaegeuk Kim <jaegeuk@...nel.org>
To:	Chao Yu <chao2.yu@...sung.com>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-f2fs-devel@...ts.sourceforge.net
Subject: Re: [f2fs-dev] [PATCH 5/5] f2fs: check the node block address of
 newly allocated nid

On Thu, Aug 20, 2015 at 05:12:03PM +0800, Chao Yu wrote:
> Hi Jaegeuk,
> 
> > -----Original Message-----
> > From: Jaegeuk Kim [mailto:jaegeuk@...nel.org]
> > Sent: Tuesday, August 18, 2015 4:46 PM
> > To: linux-kernel@...r.kernel.org; linux-fsdevel@...r.kernel.org;
> > linux-f2fs-devel@...ts.sourceforge.net
> > Cc: Jaegeuk Kim
> > Subject: [f2fs-dev] [PATCH 5/5] f2fs: check the node block address of newly allocated nid
> > 
> > This patch adds a routine which checks the block address of newly allocated nid.
> > If an nid has already allocated by other thread due to subtle data races, it
> > will result in filesystem corruption.
> > So, it needs to check whether its block address was already allocated or not
> > in prior to nid allocation as the last chance.
> > 
> > Signed-off-by: Jaegeuk Kim <jaegeuk@...nel.org>
> > ---
> >  fs/f2fs/node.c | 9 +++++++++
> >  1 file changed, 9 insertions(+)
> > 
> > diff --git a/fs/f2fs/node.c b/fs/f2fs/node.c
> > index 3cc32b8..6bef5a2 100644
> > --- a/fs/f2fs/node.c
> > +++ b/fs/f2fs/node.c
> > @@ -1573,6 +1573,8 @@ retry:
> > 
> >  	/* We should not use stale free nids created by build_free_nids */
> >  	if (nm_i->fcnt && !on_build_free_nids(nm_i)) {
> > +		struct node_info ni;
> > +
> >  		f2fs_bug_on(sbi, list_empty(&nm_i->free_nid_list));
> >  		list_for_each_entry(i, &nm_i->free_nid_list, list)
> >  			if (i->state == NID_NEW)
> > @@ -1583,6 +1585,13 @@ retry:
> >  		i->state = NID_ALLOC;
> >  		nm_i->fcnt--;
> >  		spin_unlock(&nm_i->free_nid_list_lock);
> > +
> > +		/* check nid is allocated already */
> > +		get_node_info(sbi, *nid, &ni);
> > +		if (ni.blk_addr != NULL_ADDR) {
> 
> I didn't get it, why free nid is with non-NULL blkaddr?
> Could you please explain more about this?

As I wrote in the description, I've been suffering from wrongly added free nids
which results in fs corruption. I suspect somewhat race condition in
build_free_nids, but it is very subtle to figure out exactly.
So, I wrote this patch to fix that.

The concern would be performance regarding to cold cache miss at an NAT entry.
However, I expect that it would be tolerable since get_node_info will be called
after alloc_nid later.

> 
> > +			alloc_nid_done(sbi, *nid);
> 
> Will another thread call alloc_nid_done too, making this free nid being
> released again?

No, its state became NID_ALLOC, so no other thread can pick this up till
alloc_nid_done is called.

Thanks,

> 
> Thanks,
> 
> > +			goto retry;
> > +		}
> >  		return true;
> >  	}
> >  	spin_unlock(&nm_i->free_nid_list_lock);
> > --
> > 2.1.1
> > 
> > 
> > ------------------------------------------------------------------------------
> > _______________________________________________
> > Linux-f2fs-devel mailing list
> > Linux-f2fs-devel@...ts.sourceforge.net
> > https://lists.sourceforge.net/lists/listinfo/linux-f2fs-devel
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