linux-kernel - Re: [PATCH 4/4] f2fs: optimize build_free

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Date:	Wed, 8 May 2013 14:24:55 +0800
From:	Haicheng Li <haicheng.li@...ux.intel.com>
To:	Jaegeuk Kim <jaegeuk.kim@...sung.com>
Cc:	linux-kernel@...r.kernel.org, Haicheng Li <haicheng.lee@...il.com>,
	linux-fsdevel@...r.kernel.org,
	linux-f2fs-devel@...ts.sourceforge.net
Subject: Re: [PATCH 4/4] f2fs: optimize build_free_nids()

On Tue, May 07, 2013 at 07:33:59PM +0900, Jaegeuk Kim wrote:
> 2013-05-06 (월), 23:15 +0800, Haicheng Li:
> > diff --git a/fs/f2fs/node.c b/fs/f2fs/node.c
> > index 1fe3fe2..3136224 100644
> > --- a/fs/f2fs/node.c
> > +++ b/fs/f2fs/node.c
> > @@ -1342,6 +1342,8 @@ static void build_free_nids(struct f2fs_sb_info *sbi)
> >  		if (nid >= nm_i->max_nid)
> >  			nid = 0;
> >  
> > +		if (nm_i->fcnt > 2 * MAX_FREE_NIDS)
> > +			break;
> 
> Could you explain when this can happen?

I'm thinking of this possible scenario:

as we don't hold any spinlock to protect the context, add_free_nid() could be 
called by other thread anytime, e.g. by the gc_thread_func() in background.

then nm_i->fcnt could be increased as 2 * MAX_FREE_NIDS while i < FREE_NID_PAGES.

Anything I misconsidered?

> IMO, this is an unnecessary condition check, since the below condition
> that includes FREE_NID_PAGES already limits the number of free nids.
> Thanks,

hmm, the pros is that this check may possibly avoid some (< 4) unnecessary while-loop,
the cons is that too many checks of (nm_i->fcnt > 2 * MAX_FREE_NIDS)
would make the code looking messy and fragmentary...
 
> >  		if (i++ == FREE_NID_PAGES)
> >  			break;
> >  	}
> 
> -- 
> Jaegeuk Kim
> Samsung


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