linux-ext4 - Re: [PATCH v2 3/3] ext4: Return error code from ext4_mb_good

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Date:	Tue, 2 Jun 2015 18:51:26 +0200 (CEST)
From:	Lukáš Czerner <lczerner@...hat.com>
To:	"Darrick J. Wong" <darrick.wong@...cle.com>
cc:	linux-ext4@...r.kernel.org
Subject: Re: [PATCH v2 3/3] ext4: Return error code from
 ext4_mb_good_group()

On Tue, 2 Jun 2015, Darrick J. Wong wrote:

> Date: Tue, 2 Jun 2015 09:27:18 -0700
> From: Darrick J. Wong <darrick.wong@...cle.com>
> To: Lukas Czerner <lczerner@...hat.com>
> Cc: linux-ext4@...r.kernel.org
> Subject: Re: [PATCH v2 3/3] ext4: Return error code from ext4_mb_good_group()
> 
> On Tue, Jun 02, 2015 at 02:24:50PM +0200, Lukas Czerner wrote:
> > Currently ext4_mb_good_group() only returns 0 or 1 depending on whether
> > the allocation group is suitable for use or not. However we might get
> > various errors and fail while initializing new group including -EIO
> > which would never get propagated up the call chain. This might lead to
> > an endless loop at writeback when we're trying to find a good group to
> > allocate from and we fail to initialize new group (read error for
> > example).
> > 
> > Fix this by returning proper error code from ext4_mb_good_group() and
> > using it in ext4_mb_regular_allocator(). In ext4_mb_regular_allocator()
> > we will always return only the first occurred error from
> > ext4_mb_good_group() and we only propagate it back  to the caller if we
> > do not get any other errors and we fail to allocate any blocks.
> > 
> > Note that with other modes than errors=continue, we will fail
> > immediately in ext4_mb_good_group() in case of error, however with
> > errors=continue we should try to continue using the file system, that's
> > why we're not going to fail immediately when we see an error from
> > ext4_mb_good_group(), but rather when we fail to find a suitable block
> > group to allocate from due to an problem in group initialization.
> > 
> > Signed-off-by: Lukas Czerner <lczerner@...hat.com>
> > ---
> > v2: nothing changed
> > 
> >  fs/ext4/mballoc.c | 18 ++++++++++++++----
> >  1 file changed, 14 insertions(+), 4 deletions(-)
> > 
> > diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
> > index df02951..e0e77a7 100644
> > --- a/fs/ext4/mballoc.c
> > +++ b/fs/ext4/mballoc.c
> > @@ -2034,7 +2034,7 @@ static int ext4_mb_good_group(struct ext4_allocation_context *ac,
> 
> I'd prefer a comment clarifying the possible return values.  Otherwise,
> everything (patches 1-3) looks ok to me.

Right, with this change the comment is not entirely accurate
anymore. I'll fix that.

Thanks!
-Lukas

> 
> Reviewed-by: Darrick J. Wong <darrick.wong@...cle.com>
> 
> >  	if (unlikely(EXT4_MB_GRP_NEED_INIT(grp))) {
> >  		int ret = ext4_mb_init_group(ac->ac_sb, group);
> >  		if (ret)
> > -			return 0;
> > +			return ret;
> >  	}
> >  
> >  	fragments = grp->bb_fragments;
> > @@ -2081,7 +2081,7 @@ ext4_mb_regular_allocator(struct ext4_allocation_context *ac)
> >  {
> >  	ext4_group_t ngroups, group, i;
> >  	int cr;
> > -	int err = 0;
> > +	int err = 0, first_err = 0;
> >  	struct ext4_sb_info *sbi;
> >  	struct super_block *sb;
> >  	struct ext4_buddy e4b;
> > @@ -2148,6 +2148,7 @@ repeat:
> >  		group = ac->ac_g_ex.fe_group;
> >  
> >  		for (i = 0; i < ngroups; group++, i++) {
> > +			int ret = 0;
> >  			cond_resched();
> >  			/*
> >  			 * Artificially restricted ngroups for non-extent
> > @@ -2157,8 +2158,12 @@ repeat:
> >  				group = 0;
> >  
> >  			/* This now checks without needing the buddy page */
> > -			if (!ext4_mb_good_group(ac, group, cr))
> > +			ret = ext4_mb_good_group(ac, group, cr);
> > +			if (ret <= 0) {
> > +				if (!first_err)
> > +					first_err = ret;
> >  				continue;
> > +			}
> >  
> >  			err = ext4_mb_load_buddy(sb, group, &e4b);
> >  			if (err)
> > @@ -2170,9 +2175,12 @@ repeat:
> >  			 * We need to check again after locking the
> >  			 * block group
> >  			 */
> > -			if (!ext4_mb_good_group(ac, group, cr)) {
> > +			ret = ext4_mb_good_group(ac, group, cr);
> > +			if (ret <= 0) {
> >  				ext4_unlock_group(sb, group);
> >  				ext4_mb_unload_buddy(&e4b);
> > +				if (!first_err)
> > +					first_err = ret;
> >  				continue;
> >  			}
> >  
> > @@ -2219,6 +2227,8 @@ repeat:
> >  		}
> >  	}
> >  out:
> > +	if (!err && ac->ac_status != AC_STATUS_FOUND && first_err)
> > +		err = first_err;
> >  	return err;
> >  }
> >  
> > -- 
> > 1.8.3.1
> > 
> > --
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> 
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