linux-ext4 - Re: [PATCH] e2fsck: fix multiply-claimed block quota accounting when deleting files

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Message-ID: <20170511154235.GA11505@localhost.localdomain>
Date:   Thu, 11 May 2017 11:42:35 -0400
From:   Eric Whitney <enwlinux@...il.com>
To:     Andreas Dilger <adilger@...ger.ca>
Cc:     Eric Whitney <enwlinux@...il.com>, linux-ext4@...r.kernel.org,
        tytso@....edu
Subject: Re: [PATCH] e2fsck: fix multiply-claimed block quota accounting when
 deleting files

* Andreas Dilger <adilger@...ger.ca>:
> 
> > On May 10, 2017, at 4:04 PM, Eric Whitney <enwlinux@...il.com> wrote:
> > 
> > As e2fsck processes each file in pass1, the actual file system quota is
> > increased by the number of blocks discovered in the file.  This can
> > include both non-multiply-claimed and multiply-claimed blocks, if the
> > latter exist.  However, if a file containing multiply-claimed blocks
> > is then deleted in pass1b, those blocks are not taken into account when
> > decreasing the actual quota.  In this case, the new quota values written
> > to the file system by e2fsck overstate the space actually consumed.
> > And, e2fsck must be run twice on the file system to fully correct
> > quota.
> > 
> > Fix this by counting multiply-claimed blocks as a debit to quota when
> > deleting files in pass1b.
> 
> Nice catch.  It would be good to have an e2fsck test case that checks this.
> Also, one minor code style nit (or possibly defect) below.

Yeah, there's not much test coverage in this area.  I'll look at it.

> 
> > Signed-off-by: Eric Whitney <enwlinux@...il.com>
> > ---
> > e2fsck/pass1b.c | 10 ++++++----
> > 1 file changed, 6 insertions(+), 4 deletions(-)
> > 
> > diff --git a/e2fsck/pass1b.c b/e2fsck/pass1b.c
> > index b40f026..8744fad 100644
> > --- a/e2fsck/pass1b.c
> > +++ b/e2fsck/pass1b.c
> > @@ -636,11 +636,13 @@ static int delete_file_block(ext2_filsys fs,
> > 	lc = EXT2FS_B2C(fs, blockcnt);
> > 	if (ext2fs_test_block_bitmap2(ctx->block_dup_map, *block_nr)) {
> > 		n = dict_lookup(&clstr_dict, INT_TO_VOIDPTR(c));
> > -		if (n) {
> > -			p = (struct dup_cluster *) dnode_get(n);
> > -			if (lc != pb->cur_cluster)
> > 				decrement_badcount(ctx, *block_nr, p);
> > -		} else
> > +		if (n)
> > +			if (lc != pb->cur_cluster) {
> > +				p = (struct dup_cluster *) dnode_get(n);
> > 				decrement_badcount(ctx, *block_nr, p);
> > +				pb->dup_blocks++;
> > +			}
> > +		else
> > 			com_err("delete_file_block", 0,
> > 			    _("internal error: can't find dup_blk for %llu\n"),
> > 				*block_nr);
> 
> This is tricky to know which "if" the "else" is for without the added braces,
> and to be honest I don't even know what the C standard says about this, which
> is likely why the braces were there in the first place.  I would instead
> recommend to add braces around the "else" clause to make it clear.
> 

Yes, that's a classic dangling else bug - I scrubbed too hard.  Thanks very
much for finding that.  V2 coming along shortly.

Thanks for the review,
Eric