linux-ext4 - Re: [PATCH] ext4: Fix a BUG in mb_mark

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Date:	Thu, 3 Mar 2011 11:17:46 +0100 (CET)
From:	Lukas Czerner <lczerner@...hat.com>
To:	Tao Ma <tm@....ma>
cc:	Lukas Czerner <lczerner@...hat.com>, linux-ext4@...r.kernel.org,
	"Theodore Ts'o" <tytso@....edu>
Subject: Re: [PATCH] ext4: Fix a BUG in mb_mark_used during trim.

On Thu, 3 Mar 2011, Tao Ma wrote:

> On 03/03/2011 06:01 PM, Lukas Czerner wrote:
> > On Thu, 3 Mar 2011, Tao Ma wrote:
> > 
> >    
> > > From: Tao Ma<boyu.mt@...bao.com>
> > > 
> > > In a bs=4096 volume, if we call FITRIM with the following parameter as
> > > fstrim_range(start = 102400, len = 134144000, minlen = 10240),
> > > we will trigger a BUG_ON.
> > > BUG_ON(start + len>  (e4b->bd_sb->s_blocksize<<  3));
> > > 
> > > Mar  4 00:55:52 boyu-tm kernel: ------------[ cut here ]------------
> > > Mar  4 00:55:52 boyu-tm kernel: kernel BUG at fs/ext4/mballoc.c:1506!
> > > Mar  4 01:21:09 boyu-tm kernel: Code: d4 00 00 00 00 49 89 fe 8b 56 0c 44
> > > 8b 7e 04 89 55 c4 48 8b 4f 28 89 d6 44 01 fe 48 63 d6 48 8b 41 18 48 c1 e0
> > > 03 48 39 c2 76 04<0f>  0b eb fe 48 8b 55 b0 8b 47 34 3b 42 08 74 04 0f 0b
> > > eb fe 48
> > > Mar  4 01:21:09 boyu-tm kernel: RIP  [<ffffffffa053eb42>]
> > > mb_mark_used+0x47/0x26c [ext4]
> > > Mar  4 01:21:09 boyu-tm kernel:  RSP<ffff880121e45c38>
> > > Mar  4 01:21:09 boyu-tm kernel: ---[ end trace 9f461696f6a9dcf2 ]---
> > > 
> > > The reason is that in ext4_trim_fs, the last_block is checked wrongly.
> > > if (len>= EXT4_BLOCKS_PER_GROUP(sb))
> > > 	len -= (EXT4_BLOCKS_PER_GROUP(sb) - first_block);
> > > else
> > > 	last_block = first_block + len;
> > > 
> > > So if len<  EXT4_BLOCKS_PER_GROUP while first_block + len>
> > > EXT4_BLOCKS_PER_GROUP,
> > > last_block will be set to a overflow value which exceeds
> > > EXT4_BLOCKS_PER_GROUP.
> > > 
> > > This patch fixes it and adjusts len accordingly.
> > >      
> > Oh, thanks for spotting this. One comment though.
> > 
> >    
> > > Cc: "Theodore Ts'o"<tytso@....edu>
> > > Signed-off-by: Tao Ma<boyu.mt@...bao.com>
> > > ---
> > >   fs/ext4/mballoc.c |    5 +++--
> > >   1 files changed, 3 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
> > > index 29d7d17..ed63c3e 100644
> > > --- a/fs/ext4/mballoc.c
> > > +++ b/fs/ext4/mballoc.c
> > > @@ -4889,10 +4889,11 @@ int ext4_trim_fs(struct super_block *sb, struct
> > > fstrim_range *range)
> > >   			break;
> > >   		}
> > > 
> > > -		if (len>= EXT4_BLOCKS_PER_GROUP(sb))
> > > -			len -= (EXT4_BLOCKS_PER_GROUP(sb) - first_block);
> > > +		if (first_block + len>= EXT4_BLOCKS_PER_GROUP(sb))
> > > +			last_block = EXT4_BLOCKS_PER_GROUP(sb);
> > >   		else
> > >   			last_block = first_block + len;
> > >      
> > Since last_block would not change until the last group, can we simplify
> > the condition ?
> > 
> > 		if (first_block + len<  EXT4_BLOCKS_PER_GROUP(sb))
> > 			last_block = first_block + len;
> >    
> yes, it should work. But it is a bit tricky. And I guess we need a comment
> here
> to describe the situation. Otherwise, the people who read the code would
> consider
> why last_block won't be reset in case first _block + len >=
> EXT4_BLOCKS_PER_GROUP
> every time. Agree?

Agree, comment would be useful.

Thanks!
-Lukas

> >    
> > > +		len -= last_block - first_block;
> > > 
> > >   		if (e4b.bd_info->bb_free>= minlen) {
> > >   			cnt = ext4_trim_all_free(sb,&e4b, first_block,
> > > 
> > >      
> > Thanks!
> > -Lukas
> >    
> Regards,
> Tao
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