Re: [PATCH] ext4: fix wrong size computation in ext4_mb_normalize_request()

["Lukáš Czerner" <lczerner@redhat.com>]

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On Fri, 11 Jul 2014, Xiaoguang Wang wrote:

> Date: Fri, 11 Jul 2014 11:41:08 +0800
> From: Xiaoguang Wang <wangxg.fnst@cn.fujitsu.com>
> To: Lukáš Czerner <lczerner@redhat.com>
> Cc: linux-ext4@vger.kernel.org, tytso@mit.edu, mlombard@redhat.com
> Subject: Re: [PATCH] ext4: fix wrong size computation in
>     ext4_mb_normalize_request()
> 
> Hi,
> 
> On 07/10/2014 05:25 PM, Lukáš Czerner wrote:
> > On Thu, 10 Jul 2014, Xiaoguang Wang wrote:
> > 
> >> Date: Thu, 10 Jul 2014 13:40:36 +0800
> >> From: Xiaoguang Wang <wangxg.fnst@cn.fujitsu.com>
> >> To: linux-ext4@vger.kernel.org
> >> Cc: tytso@mit.edu, mlombard@redhat.com,
> >>     Xiaoguang Wang <wangxg.fnst@cn.fujitsu.com>
> >> Subject: [PATCH] ext4: fix wrong size computation in
> >>     ext4_mb_normalize_request()
> >>
> >> As the member fe_len defined in struct ext4_free_extent is expressed as
> >> number of clusters, the variable "size" computation is wrong, we need to
> >> first translate it to block number, then to bytes.
> >>
> >> Signed-off-by: Xiaoguang Wang <wangxg.fnst@cn.fujitsu.com>
> >> ---
> >>  fs/ext4/mballoc.c | 3 ++-
> >>  1 file changed, 2 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
> >> index 7f72f50..9a543b5 100644
> >> --- a/fs/ext4/mballoc.c
> >> +++ b/fs/ext4/mballoc.c
> >> @@ -3076,7 +3076,8 @@ ext4_mb_normalize_request(struct ext4_allocation_context *ac,
> >>  		size = 8 * 1024 * 1024;
> >>  	} else {
> >>  		start_off = (loff_t)ac->ac_o_ex.fe_logical << bsbits;
> >> -		size	  = ac->ac_o_ex.fe_len << bsbits;
> >> +		size	  = EXT4_C2B(EXT4_SB(ac->ac_sb),
> >> +				     ac->ac_o_ex.fe_len) << bsbits;
> > 
> > There is a patch out there up for more discussion which rewrites a
> > whole bunch of code in ext4_mb_normalize_request(). But more
> > importantly this fix, while correct is not going to change anything
> > since this condition will never be run.
> 
> Yeah, I have read your patch and related discussions, indeed this condition
> would never be run, thanks for your explanation. But given that currently your
> patch is not merged, still this code is wrong and should be fixed.
> I'm new to ext4:) Whether should I continue to send a v2 version to fix this issue? thanks!

Please do, it's still worth fixing.

Thanks!
-Lukas

> 
> Regards,
> Xiaoguang Wang
> > 
> > Btw, I take back the correct part since it seems that there is a
> > a possibility of overflow. This should be better.
> > 
> > 		size	  = (loff_t)EXT4_C2B(EXT4_SB(ac->ac_sb),
> > 					     ac->ac_o_ex.fe_len) << bsbits;
> > 
> > Thanks!
> > -Lukas
> > 
> >>  	}
> >>  	size = size >> bsbits;
> >>  	start = start_off >> bsbits;
> >>
> > .
> > 
> 
>