Re: [PATCH] btrfs: make sure we stay inside the bvec during __btrfs_lookup_bio_sums

[Chandan Rajendra <chandan@linux.vnet.ibm.com>]

On Monday 21 Mar 2016 11:00:14 Chris Mason wrote:
> Hi everyone,
> 
> I realized last week that CONFIG_DEBUG_PAGEALLOC had dropped out of my
> config, and hit a crash inside __btrfs_lookup_bio_sums once I enabled it
> again.  It's hard for this bug to cause problems because Chandan's inner
> loop is always done at the same time the outer loop is done.  Without my
> goto, it's just exiting normally, but only after reading bvec->bv_len
> (which isn't valid).
> 
> I have this on top of my integration-4.6.  Once things pass I'll
> send a pull later today or Tuesday morning:
> 
> Commit c40a3d38aff4e1c (Btrfs: Compute and look up csums based on
> sectorsized blocks) changes around how we walk the bios while looking up
> crcs.  There's an inner loop that is jumping to the next bvec based on
> sectors and before it derefs the next bvec, it needs to make sure we're
> still in the bio.
> 
> In this case, the outer loop would have decided to stop moving forward
> too, and the bvec deref is never actually used for anything.  But
> CONFIG_DEBUG_PAGEALLOC catches it because we're outside our bio.
>

Chris, Thanks for finding the issue and fixing it. From now onwards, I will
make sure that I have CONFIG_DEBUG_PAGEALLOC enabled on my test kernel build
config.

Reviewed-by: Chandan Rajendra <chandan@linux.vnet.ibm.com>

-- 
chandan