Re: [PATCH 2/2] Btrfs: fix memory leak on extent map after fsync

[Liu Bo <bo.li.liu@oracle.com>]

On Fri, Jan 25, 2013 at 09:38:06AM +0800, Liu Bo wrote:
> On Thu, Jan 24, 2013 at 11:44:33AM -0500, Josef Bacik wrote:
> > On Tue, Jan 08, 2013 at 07:49:21AM -0700, Liu Bo wrote:
> > > During fsync, we put the changed parts(i.e. extent map) into the log tree,
> > > and we ship these parts from a list of modified_extents to a local list
> > > to process, of course, we must increment the refs of the extent maps to
> > > avoid it from getting evicted from cache.
> > > 
> > > The problem is
> > > we don't hold the tree writer lock all the time of iterating the local list,
> > > and it is possible that other threads hack in and delete the extent map from
> > > the local list silently.  So we'll end up with memory leak here.
> > > 
> > > I hit this when testing xfstest 274 with mount options 'autodefrag,compress=zlib'.
> > > 
> > > With this fix, the memory leak has gone away.
> > 
> > This isn't going to work, we use the LOGGING flag to make sure the em isn't
> > merged as well.  Thanks,

Well, never mind, I've seen the fix in your btrfs-next.  But it'd be
better if you can also send it to the list where everyone can review it
easily.

thanks,
liubo

> 
> A quick grep shows,
> 
>    1     16  fs/btrfs/extent_map.h <<GLOBAL>>
>              #define EXTENT_FLAG_LOGGING 4
>    2    406  fs/btrfs/extent_map.c <<remove_extent_mapping>>
>              if (!test_bit(EXTENT_FLAG_LOGGING, &em->flags))
>    3   3403  fs/btrfs/tree-log.c <<btrfs_log_changed_extents>>
>              set_bit(EXTENT_FLAG_LOGGING, &em->flags);
>    4   3413  fs/btrfs/tree-log.c <<btrfs_log_changed_extents>>
>              clear_bit(EXTENT_FLAG_LOGGING, &em->flags);
> 
> how does the flag avoid merging em?
> 
> Seems we lost the check.
> 
> static int mergable_maps(struct extent_map *prev, struct extent_map *next)       
> {                                                                                
>         if (test_bit(EXTENT_FLAG_PINNED, &prev->flags))                          
>                 return 0;                                                        
>                                                                                  
>         /*                                                                       
>          * don't merge compressed extents, we need to know their                 
>          * actual size                                                           
>          */                                                                      
>         if (test_bit(EXTENT_FLAG_COMPRESSED, &prev->flags))                      
>                 return 0;                                                        
> 
> 	...
> }
>                                                                                  
> thanks,
> liubo
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