Re: [PATCH 0/7] eb reference count cleanups

[Alex Lyakas <alex@zadara.com>]

Hi Nikolay, David,

Isn't patch 5 (btrfs: Remove extra reference count bumps in
btrfs_compare_trees) fixing a memory leak, and hence should be tagged
as "stable"? I am specifically interested in 4.14.x.

The comment says "remove redundant calls to extent_buffer_get since
they don't really add any value". But with the extra ref-count, the
extent buffer will not be properly freed and will cause a memory leak,
won't it?

Thanks,
Alex.



On Tue, Nov 6, 2018 at 4:30 PM David Sterba <dsterba@suse.cz> wrote:
>
> On Wed, Aug 15, 2018 at 06:26:50PM +0300, Nikolay Borisov wrote:
> > Here is a series which simplifies the way eb are used in EXTENT_BUFFER_UNMAPPED
> > context. The end goal was to remove the special "if we have ref count of 2 and
> > EXTENT_BUFFER_UNMAPPED flag then act as if this is the last ref and free the
> > buffer" case. To enable this the first 6 patches modify call sites which
> > needlessly bump the reference count.
> >
> > Patch 1 & 2 remove some btree locking when we are operating on unmapped extent
> > buffers. Each patch's changelog explains why this is safe to do .
> >
> > Patch 3,4,5 and 6 remove redundant calls to extent_buffer_get since they don't
> > really add any value. In all 3 cases having a reference count of 1 is sufficient
> > for the eb to be freed via btrfs_release_path.
> >
> > Patch 7 removes the special handling of EXTENT_BUFFER_UNMAPPED flag in
> > free_extent_buffer. Also adjust the selftest code to account for this change
> > by calling one extra time free_extent_buffer. Also document which references
> > are being dropped. All in all this shouldn't have any functional bearing.
> >
> > This was tested with multiple full xfstest runs as well as unloading the btrfs
> > module after each one to trigger the leak check and ensure no eb's are leaked.
> > I've also run it through btrfs' selftests multiple times with no problems.
> >
> > With this set applied EXTENT_BUFFER_UNMAPPED seems to be relevant only for
> > selftest which leads me to believe it can be removed altogether. I will
> > investigate this next but in the meantime this series should be good to go.
>
> Besides the 8/7 patch, the rest was in for-next for a long time so I'm
> merging that to misc-next, targeting 4.21. I'll do one last pass
> thhrough fstests with the full set and then upddate and push the branch
> so there might be some delay before it appears in the public repo.
> Thanks for the cleanup.