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

[Alex Lyakas <alex@zadara.com>]

Hi Nikolay,

In my kernel (4.14.x) the flag is called EXTENT_BUFFER_DUMMY, and
indeed I see that there is an extra dec-ref for them in
free_extent_buffer().

Thanks for clearing that up,
Alex.



On Wed, Feb 6, 2019 at 4:36 PM Nikolay Borisov <nborisov@suse.com> wrote:
>
>
>
> On 6.02.19 г. 16:26 ч., Alex Lyakas wrote:
> > 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?
>
> No, take a look into the logic of free_extent_buffer and see there is
> special handling for cloned buffers. And in fact, the series this patch
> was part of exactly removed this special handling since it's rather
> non-intuitive, your email being case in point.
>
> >
> > 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.
> >