RE: [PATCH] staging/zcache: Fix/improve zcache writeback code, tie to a config option

[Dan Magenheimer <dan.magenheimer@oracle.com>]

> From: Ric Mason [mailto:ric.masonn@gmail.com]
> Subject: Re: [PATCH] staging/zcache: Fix/improve zcache writeback code, tie to a config option
> 
> On 02/07/2013 02:27 AM, Dan Magenheimer wrote:
> > It was observed by Andrea Arcangeli in 2011 that zcache can get "full"
> > and there must be some way for compressed swap pages to be (uncompressed
> > and then) sent through to the backing swap disk.  A prototype of this
> > functionality, called "unuse", was added in 2012 as part of a major update
> > to zcache (aka "zcache2"), but was left unfinished due to the unfortunate
> > temporary fork of zcache.
> >
> > This earlier version of the code had an unresolved memory leak
> > and was anyway dependent on not-yet-upstream frontswap and mm changes.
> > The code was meanwhile adapted by Seth Jennings for similar
> > functionality in zswap (which he calls "flush").  Seth also made some
> > clever simplifications which are herein ported back to zcache.  As a
> > result of those simplifications, the frontswap changes are no longer
> > necessary, but a slightly different (and simpler) set of mm changes are
> > still required [1].  The memory leak is also fixed.
> >
> > Due to feedback from akpm in a zswap thread, this functionality in zcache
> > has now been renamed from "unuse" to "writeback".
> >
> > Although this zcache writeback code now works, there are open questions
> > as how best to handle the policy that drives it.  As a result, this
> > patch also ties writeback to a new config option.  And, since the
> > code still depends on not-yet-upstreamed mm patches, to avoid build
> > problems, the config option added by this patch temporarily depends
> > on "BROKEN"; this config dependency can be removed in trees that
> > contain the necessary mm patches.
> >
> > [1] https://lkml.org/lkml/2013/1/29/540/ https://lkml.org/lkml/2013/1/29/539/
> 
> shrink_zcache_memory:
> 
> while(nr_evict-- > 0) {
>      page = zcache_evict_eph_pageframe();
>      if (page == NULL)
>          break;
>      zcache_free_page(page);
> }
> 
> zcache_evict_eph_pageframe
> ->zbud_evict_pageframe_lru
>      ->zbud_evict_tmem
>          ->tmem_flush_page
>              ->zcache_pampd_free
>                  ->zcache_free_page  <- zbudpage has already been free here
> 
> If the zcache_free_page called in shrink_zcache_memory can be treated as
> a double free?

Thanks for the code review and sorry for the delay...

zcache_pampd_free() only calls zcache_free_page() if page is non-NULL,
but in this code path I think when zcache_pampd_free() calls
zbud_free_and_delist(), that function determines that the zbudpage
is dying and returns NULL.

So unless I am misunderstanding (or misreading the code), there
is no double free.

Thanks,
Dan

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