Re: [PATCH] staging/zcache: Fix/improve zcache writeback code, tieto a config option

[Ric Mason]

On 03/01/2013 06:29 AM, Dan Magenheimer wrote:
> > From: Ric Mason [mailto:ric.masonn@xxxxxxxxx]
> > 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.

Oh, I see. Thanks for your response. :)

> Thanks,
> Dan

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