Re: [PATCH 4/5] [RFC][HACK] Add LRU_VOLATILE support to the VM

Minchan Kim
2012-08-07T01:43:54Z

On Mon, Aug 06, 2012 at 06:26:03PM -0700, Dan Magenheimer wrote:
> > From: Minchan Kim [mailto:minchan@kernel.org]
> > Subject: Re: [PATCH 4/5] [RFC][HACK] Add LRU_VOLATILE support to the VM
> >
> > On Mon, Aug 06, 2012 at 08:46:18AM -0700, Dan Magenheimer wrote:
> > > > From: Minchan Kim [mailto:minchan@kernel.org]
> > > > To: John Stultz
> > > > Subject: Re: [PATCH 4/5] [RFC][HACK] Add LRU_VOLATILE support to the VM
> > >
> > > Hi Minchan --
> > >
> > > Thanks for cc'ing me on this!
> > >
> > > > Targets for the LRU list could be following as in future
> > > >
> > > > 1. volatile pages in this patchset.
> > > > 2. ephemeral pages of tmem
> > > > 3. madivse(DONTNEED)
> > > > 4. fadvise(NOREUSE)
> > > > 5. PG_reclaimed pages
> > > > 6. clean pages if we write CFLRU(clean first LRU)
> > > >
> > > > So if any guys have objection, please raise your hands
> > > > before further progress.
> > >
> > > I agree that the existing shrinker mechanism is too primitive
> > > and the kernel needs to take into account more factors in
> > > deciding how to quickly reclaim pages from a broader set
> > > of sources. However, I think it is important to ensure
> > > that both the "demand" side and the "supply" side are
> > > studied. There has to be some kind of prioritization policy
> > > among all the RAM consumers so that a lower-priority
> > > alloc_page doesn't cause a higher-priority "volatile" page
> > > to be consumed. I suspect this policy will be VERY hard to
> > > define and maintain.
> >
> > Yes. It's another story.
> > At the moment, VM doesn't consider such priority-inversion problem
> > excpet giving the more memory to privileged processes. It's so simple
> > but works well till now.
>
> I think it is very important that both stories must be
> solved together. See below...
>
> > > Related, ephemeral pages in tmem are not truly volatile
> >
> > "volatile" term is used by John for only his special patch so
> > I like Ereclaim(Easy Reclaim) rather than volatile.
>
> If others agree, that's fine. However, the "E" prefix is
> currently used differently in common English (for example,
> for e-books). Maybe "ezreclaim"?

Looks better. I will use that term from now on.
Thanks!

>
> > > as there is always at least one tmem data structure pointing
> > > to it. I haven't followed this thread previously so my apologies
> > > if it already has this, but the LRU_VOLATILE list might
> > > need to support a per-page "garbage collection" callback.
> >
> > Right. That's why this patch provides purgepage in address_space_operations.
> > I think zcache could attach own address_space_operations to the page
> > which is allocated by zbud for instance, zcache_purgepage which is called by VM
> > when the page is reclaimed. So zcache don't need custom LRU policy(but still need
> > linked list for managing zbuddy) and pass the decision to the VM.
>
> The simple VM decisions are going to need a lot more intelligence
> (and data?) to drive which page to reclaim. For example, is it better
> to reclaim a pageframe that contains two compressed pages of ephemeral data
> or a pageframe that has one active (or inactive) file page? Such
> a policy is not "Easy". ;-)

I should have said more cleary.
VM just pick a page in tail of ezreclaim list and and then just reclaim.
So rotation of the active page or two compresssed pages should be implemented
by smart zcache which can do anyting.

>
> (Also, BTW, zcache pages aren't in any address space so don't have
> an address_space_operations... because it is not possible to directly
> address the data in a compressed page.)

I mean we can make just fake address_space_operations like this.

static struct address_space_operations zcache_aop = {
.purgepage = zcache_purge_page,
};

static struct address_space zcache_address_space = {
.a_ops = &zcache_aop,
};

struct page *page = alloc_page();
page->mapping = &zcache_address_space;

>
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--
Kind regards,
Minchan Kim