linux-kernel - Re: [RFC PATCH 0/7] Memory hotplug/hotremove at subsection size

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Date:   Thu, 6 May 2021 21:38:37 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Zi Yan <ziy@...dia.com>, Oscar Salvador <osalvador@...e.de>,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Thomas Gleixner <tglx@...utronix.de>, x86@...nel.org,
        Andy Lutomirski <luto@...nel.org>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Rapoport <rppt@...nel.org>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Michal Hocko <mhocko@...e.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Wei Yang <richard.weiyang@...ux.alibaba.com>,
        linux-ia64@...r.kernel.org, linux-kernel@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, linux-mm@...ck.org
Subject: Re: [RFC PATCH 0/7] Memory hotplug/hotremove at subsection size

On 06.05.21 21:30, Matthew Wilcox wrote:
> On Thu, May 06, 2021 at 09:10:52PM +0200, David Hildenbrand wrote:
>> I have to admit that I am not really a friend of that. I still think our
>> target goal should be to have gigantic THP *in addition to* ordinary THP.
>> Use gigantic THP where enabled and possible, and just use ordinary THP
>> everywhere else. Having one pageblock granularity is a real limitation IMHO
>> and requires us to hack the system to support it to some degree.
> 
> You're thinking too small with only two THP sizes ;-)  I'm aiming to

Well, I raised in my other mail that we will have multiple different use 
cases, including multiple different THP e.g., on aarch64 ;)

> support arbitrary power-of-two memory allocations.  I think there's a
> fruitful discussion to be had about how that works for anonymous memory --
> with page cache, we have readahead to tell us when our predictions of use
> are actually fulfilled.  It doesn't tell us what percentage of the pages

Right, and I think we have to think about a better approach than just 
increasing the pageblock_order.

> allocated were actually used, but it's a hint.  It's a big lift to go from
> 2MB all the way to 1GB ... if you can look back to see that the previous
> 1GB was basically fully populated, then maybe jump up from allocating
> 2MB folios to allocating a 1GB folio, but wow, that's a big step.
> 
> This goal really does mean that we want to allocate from the page
> allocator, and so we do want to grow MAX_ORDER.  I suppose we could
> do somethig ugly like
> 
> 	if (order <= MAX_ORDER)
> 		alloc_page()
> 	else
> 		alloc_really_big_page()
> 
> but that feels like unnecessary hardship to place on the user.

I had something similar for the sort term in mind, relying on 
alloc_contig_pages() (and maybe ZONE_MOVABLE to make allocations more 
likely to succeed). Devil's in the details (page migration, ...).


-- 
Thanks,

David / dhildenb