linux-kernel - Re: [PATCHv7 02/14] mm: Add support for unaccepted memory

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Date:   Fri, 5 Aug 2022 20:17:52 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Dave Hansen <dave.hansen@...el.com>,
        David Hildenbrand <david@...hat.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        Sean Christopherson <seanjc@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Joerg Roedel <jroedel@...e.de>,
        Ard Biesheuvel <ardb@...nel.org>
Cc:     Andi Kleen <ak@...ux.intel.com>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        David Rientjes <rientjes@...gle.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Dario Faggioli <dfaggioli@...e.com>,
        Mike Rapoport <rppt@...nel.org>, marcelo.cerri@...onical.com,
        tim.gardner@...onical.com, khalid.elmously@...onical.com,
        philip.cox@...onical.com, x86@...nel.org, linux-mm@...ck.org,
        linux-coco@...ts.linux.dev, linux-efi@...r.kernel.org,
        linux-kernel@...r.kernel.org, Mike Rapoport <rppt@...ux.ibm.com>,
        Mel Gorman <mgorman@...hsingularity.net>
Subject: Re: [PATCHv7 02/14] mm: Add support for unaccepted memory

On 8/5/22 16:41, Dave Hansen wrote:
> On 8/5/22 06:38, Vlastimil Babka wrote:
>>> I'm sure we could optimize for the !unaccepted memory via static keys
>>> also in this version with some checks at the right places if we find
>>> this to hurt performance?
>> It would be great if we would at least somehow hit the necessary code only
>> when dealing with a >=pageblock size block. The bitmap approach and
>> accepting everything smaller uprofront actually seems rather compatible. Yet
>> in the current patch we e.g. check PageUnaccepted(buddy) on every buddy size
>> while merging.
> 
> Needing to check PageUnaccepted() during the merge is fallout from
> moving the acceptance to post_alloc_hook().  I _think_ an earlier
> version of this did page acceptance under the zone lock, closer to where
> the page comes off the 2M/4M lists.
> 
> But, page acceptance is horribly slow, so I asked Kirill to move it out
> from under the zone lock.  Doing it in post_alloc_hook() (after the zone
> lock is dropped) makes a lot of sense since we do zeroing in there and
> zeroing is also nice and slow.
> 
> But, post_alloc_hook() is long after the 2M page has been split and that
> means that we have to deal with potentially unaccepted pages during merges.
> 
> I think there are three basic options:
> 
> 1. This patch: Do acceptance after the zone lock is dropped and deal
>    with mixed-acceptance merges
> 2. Do acceptance under the zone lock as pages come off the 2M/4M lists,
>    but before the page is split.

Rather not, as acceptance can be slow and we shouldn't hog the zone lock
while doing it.

> 3. Pull the page off the 2M/4M lists, drop the zone lock, accept it,
>    then put it back.

Worth trying, IMHO. Perhaps easier to manage if the lists are distinct from
the normal ones, as I suggested.

> I'm not sure any of those other options are better.