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Date:   Fri, 5 Aug 2022 07:41:38 -0700
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Subject: Re: [PATCHv7 02/14] mm: Add support for unaccepted memory
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To:     Vlastimil Babka <vbabka@suse.cz>,
        David Hildenbrand <david@redhat.com>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Borislav Petkov <bp@alien8.de>,
        Andy Lutomirski <luto@kernel.org>,
        Sean Christopherson <seanjc@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Joerg Roedel <jroedel@suse.de>,
        Ard Biesheuvel <ardb@kernel.org>
Cc:     Andi Kleen <ak@linux.intel.com>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@linux.intel.com>,
        David Rientjes <rientjes@google.com>,
        Tom Lendacky <thomas.lendacky@amd.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Ingo Molnar <mingo@redhat.com>,
        Dario Faggioli <dfaggioli@suse.com>,
        Mike Rapoport <rppt@kernel.org>, marcelo.cerri@canonical.com,
        tim.gardner@canonical.com, khalid.elmously@canonical.com,
        philip.cox@canonical.com, x86@kernel.org, linux-mm@kvack.org,
        linux-coco@lists.linux.dev, linux-efi@vger.kernel.org,
        linux-kernel@vger.kernel.org, Mike Rapoport <rppt@linux.ibm.com>,
        Mel Gorman <mgorman@techsingularity.net>
References: <20220614120231.48165-1-kirill.shutemov@linux.intel.com>
 <20220614120231.48165-3-kirill.shutemov@linux.intel.com>
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 <cb9d3310-3bc0-8ecf-5e71-becce980235f@redhat.com>
 <f936b024-43e1-5390-e33f-ad7d355a2802@suse.cz>
From:   Dave Hansen <dave.hansen@intel.com>
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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.
3. Pull the page off the 2M/4M lists, drop the zone lock, accept it,
   then put it back.

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