linux-kernel - Re: [RFC PATCH 1/3] mm: support hugetlb free page reporting

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Date:   Tue, 22 Dec 2020 14:55:28 -0800
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Alexander Duyck <alexander.duyck@...il.com>,
        Alexander Duyck <alexander.h.duyck@...ux.intel.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Dan Williams <dan.j.williams@...el.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        David Hildenbrand <david@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Michal Hocko <mhocko@...e.com>,
        Liang Li <liliangleo@...iglobal.com>,
        Liang Li <liliang324@...il.com>, linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        virtualization@...ts.linux-foundation.org, qemu-devel@...gnu.org
Subject: Re: [RFC PATCH 1/3] mm: support hugetlb free page reporting

On 12/22/20 11:59 AM, Alexander Duyck wrote:
> On Mon, Dec 21, 2020 at 11:47 PM Liang Li <liliang.opensource@...il.com> wrote:
>> +
>> +       if (huge_page_order(h) > MAX_ORDER)
>> +               budget = HUGEPAGE_REPORTING_CAPACITY;
>> +       else
>> +               budget = HUGEPAGE_REPORTING_CAPACITY * 32;
> 
> Wouldn't huge_page_order always be more than MAX_ORDER? Seems like we
> don't even really need budget since this should probably be pulling
> out no more than one hugepage at a time.

On standard x86_64 configs, 2MB huge pages are of order 9 < MAX_ORDER (11).
What is important for hugetlb is the largest order that can be allocated
from buddy.  Anything bigger is considered a gigantic page and has to be
allocated differently.

If the code above is trying to distinguish between huge and gigantic pages,
it is off by 1.  The largest order that can be allocated from the buddy is
(MAX_ORDER - 1).  So, the check should be '>='.

-- 
Mike Kravetz