linux-kernel - Re: [PATCH v5 21/21] mm/hugetlb: Disable freeing vmemmap if struct page size is not power of two

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Message-ID: <20201122190002.GH4327@casper.infradead.org>
Date:   Sun, 22 Nov 2020 19:00:02 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Muchun Song <songmuchun@...edance.com>, corbet@....net,
        mike.kravetz@...cle.com, tglx@...utronix.de, mingo@...hat.com,
        bp@...en8.de, x86@...nel.org, hpa@...or.com,
        dave.hansen@...ux.intel.com, luto@...nel.org, peterz@...radead.org,
        viro@...iv.linux.org.uk, akpm@...ux-foundation.org,
        paulmck@...nel.org, mchehab+huawei@...nel.org,
        pawan.kumar.gupta@...ux.intel.com, rdunlap@...radead.org,
        oneukum@...e.com, anshuman.khandual@....com, jroedel@...e.de,
        almasrymina@...gle.com, rientjes@...gle.com, osalvador@...e.de,
        song.bao.hua@...ilicon.com, duanxiongchun@...edance.com,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v5 21/21] mm/hugetlb: Disable freeing vmemmap if struct
 page size is not power of two

On Fri, Nov 20, 2020 at 09:25:52AM +0100, Michal Hocko wrote:
> On Fri 20-11-20 14:43:25, Muchun Song wrote:
> > We only can free the unused vmemmap to the buddy system when the
> > size of struct page is a power of two.
> 
> Can we actually have !power_of_2 struct pages?

Yes.  On x86-64, if you don't enable MEMCG, it's 56 bytes.  On SPARC64,
if you do enable MEMCG, it's 72 bytes.  On 32-bit systems, it's
anything from 32-44 bytes, depending on MEMCG, WANT_PAGE_VIRTUAL and
LAST_CPUPID_NOT_IN_PAGE_FLAGS.