linux-kernel - Re: [PATCH v8 1/4] mm: hugetlb_vmemmap: introduce CONFIG_HUGETLB_PAGE_HAS_OPTIMIZE

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Date:   Thu, 14 Apr 2022 11:10:01 +0800
From:   Muchun Song <songmuchun@...edance.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     corbet@....net, mike.kravetz@...cle.com, mcgrof@...nel.org,
        keescook@...omium.org, yzaikin@...gle.com, osalvador@...e.de,
        david@...hat.com, masahiroy@...nel.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        duanxiongchun@...edance.com, smuchun@...il.com
Subject: Re: [PATCH v8 1/4] mm: hugetlb_vmemmap: introduce
 CONFIG_HUGETLB_PAGE_HAS_OPTIMIZE_VMEMMAP

On Wed, Apr 13, 2022 at 12:08:04PM -0700, Andrew Morton wrote:
> On Wed, 13 Apr 2022 22:47:45 +0800 Muchun Song <songmuchun@...edance.com> wrote:
> 
> > If the size of "struct page" is not the power of two but with the feature
> > of minimizing overhead of struct page associated with each HugeTLB is
> > enabled, then the vmemmap pages of HugeTLB will be corrupted after
> > remapping (panic is about to happen in theory).  But this only exists when
> > !CONFIG_MEMCG && !CONFIG_SLUB on x86_64.  However, it is not a conventional
> > configuration nowadays.  So it is not a real word issue, just the result
> > of a code review.
> 
> The patch does add a whole bunch of tricky junk to address something
> which won't happen.  How about we simply disable
> CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP if (!CONFIG_MEMCG &&
> !CONFIG_SLUB)?
>
 
I'm afraid not. The size of 'struct page' also depends on
LAST_CPUPID_NOT_IN_PAGE_FLAGS which could be defined
when CONFIG_NODES_SHIFT or CONFIG_KASAN_SW_TAGS
or CONFIG_NR_CPUS is configured with a large value.  Then
the size would be more than 64 bytes.

Seems like the approach [1] is more simple and feasible,
which also could prevent the users from doing unexpected
configurations, however, it is objected by Masahiro.
Shall we look back at the approach again?

Thanks.