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Date:   Tue, 19 Apr 2022 14:23:56 +0800
From:   Muchun Song <songmuchun@bytedance.com>
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     corbet@lwn.net, mike.kravetz@oracle.com, mcgrof@kernel.org,
        keescook@chromium.org, yzaikin@google.com, osalvador@suse.de,
        david@redhat.com, masahiroy@kernel.org, linux-doc@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        duanxiongchun@bytedance.com, smuchun@gmail.com
Subject: Re: [PATCH v8 1/4] mm: hugetlb_vmemmap: introduce
 CONFIG_HUGETLB_PAGE_HAS_OPTIMIZE_VMEMMAP
Message-ID: <Yl5VfI1h6nUd8s8q@FVFYT0MHHV2J.usts.net>
References: <20220413144748.84106-1-songmuchun@bytedance.com>
 <20220413144748.84106-2-songmuchun@bytedance.com>
 <20220413120804.3570dc230a958f4923e3f3c3@linux-foundation.org>
 <YleQiQW7gFTO7SMk@FVFYT0MHHV2J.usts.net>
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On Thu, Apr 14, 2022 at 11:10:01AM +0800, Muchun Song wrote:
> 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@bytedance.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,

Sorry, forgot to post the Link.

[1] https://lore.kernel.org/all/20220323125523.79254-2-songmuchun@bytedance.com/

> which also could prevent the users from doing unexpected
> configurations, however, it is objected by Masahiro.
> Shall we look back at the approach again?
>

Hi all,

Friendly ping.

I have implemented 3 approaches to address this issue.

  1) V8 has added a lot of tricky code.
  2) V5 has added a feadback from Kbuild to Kconfig, as Masahiro
     said, it is terrible.
  3) V1 [2] has added a check of is_power_of_2() into hugetlb_vmemmap.c.

Iterated and explored through 8 versions, v1 seems to be the easiest way
to address this.  I think reusing v1 may be the best choice now.
What do you think?

[2] https://lore.kernel.org/all/20220228071022.26143-2-songmuchun@bytedance.com/

Thanks.