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Date:   Thu, 12 May 2022 21:59:37 +0800
From:   Muchun Song <songmuchun@bytedance.com>
To:     David Hildenbrand <david@redhat.com>
Cc:     corbet@lwn.net, mike.kravetz@oracle.com, akpm@linux-foundation.org,
        mcgrof@kernel.org, keescook@chromium.org, yzaikin@google.com,
        osalvador@suse.de, 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 v10 2/4] mm: memory_hotplug: override memmap_on_memory
 when hugetlb_free_vmemmap=on
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On Thu, May 12, 2022 at 03:04:57PM +0200, David Hildenbrand wrote:
> On 12.05.22 14:50, Muchun Song wrote:
> > On Thu, May 12, 2022 at 09:36:15AM +0200, David Hildenbrand wrote:
> >> On 09.05.22 08:27, Muchun Song wrote:
> >>> Optimizing HugeTLB vmemmap pages is not compatible with allocating memmap on
> >>> hot added memory. If "hugetlb_free_vmemmap=on" and
> >>> memory_hotplug.memmap_on_memory" are both passed on the kernel command line,
> >>> optimizing hugetlb pages takes precedence. 
> >>
> >> Why?
> >>
> > 
> > Because both two features are not compatible since hugetlb_free_vmemmap cannot
> > optimize the vmemmap pages allocated from alternative allocator (when
> > memory_hotplug.memmap_on_memory=1). So when the feature of hugetlb_free_vmemmap
> > is introduced, I made hugetlb_free_vmemmap take precedence.  BTW, I have a plan
> > to remove this restriction, I'll post it out ASAP.
> 
> I was asking why vmemmap optimization should take precedence.
> memmap_on_memory makes it more likely to succeed memory hotplug in
> close-to-OOM situations -- which is IMHO more important than a vmemmap
> optimization.
>

I thought the users who enable hugetlb_free_vmemmap value memory
savings more, so I made a decision in commit 4bab4964a59f.  Seems
I made a bad decision from your description.
 
> But anyhow, the proper approach should most probably be to simply not
> mess with the vmemmap if we stumble over a vmemmap that's special due to
> memmap_on_memory. I assume that's what you're talking about sending out.
>

I mean I want to have hugetlb_vmemmap.c do the check whether the section
which the HugeTLB pages belong to can be optimized instead of making
hugetlb_free_vmemmap take precedence.  E.g. If the section's vmemmap pages
are allocated from the added memory block itself, hugetlb_free_vmemmap will
refuse to optimize the vmemmap, otherwise, do the optimization.  Then
both kernel parameters are compatible.  I have done those patches, but
haven't send them out.

Thanks.