linux-kernel - Re: [PATCH v5 2/2] mm: memory_hotplug: make hugetlb_optimize_vmemmap compatible with memmap_on

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Message-ID: <YrCA9JIet7RulSPo@FVFYT0MHHV2J.usts.net>
Date:   Mon, 20 Jun 2022 22:15:16 +0800
From:   Muchun Song <songmuchun@...edance.com>
To:     Oscar Salvador <osalvador@...e.de>
Cc:     akpm@...ux-foundation.org, corbet@....net, david@...hat.com,
        mike.kravetz@...cle.com, paulmck@...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 v5 2/2] mm: memory_hotplug: make hugetlb_optimize_vmemmap
 compatible with memmap_on_memory

On Mon, Jun 20, 2022 at 03:19:33PM +0200, Oscar Salvador wrote:
> On Mon, Jun 20, 2022 at 07:06:16PM +0800, Muchun Song wrote:
> > +		/*
> > +		 * The READ_ONCE() is used to stabilize *pmdp in a register or
> > +		 * on the stack so that it will stop changing under the code.
> > +		 * The only concurrent operation where it can be changed is
> > +		 * split_vmemmap_huge_pmd() (*pmdp will be stable after this
> > +		 * operation).
> > +		 */
> > +		pmd = READ_ONCE(*pmdp);
> > +		if (pmd_leaf(pmd))
> > +			vmemmap_page = pmd_page(pmd) + pte_index(vaddr);
> > +		else
> > +			vmemmap_page = pte_page(*pte_offset_kernel(pmdp, vaddr));
> 
> I was about to suggest to get rid of the else branch because on x86_64
> we can only allocate PMD_SIZE chunks when using an alternative allocator,
> meaning that anything which is not a pmd_leaf can't be a PageVmemmapSelfHosted.
>

You are right. However, I think relaying on this condition is fragile and
not straightforward compared to the check of PageVmemmapSelfHosted(). And
the else branch is not in a hot path. So I'd like to stay with it. Does
this make sense for you?
 
> But then I went to check the other platform that supports memmap_on_memory (arm64),
> and in there I can see that we fallback to populate basepages with altmap should
> we fail to allocate a PMD_SIZE chunk.
> 

I think it cannot be fail for memmap_on_memory case.

Thanks.