linux-kernel - Re: [PATCH 3/4] mm: hugetlb_vmemmap: move PageVmemmapSelfHosted() check to split_vmemmap_huge

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Date:   Tue, 5 Dec 2023 10:46:52 +0800
From:   Muchun Song <muchun.song@...ux.dev>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     Muchun Song <songmuchun@...edance.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux-MM <linux-mm@...ck.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/4] mm: hugetlb_vmemmap: move PageVmemmapSelfHosted()
 check to split_vmemmap_huge_pmd()



> On Dec 5, 2023, at 07:27, Mike Kravetz <mike.kravetz@...cle.com> wrote:
> 
> On 11/27/23 16:46, Muchun Song wrote:
>> To check a page whether it is self-hosted needs to traverse the page table (e.g.
>> pmd_off_k()), however, we already have done this in the next calling of
>> vmemmap_remap_range().  Moving PageVmemmapSelfHosted() check to vmemmap_pmd_entry()
>> could simplify the code a bit.
>> 
>> Signed-off-by: Muchun Song <songmuchun@...edance.com>
>> ---
>> mm/hugetlb_vmemmap.c | 70 +++++++++++++++-----------------------------
>> 1 file changed, 24 insertions(+), 46 deletions(-)
>> 
>> diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
>> index ef14356855d13..ce920ca6c90ee 100644
>> --- a/mm/hugetlb_vmemmap.c
>> +++ b/mm/hugetlb_vmemmap.c
>> @@ -95,6 +95,7 @@ static int vmemmap_split_pmd(pmd_t *pmd, struct page *head, unsigned long start,
>> static int vmemmap_pmd_entry(pmd_t *pmd, unsigned long addr,
>>      unsigned long next, struct mm_walk *walk)
>> {
>> + int ret = 0;
>> struct page *head;
>> struct vmemmap_remap_walk *vmemmap_walk = walk->private;
>> 
>> @@ -104,9 +105,30 @@ static int vmemmap_pmd_entry(pmd_t *pmd, unsigned long addr,
>> 
>> spin_lock(&init_mm.page_table_lock);
>> head = pmd_leaf(*pmd) ? pmd_page(*pmd) : NULL;
> 
> I was going to ask why you dropped the:
> if (IS_ENABLED(CONFIG_MEMORY_HOTPLUG))
> However, without the extra page table reads it really is not necessary.

I thought the compiler is smart enough to find out this code
is unnecessary so that it will be dropped. But I am wrong,
I did a test showing the compiler does optimize the code
when CONFIG_MEMORY_HOTPLUG is disabled, but it is not
enough compared with adding the check of "IS_ENABLED(CONFIG_MEMORY_HOTPLUG)".
I will add the check back in a separate patch (I suspect it is easy
for Andrew to merge).

> 
> Reviewed-by: Mike Kravetz <mike.kravetz@...cle.com>

Thanks.

> -- 
> Mike Kravetz