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Subject: Re: [PATCH 3/4] mm: hugetlb_vmemmap: move PageVmemmapSelfHosted()
 check to split_vmemmap_huge_pmd()
From:   Muchun Song <muchun.song@linux.dev>
In-Reply-To: <20231204232728.GD4129@monkey>
Date:   Tue, 5 Dec 2023 10:46:52 +0800
Cc:     Muchun Song <songmuchun@bytedance.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linux-MM <linux-mm@kvack.org>, linux-kernel@vger.kernel.org
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Message-Id: <D2B8146D-0A41-4722-9E25-9D3F818C86C9@linux.dev>
References: <20231127084645.27017-1-songmuchun@bytedance.com>
 <20231127084645.27017-4-songmuchun@bytedance.com>
 <20231204232728.GD4129@monkey>
To:     Mike Kravetz <mike.kravetz@oracle.com>
Precedence: bulk


> On Dec 5, 2023, at 07:27, Mike Kravetz <mike.kravetz@oracle.com> =
wrote:
>=20
> 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.
>>=20
>> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
>> ---
>> mm/hugetlb_vmemmap.c | 70 =
+++++++++++++++-----------------------------
>> 1 file changed, 24 insertions(+), 46 deletions(-)
>>=20
>> 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 =3D 0;
>> struct page *head;
>> struct vmemmap_remap_walk *vmemmap_walk =3D walk->private;
>>=20
>> @@ -104,9 +105,30 @@ static int vmemmap_pmd_entry(pmd_t *pmd, =
unsigned long addr,
>>=20
>> spin_lock(&init_mm.page_table_lock);
>> head =3D pmd_leaf(*pmd) ? pmd_page(*pmd) : NULL;
>=20
> 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).

>=20
> Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>

Thanks.

> --=20
> Mike Kravetz