linux-kernel - Re: [PATCH 09/12] hugetlb_vmemmap: Optimistically set Optimized flag

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Message-ID: <8e298c9f-1ef3-5c99-d7b5-47fd6703cf83@linux.dev>
Date:   Wed, 30 Aug 2023 15:26:11 +0800
From:   Muchun Song <muchun.song@...ux.dev>
To:     Mike Kravetz <mike.kravetz@...cle.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Cc:     Muchun Song <songmuchun@...edance.com>,
        Joao Martins <joao.m.martins@...cle.com>,
        Oscar Salvador <osalvador@...e.de>,
        David Hildenbrand <david@...hat.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        David Rientjes <rientjes@...gle.com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Naoya Horiguchi <naoya.horiguchi@...ux.dev>,
        Barry Song <song.bao.hua@...ilicon.com>,
        Michal Hocko <mhocko@...e.com>,
        Matthew Wilcox <willy@...radead.org>,
        Xiongchun Duan <duanxiongchun@...edance.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 09/12] hugetlb_vmemmap: Optimistically set Optimized flag



On 2023/8/26 03:04, Mike Kravetz wrote:
> At the beginning of hugetlb_vmemmap_optimize, optimistically set
> the HPageVmemmapOptimized flag in the head page.  Clear the flag
> if the operation fails.
>
> No change in behavior.  However, this will become important in
> subsequent patches where we batch delay TLB flushing.  We need to
> make sure the content in the old and new vmemmap pages are the same.

Sorry, I didn't get the point here. Could you elaborate it?

>
> Signed-off-by: Mike Kravetz <mike.kravetz@...cle.com>
> ---
>   mm/hugetlb_vmemmap.c | 8 +++++---
>   1 file changed, 5 insertions(+), 3 deletions(-)
>
> diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
> index e390170c0887..500a118915ff 100644
> --- a/mm/hugetlb_vmemmap.c
> +++ b/mm/hugetlb_vmemmap.c
> @@ -566,7 +566,9 @@ static void __hugetlb_vmemmap_optimize(const struct hstate *h,
>   	if (!vmemmap_should_optimize(h, head))
>   		return;
>   
> +	/* Optimistically assume success */
>   	static_branch_inc(&hugetlb_optimize_vmemmap_key);
> +	SetHPageVmemmapOptimized(head);
>   
>   	vmemmap_end	= vmemmap_start + hugetlb_vmemmap_size(h);
>   	vmemmap_reuse	= vmemmap_start;
> @@ -577,10 +579,10 @@ static void __hugetlb_vmemmap_optimize(const struct hstate *h,
>   	 * to the page which @vmemmap_reuse is mapped to, then free the pages
>   	 * which the range [@vmemmap_start, @vmemmap_end] is mapped to.
>   	 */
> -	if (vmemmap_remap_free(vmemmap_start, vmemmap_end, vmemmap_reuse, bulk_pages))
> +	if (vmemmap_remap_free(vmemmap_start, vmemmap_end, vmemmap_reuse, bulk_pages)) {
>   		static_branch_dec(&hugetlb_optimize_vmemmap_key);
> -	else
> -		SetHPageVmemmapOptimized(head);
> +		ClearHPageVmemmapOptimized(head);
> +	}
>   }
>   
>   /**