linux-kernel - Re: [PATCH 4/6] mm: memory: use a folio in do_cow

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Message-ID: <ff007e3a-9951-bd6e-2780-e46e70321665@oracle.com>
Date:   Tue, 7 Nov 2023 10:27:24 -0800
From:   Sidhartha Kumar <sidhartha.kumar@...cle.com>
To:     Kefeng Wang <wangkefeng.wang@...wei.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Matthew Wilcox <willy@...radead.org>,
        David Hildenbrand <david@...hat.com>
Subject: Re: [PATCH 4/6] mm: memory: use a folio in do_cow_page()

On 11/7/23 5:52 AM, Kefeng Wang wrote:
> Use folio_prealloc() helper and convert to use a folio in
> do_cow_page(), which save five compound_head() calls.
> 
> Signed-off-by: Kefeng Wang <wangkefeng.wang@...wei.com>
> ---
>   mm/memory.c | 16 ++++++----------
>   1 file changed, 6 insertions(+), 10 deletions(-)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index 062136d25da3..5ecee3eac29d 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -4648,6 +4648,7 @@ static vm_fault_t do_read_fault(struct vm_fault *vmf)
>   static vm_fault_t do_cow_fault(struct vm_fault *vmf)
>   {
>   	struct vm_area_struct *vma = vmf->vma;
> +	struct folio *folio;
>   	vm_fault_t ret;
>   
>   	ret = vmf_can_call_fault(vmf);
> @@ -4656,16 +4657,11 @@ static vm_fault_t do_cow_fault(struct vm_fault *vmf)
>   	if (ret)
>   		return ret;
>   
> -	vmf->cow_page = alloc_page_vma(GFP_HIGHUSER_MOVABLE, vma, vmf->address);
> -	if (!vmf->cow_page)
> +	folio = folio_prealloc(vma->vm_mm, vma, vmf->address, false);
> +	if (!folio)
>   		return VM_FAULT_OOM;
>   
> -	if (mem_cgroup_charge(page_folio(vmf->cow_page), vma->vm_mm,
> -				GFP_KERNEL)) {
> -		put_page(vmf->cow_page);
> -		return VM_FAULT_OOM;
> -	}
> -	folio_throttle_swaprate(page_folio(vmf->cow_page), GFP_KERNEL);

Hi Kefeng,

I was wondering why this block of code is no longer needed after using a 
folio?

Thanks,
Sidhartha Kumar

> +	vmf->cow_page = &folio->page;
>   
>   	ret = __do_fault(vmf);
>   	if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE | VM_FAULT_RETRY)))
> @@ -4674,7 +4670,7 @@ static vm_fault_t do_cow_fault(struct vm_fault *vmf)
>   		return ret;
>   
>   	copy_user_highpage(vmf->cow_page, vmf->page, vmf->address, vma);
> -	__SetPageUptodate(vmf->cow_page);
> +	__folio_mark_uptodate(folio);
>   
>   	ret |= finish_fault(vmf);
>   	unlock_page(vmf->page);
> @@ -4683,7 +4679,7 @@ static vm_fault_t do_cow_fault(struct vm_fault *vmf)
>   		goto uncharge_out;
>   	return ret;
>   uncharge_out:
> -	put_page(vmf->cow_page);
> +	folio_put(folio);
>   	return ret;
>   }
>