linux-kernel - Re: [PATCH 1/7] mm/hugetlb: Handle FOLL_DUMP well in follow_page

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Date:   Fri, 16 Jun 2023 10:08:26 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Peter Xu <peterx@...hat.com>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Cc:     Matthew Wilcox <willy@...radead.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        John Hubbard <jhubbard@...dia.com>,
        Mike Rapoport <rppt@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        James Houghton <jthoughton@...gle.com>,
        Hugh Dickins <hughd@...gle.com>
Subject: Re: [PATCH 1/7] mm/hugetlb: Handle FOLL_DUMP well in
 follow_page_mask()

On 13.06.23 23:53, Peter Xu wrote:
> Firstly, the no_page_table() is meaningless for hugetlb which is a no-op
> there, because a hugetlb page always satisfies:
> 
>    - vma_is_anonymous() == false
>    - vma->vm_ops->fault != NULL
> 
> So we can already safely remove it in hugetlb_follow_page_mask(), alongside
> with the page* variable.
> 
> Meanwhile, what we do in follow_hugetlb_page() actually makes sense for a
> dump: we try to fault in the page only if the page cache is already
> allocated.  Let's do the same here for follow_page_mask() on hugetlb.
> 
> It should so far has zero effect on real dumps, because that still goes
> into follow_hugetlb_page().  But this may start to influence a bit on
> follow_page() users who mimics a "dump page" scenario, but hopefully in a
> good way.  This also paves way for unifying the hugetlb gup-slow.
> 
> Signed-off-by: Peter Xu <peterx@...hat.com>
> ---
>   mm/gup.c     | 9 ++-------
>   mm/hugetlb.c | 9 +++++++++
>   2 files changed, 11 insertions(+), 7 deletions(-)
> 
> diff --git a/mm/gup.c b/mm/gup.c
> index dbe96d266670..aa0668505d61 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -781,7 +781,6 @@ static struct page *follow_page_mask(struct vm_area_struct *vma,
>   			      struct follow_page_context *ctx)
>   {
>   	pgd_t *pgd;
> -	struct page *page;
>   	struct mm_struct *mm = vma->vm_mm;
>   
>   	ctx->page_mask = 0;
> @@ -794,12 +793,8 @@ static struct page *follow_page_mask(struct vm_area_struct *vma,
>   	 * hugetlb_follow_page_mask is only for follow_page() handling here.
>   	 * Ordinary GUP uses follow_hugetlb_page for hugetlb processing.
>   	 */
> -	if (is_vm_hugetlb_page(vma)) {
> -		page = hugetlb_follow_page_mask(vma, address, flags);
> -		if (!page)
> -			page = no_page_table(vma, flags);
> -		return page;
> -	}
> +	if (is_vm_hugetlb_page(vma))
> +		return hugetlb_follow_page_mask(vma, address, flags);
>   
>   	pgd = pgd_offset(mm, address);
>   
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 270ec0ecd5a1..82dfdd96db4c 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -6501,6 +6501,15 @@ struct page *hugetlb_follow_page_mask(struct vm_area_struct *vma,
>   	spin_unlock(ptl);
>   out_unlock:
>   	hugetlb_vma_unlock_read(vma);
> +
> +	/*
> +	 * Fixup retval for dump requests: if pagecache doesn't exist,
> +	 * don't try to allocate a new page but just skip it.
> +	 */
> +	if (!page && (flags & FOLL_DUMP) &&
> +	    !hugetlbfs_pagecache_present(h, vma, address))
> +		page = ERR_PTR(-EFAULT);
> +
>   	return page;
>   }
>   

Makes sense to me:

Reviewed-by: David Hildenbrand <david@...hat.com>

-- 
Cheers,

David / dhildenb