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Date:   Mon, 17 Apr 2023 10:09:36 -0300
From:   Jason Gunthorpe <jgg@nvidia.com>
To:     Lorenzo Stoakes <lstoakes@gmail.com>
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Subject: Re: [PATCH 3/7] mm/gup: remove vmas parameter from
 get_user_pages_remote()
Message-ID: <ZD1FECftWekha6Do@nvidia.com>
References: <cover.1681508038.git.lstoakes@gmail.com>
 <5a4cf1ebf1c6cdfabbf2f5209facb0180dd20006.1681508038.git.lstoakes@gmail.com>
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On Sat, Apr 15, 2023 at 12:27:31AM +0100, Lorenzo Stoakes wrote:
> The only instances of get_user_pages_remote() invocations which used the
> vmas parameter were for a single page which can instead simply look up the
> VMA directly. In particular:-
> 
> - __update_ref_ctr() looked up the VMA but did nothing with it so we simply
>   remove it.
> 
> - __access_remote_vm() was already using vma_lookup() when the original
>   lookup failed so by doing the lookup directly this also de-duplicates the
>   code.
> 
> This forms part of a broader set of patches intended to eliminate the vmas
> parameter altogether.
> 
> Signed-off-by: Lorenzo Stoakes <lstoakes@gmail.com>
> ---
>  arch/arm64/kernel/mte.c   |  5 +++--
>  arch/s390/kvm/interrupt.c |  2 +-
>  fs/exec.c                 |  2 +-
>  include/linux/mm.h        |  2 +-
>  kernel/events/uprobes.c   | 10 +++++-----
>  mm/gup.c                  | 12 ++++--------
>  mm/memory.c               |  9 +++++----
>  mm/rmap.c                 |  2 +-
>  security/tomoyo/domain.c  |  2 +-
>  virt/kvm/async_pf.c       |  3 +--
>  10 files changed, 23 insertions(+), 26 deletions(-)
> 
> diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
> index f5bcb0dc6267..74d8d4007dec 100644
> --- a/arch/arm64/kernel/mte.c
> +++ b/arch/arm64/kernel/mte.c
> @@ -437,8 +437,9 @@ static int __access_remote_tags(struct mm_struct *mm, unsigned long addr,
>  		struct page *page = NULL;
>  
>  		ret = get_user_pages_remote(mm, addr, 1, gup_flags, &page,
> -					    &vma, NULL);
> -		if (ret <= 0)
> +					    NULL);
> +		vma = vma_lookup(mm, addr);
> +		if (ret <= 0 || !vma)
>  			break;

Given the slightly tricky error handling, it would make sense to turn
this pattern into a helper function:

page = get_single_user_page_locked(mm, addr, gup_flags, &vma);
if (IS_ERR(page))
  [..]

static inline struct page *get_single_user_page_locked(struct mm_struct *mm,
   unsigned long addr, int gup_flags, struct vm_area_struct **vma)
{
	struct page *page;
	int ret;

	ret = get_user_pages_remote(*mm, addr, 1, gup_flags, &page, NULL, NULL);
	if (ret < 0)
	   return ERR_PTR(ret);
	if (WARN_ON(ret == 0))
	   return ERR_PTR(-EINVAL);
        *vma = vma_lookup(mm, addr);
	if (WARN_ON(!*vma) {
	   put_user_page(page);
	   return ERR_PTR(-EINVAL);
        }
	return page;
}

It could be its own patch so this change was just a mechanical removal
of NULL

Jason