linux-kernel - Re: [PATCH 4/4] mm/gup: remove get_user_pages

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Date:   Mon, 31 Jan 2022 09:36:12 -0400
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Peter Xu <peterx@...hat.com>, Jan Kara <jack@...e.cz>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org
Subject: Re: [PATCH 4/4] mm/gup: remove get_user_pages_locked()

On Sun, Jan 30, 2022 at 09:17:52PM -0800, John Hubbard wrote:

> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index 028e8dd82b44..040d88354cfa 100644
> +++ b/mm/mempolicy.c
> @@ -907,17 +907,15 @@ static void get_policy_nodemask(struct mempolicy *p, nodemask_t *nodes)
>  static int lookup_node(struct mm_struct *mm, unsigned long addr)
>  {
>  	struct page *p = NULL;
> -	int err;
> +	int ret;
>  
> -	int locked = 1;
> -	err = get_user_pages_locked(addr & PAGE_MASK, 1, 0, &p, &locked);
> -	if (err > 0) {
> -		err = page_to_nid(p);
> +	mmap_assert_locked(mm);
> +	ret = get_user_pages(addr & PAGE_MASK, 1, 0, &p, NULL);
> +	if (ret > 0) {
> +		ret = page_to_nid(p);
>  		put_page(p);
>  	}
> -	if (locked)
> -		mmap_read_unlock(mm);
> -	return err;
> +	return ret;
>  }
>  
>  /* Retrieve NUMA policy */
> @@ -968,15 +966,15 @@ static long do_get_mempolicy(int *policy, nodemask_t *nmask,
>  	if (flags & MPOL_F_NODE) {
>  		if (flags & MPOL_F_ADDR) {
>  			/*
> -			 * Take a refcount on the mpol, lookup_node()
> -			 * will drop the mmap_lock, so after calling
> -			 * lookup_node() only "pol" remains valid, "vma"
> -			 * is stale.
> +			 * Take a refcount on the mpol, because we are about to
> +			 * drop the mmap_lock, after which only "pol" remains
> +			 * valid, "vma" is stale.
>  			 */
>  			pol_refcount = pol;
>  			vma = NULL;
>  			mpol_get(pol);
>  			err = lookup_node(mm, addr);
> +			mmap_read_unlock(mm);

How about move the mmap_read_unlock up one line and then use
get_user_pages_fast()

I'm guessing in most cases here the PTE will be present so that should
be a net win?

Jason