linux-kernel - Re: [PATCH v1 2/3] mm/gup: use gup_can_follow

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Message-ID: <1892f6de-fd22-0e8b-3ff6-4c8641e1c68e@redhat.com>
Date:   Fri, 26 Aug 2022 16:59:05 +0200
From:   David Hildenbrand <david@...hat.com>
To:     linux-kernel@...r.kernel.org
Cc:     linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...e.de>, Jason Gunthorpe <jgg@...dia.com>,
        John Hubbard <jhubbard@...dia.com>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Hugh Dickins <hughd@...gle.com>, Peter Xu <peterx@...hat.com>
Subject: Re: [PATCH v1 2/3] mm/gup: use gup_can_follow_protnone() also in
 GUP-fast

On 25.08.22 18:46, David Hildenbrand wrote:
> There seems to be no reason why FOLL_FORCE during GUP-fast would have to
> fallback to the slow path when stumbling over a PROT_NONE mapped page. We
> only have to trigger hinting faults in case FOLL_FORCE is not set, and any
> kind of fault handling naturally happens from the slow path -- where
> NUMA hinting accounting/handling would be performed.
> 
> Note that the comment regarding THP migration is outdated:
> commit 2b4847e73004 ("mm: numa: serialise parallel get_user_page against
> THP migration") described that this was required for THP due to lack of PMD
> migration entries. Nowadays, we do have proper PMD migration entries in
> place -- see set_pmd_migration_entry(), which does a proper
> pmdp_invalidate() when placing the migration entry.
> 
> So let's just reuse gup_can_follow_protnone() here to make it
> consistent and drop the somewhat outdated comments.
> 
> Signed-off-by: David Hildenbrand <david@...hat.com>
> ---
>  mm/gup.c | 14 +++-----------
>  1 file changed, 3 insertions(+), 11 deletions(-)
> 
> diff --git a/mm/gup.c b/mm/gup.c
> index a1355dbd848e..dfef23071dc8 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -2350,11 +2350,7 @@ static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end,
>  		struct page *page;
>  		struct folio *folio;
>  
> -		/*
> -		 * Similar to the PMD case below, NUMA hinting must take slow
> -		 * path using the pte_protnone check.
> -		 */
> -		if (pte_protnone(pte))
> +		if (pte_protnone(pte) && !gup_can_follow_protnone(flags))
>  			goto pte_unmap;
>  
>  		if (!pte_access_permitted(pte, flags & FOLL_WRITE))
> @@ -2736,12 +2732,8 @@ static int gup_pmd_range(pud_t *pudp, pud_t pud, unsigned long addr, unsigned lo
>  
>  		if (unlikely(pmd_trans_huge(pmd) || pmd_huge(pmd) ||
>  			     pmd_devmap(pmd))) {
> -			/*
> -			 * NUMA hinting faults need to be handled in the GUP
> -			 * slowpath for accounting purposes and so that they
> -			 * can be serialised against THP migration.
> -			 */
> -			if (pmd_protnone(pmd))
> +			if (pmd_protnone(pmd) &&
> +			    !gup_can_follow_protnone(flags))
>  				return 0;
>  
>  			if (!gup_huge_pmd(pmd, pmdp, addr, next, flags,


I just stumbled over something interesting. If we have a pte_protnone()
entry, ptep_clear_flush() might not flush, because the !pte_accessible()
 does not hold.

Consequently, we could be in trouble when using ptep_clear_flush() on a
pte_protnone() PTE to make sure that GUP cannot run anymore.

Will give this a better thought, but most probably I'll replace this
patch by a proper documentation update here.

-- 
Thanks,

David / dhildenb