linux-kernel - Re: [PATCH v1 2/9] mm/memory: handle !page case in zap_present

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Date: Tue, 30 Jan 2024 08:20:02 +0000
From: Ryan Roberts <ryan.roberts@....com>
To: David Hildenbrand <david@...hat.com>, linux-kernel@...r.kernel.org
Cc: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
 Matthew Wilcox <willy@...radead.org>,
 Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>,
 "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
 Nick Piggin <npiggin@...il.com>, Peter Zijlstra <peterz@...radead.org>,
 Michael Ellerman <mpe@...erman.id.au>,
 Christophe Leroy <christophe.leroy@...roup.eu>,
 "Naveen N. Rao" <naveen.n.rao@...ux.ibm.com>,
 Heiko Carstens <hca@...ux.ibm.com>, Vasily Gorbik <gor@...ux.ibm.com>,
 Alexander Gordeev <agordeev@...ux.ibm.com>,
 Christian Borntraeger <borntraeger@...ux.ibm.com>,
 Sven Schnelle <svens@...ux.ibm.com>, Arnd Bergmann <arnd@...db.de>,
 linux-arch@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
 linux-s390@...r.kernel.org
Subject: Re: [PATCH v1 2/9] mm/memory: handle !page case in zap_present_pte()
 separately

On 29/01/2024 14:32, David Hildenbrand wrote:
> We don't need uptodate accessed/dirty bits, so in theory we could
> replace ptep_get_and_clear_full() by an optimized ptep_clear_full()
> function. Let's rely on the provided pte.
> 
> Further, there is no scenario where we would have to insert uffd-wp
> markers when zapping something that is not a normal page (i.e., zeropage).
> Add a sanity check to make sure this remains true.
> 
> should_zap_folio() no longer has to handle NULL pointers. This change
> replaces 2/3 "!page/!folio" checks by a single "!page" one.
> 
> Signed-off-by: David Hildenbrand <david@...hat.com>

Reviewed-by: Ryan Roberts <ryan.roberts@....com>

> ---
>  mm/memory.c | 20 ++++++++++----------
>  1 file changed, 10 insertions(+), 10 deletions(-)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index 50a6c79c78fc..69502cdc0a7d 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -1497,10 +1497,6 @@ static inline bool should_zap_folio(struct zap_details *details,
>  	if (should_zap_cows(details))
>  		return true;
>  
> -	/* E.g. the caller passes NULL for the case of a zero folio */
> -	if (!folio)
> -		return true;
> -
>  	/* Otherwise we should only zap non-anon folios */
>  	return !folio_test_anon(folio);
>  }
> @@ -1543,19 +1539,23 @@ static inline void zap_present_pte(struct mmu_gather *tlb,
>  	struct page *page;
>  
>  	page = vm_normal_page(vma, addr, ptent);
> -	if (page)
> -		folio = page_folio(page);
> +	if (!page) {
> +		/* We don't need up-to-date accessed/dirty bits. */
> +		ptep_get_and_clear_full(mm, addr, pte, tlb->fullmm);
> +		arch_check_zapped_pte(vma, ptent);
> +		tlb_remove_tlb_entry(tlb, pte, addr);
> +		VM_WARN_ON_ONCE(userfaultfd_wp(vma));
> +		ksm_might_unmap_zero_page(mm, ptent);
> +		return;
> +	}
>  
> +	folio = page_folio(page);
>  	if (unlikely(!should_zap_folio(details, folio)))
>  		return;
>  	ptent = ptep_get_and_clear_full(mm, addr, pte, tlb->fullmm);
>  	arch_check_zapped_pte(vma, ptent);
>  	tlb_remove_tlb_entry(tlb, pte, addr);
>  	zap_install_uffd_wp_if_needed(vma, addr, pte, details, ptent);
> -	if (unlikely(!page)) {
> -		ksm_might_unmap_zero_page(mm, ptent);
> -		return;
> -	}
>  
>  	if (!folio_test_anon(folio)) {
>  		if (pte_dirty(ptent)) {