linux-kernel - Re: [PATCH v3 12/15] mm/memory: pass PTE to copy_present

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Date: Wed, 14 Feb 2024 23:40:22 +0100
From: David Hildenbrand <david@...hat.com>
To: linux-kernel@...r.kernel.org
Cc: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
 Matthew Wilcox <willy@...radead.org>, Ryan Roberts <ryan.roberts@....com>,
 Russell King <linux@...linux.org.uk>,
 Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>,
 Dinh Nguyen <dinguyen@...nel.org>, Michael Ellerman <mpe@...erman.id.au>,
 Nicholas Piggin <npiggin@...il.com>,
 Christophe Leroy <christophe.leroy@...roup.eu>,
 "Aneesh Kumar K.V" <aneesh.kumar@...nel.org>,
 "Naveen N. Rao" <naveen.n.rao@...ux.ibm.com>,
 Paul Walmsley <paul.walmsley@...ive.com>, Palmer Dabbelt
 <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>,
 Alexander Gordeev <agordeev@...ux.ibm.com>,
 Gerald Schaefer <gerald.schaefer@...ux.ibm.com>,
 Heiko Carstens <hca@...ux.ibm.com>, Vasily Gorbik <gor@...ux.ibm.com>,
 Christian Borntraeger <borntraeger@...ux.ibm.com>,
 Sven Schnelle <svens@...ux.ibm.com>, "David S. Miller"
 <davem@...emloft.net>, linux-arm-kernel@...ts.infradead.org,
 linuxppc-dev@...ts.ozlabs.org, linux-riscv@...ts.infradead.org,
 linux-s390@...r.kernel.org, sparclinux@...r.kernel.org
Subject: Re: [PATCH v3 12/15] mm/memory: pass PTE to copy_present_pte()

On 29.01.24 13:46, David Hildenbrand wrote:
> We already read it, let's just forward it.
> 
> This patch is based on work by Ryan Roberts.
> 
> Reviewed-by: Ryan Roberts <ryan.roberts@....com>
> Signed-off-by: David Hildenbrand <david@...hat.com>
> ---
>   mm/memory.c | 7 +++----
>   1 file changed, 3 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index a3bdb25f4c8d..41b24da5be38 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -959,10 +959,9 @@ static inline void __copy_present_pte(struct vm_area_struct *dst_vma,
>    */
>   static inline int
>   copy_present_pte(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma,
> -		 pte_t *dst_pte, pte_t *src_pte, unsigned long addr, int *rss,
> -		 struct folio **prealloc)
> +		 pte_t *dst_pte, pte_t *src_pte, pte_t pte, unsigned long addr,
> +		 int *rss, struct folio **prealloc)
>   {
> -	pte_t pte = ptep_get(src_pte);
>   	struct page *page;
>   	struct folio *folio;
>   
> @@ -1103,7 +1102,7 @@ copy_pte_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma,
>   		}
>   		/* copy_present_pte() will clear `*prealloc' if consumed */
>   		ret = copy_present_pte(dst_vma, src_vma, dst_pte, src_pte,
> -				       addr, rss, &prealloc);
> +				       ptent, addr, rss, &prealloc);
>   		/*
>   		 * If we need a pre-allocated page for this pte, drop the
>   		 * locks, allocate, and try again.

The following fixup for that device-exclusive thingy on top (fixing a hmm
selftest I just discovered to be broken).

 From 8f9e44f25087dc71890b8d9bd680375691232e85 Mon Sep 17 00:00:00 2001
From: David Hildenbrand <david@...hat.com>
Date: Wed, 14 Feb 2024 23:09:29 +0100
Subject: [PATCH] fixup: mm/memory: pass PTE to copy_present_pte()

For device-exclusive nonswp entries (is_device_exclusive_entry()),
copy_nonpresent_pte() can turn the PTEs into actual present PTEs while
holding the page table lock.

We hae to re-read the PTE after that operation, such that we won't be
working on the stale non-present PTE, assuming it would be present.

This fixes the hmm "exclusive_cow" selftest.

  ./run_vmtests.sh -t hmm
  # #  RUN           hmm.hmm_device_private.exclusive_cow ...
  # #            OK  hmm.hmm_device_private.exclusive_cow
  # ok 23 hmm.hmm_device_private.exclusive_cow

Signed-off-by: David Hildenbrand <david@...hat.com>
---
  mm/memory.c | 2 ++
  1 file changed, 2 insertions(+)

diff --git a/mm/memory.c b/mm/memory.c
index 3b8e56eb08a3..29a75f38df7c 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1208,6 +1208,8 @@ copy_pte_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma,
  				progress += 8;
  				continue;
  			}
+			ptent = ptep_get(src_pte);
+			VM_WARN_ON_ONCE(!pte_present(ptent));

  			/*
  			 * Device exclusive entry restored, continue by copying
-- 
2.43.0

-- 
Cheers,

David / dhildenb