linux-kernel - Re: [PATCH v2 08/14] mm: copy_pte_range() use pte_offset_map_rw

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Message-ID: <4101a941-6286-4128-a16c-29c7cffcbe8c@redhat.com>
Date: Thu, 29 Aug 2024 17:36:07 +0200
From: David Hildenbrand <david@...hat.com>
To: Qi Zheng <zhengqi.arch@...edance.com>, hughd@...gle.com,
 willy@...radead.org, muchun.song@...ux.dev, vbabka@...nel.org,
 akpm@...ux-foundation.org, rppt@...nel.org, vishal.moola@...il.com,
 peterx@...hat.com, ryan.roberts@....com, christophe.leroy2@...soprasteria.com
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 linux-arm-kernel@...ts.infradead.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v2 08/14] mm: copy_pte_range() use
 pte_offset_map_rw_nolock()

On 22.08.24 09:13, Qi Zheng wrote:
> In copy_pte_range(), we may modify the src_pte entry after holding the
> src_ptl, so convert it to using pte_offset_map_rw_nolock(). But since we
> already hold the write lock of mmap_lock, there is no need to get pmdval
> to do pmd_same() check, just pass a dummy variable to it.
> 
> Signed-off-by: Qi Zheng <zhengqi.arch@...edance.com>
> ---
>   mm/memory.c | 11 ++++++++++-
>   1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index 7b6071a0e21e2..30d98025b2a40 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -1083,6 +1083,7 @@ copy_pte_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma,
>   	struct mm_struct *src_mm = src_vma->vm_mm;
>   	pte_t *orig_src_pte, *orig_dst_pte;
>   	pte_t *src_pte, *dst_pte;
> +	pmd_t dummy_pmdval;
>   	pte_t ptent;
>   	spinlock_t *src_ptl, *dst_ptl;
>   	int progress, max_nr, ret = 0;
> @@ -1108,7 +1109,15 @@ copy_pte_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma,
>   		ret = -ENOMEM;
>   		goto out;
>   	}
> -	src_pte = pte_offset_map_nolock(src_mm, src_pmd, addr, &src_ptl);
> +
> +	/*
> +	 * Use the maywrite version to indicate that dst_pte will be modified,
> +	 * but since we already hold the write lock of mmap_lock, there is no
> +	 * need to get pmdval to do pmd_same() check, just pass a dummy variable
> +	 * to it.

As we hold the mmap lock write lock, I assume it will prevent any page 
table removal, because they need *at least* the mmap lock in read mode, 
right?

We should probably document the rules for removing a page table -- which 
locks must be held in which mode (if not already done).

-- 
Cheers,

David / dhildenb