linux-kernel - Re: [PATCH v2 06/14] mm: handle_pte_fault() use pte_offset_map_rw

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Message-Id: <6F8142C5-9579-4F5A-B654-62E1E3183F0E@linux.dev>
Date: Thu, 29 Aug 2024 15:30:26 +0800
From: Muchun Song <muchun.song@...ux.dev>
To: Qi Zheng <zhengqi.arch@...edance.com>
Cc: David Hildenbrand <david@...hat.com>,
 Hugh Dickins <hughd@...gle.com>,
 Matthew Wilcox <willy@...radead.org>,
 "Vlastimil Babka (SUSE)" <vbabka@...nel.org>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Mike Rapoport <rppt@...nel.org>,
 vishal.moola@...il.com,
 peterx@...hat.com,
 ryan.roberts@....com,
 christophe.leroy2@...soprasteria.com,
 linux-kernel@...r.kernel.org,
 linux-mm@...ck.org,
 linux-arm-kernel@...ts.infradead.org,
 linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v2 06/14] mm: handle_pte_fault() use
 pte_offset_map_rw_nolock()



> On Aug 22, 2024, at 15:13, Qi Zheng <zhengqi.arch@...edance.com> wrote:
> 
> In handle_pte_fault(), we may modify the vmf->pte after acquiring the
> vmf->ptl, so convert it to using pte_offset_map_rw_nolock(). But since we
> will do the pte_same() check, so 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>

Reviewed-by: Muchun Song <muchun.song@...ux.dev>

A nit below.

> ---
> mm/memory.c | 12 ++++++++++--
> 1 file changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index 93c0c25433d02..7b6071a0e21e2 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -5499,14 +5499,22 @@ static vm_fault_t handle_pte_fault(struct vm_fault *vmf)
> 		vmf->pte = NULL;
> 		vmf->flags &= ~FAULT_FLAG_ORIG_PTE_VALID;
> 	} else {
> + 		pmd_t dummy_pmdval;
> +
> 	/*
> 	 * A regular pmd is established and it can't morph into a huge
> 	 * pmd by anon khugepaged, since that takes mmap_lock in write
> 	 * mode; but shmem or file collapse to THP could still morph
> 	 * it into a huge pmd: just retry later if so.
> +	 *
> +	 * Use the maywrite version to indicate that vmf->pte will be

Not "will be", should be "may be".

> +	 * modified, but since we will use pte_same() to detect the
> +	 * change of the pte entry, there is no need to get pmdval, so
> +	 * just pass a dummy variable to it.
> 	 */
> - 	vmf->pte = pte_offset_map_nolock(vmf->vma->vm_mm, vmf->pmd,
> -					 vmf->address, &vmf->ptl);
> +	vmf->pte = pte_offset_map_rw_nolock(vmf->vma->vm_mm, vmf->pmd,
> +					    vmf->address, &dummy_pmdval,
> +					    &vmf->ptl);
> 	if (unlikely(!vmf->pte))
> 		return 0;
> 	vmf->orig_pte = ptep_get_lockless(vmf->pte);
> -- 
> 2.20.1
>