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Message-ID: <d8e2ef66-c345-43f0-9989-f20f16df34d3@linux.dev>
Date: Thu, 5 Sep 2024 16:57:44 +0800
From: Muchun Song <muchun.song@...ux.dev>
To: Qi Zheng <zhengqi.arch@...edance.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
linux-arm-kernel@...ts.infradead.org, linuxppc-dev@...ts.ozlabs.org,
david@...hat.com, hughd@...gle.com, willy@...radead.org, vbabka@...nel.org,
akpm@...ux-foundation.org, rppt@...nel.org, vishal.moola@...il.com,
peterx@...hat.com, ryan.roberts@....com, christophe.leroy2@...soprasteria.com
Subject: Re: [PATCH v3 08/14] mm: copy_pte_range() use
pte_offset_map_rw_nolock()
On 2024/9/4 16:40, 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(). Since we may
> free the PTE page in retract_page_tables() without holding the read lock
> of mmap_lock, so we still need to get pmdval and do pmd_same() check after
> the ptl is held.
See commit 3db82b9374ca92, copy_pte_range and retract_page_tables
are using vma->anon_vma to be exclusive.
retract_page_tables() copy_page_range()
vma_interval_tree_foreach() if (!vma_needs_copy())
if (READ_ONCE(vma->anon_vma)) return 0;
continue; copy_pte_range()
So I think mmap write lock here is also used for keeping ->anon_vma stable.
And we do not need pmd_same().
Muchun,
Thanks.
>
> Signed-off-by: Qi Zheng <zhengqi.arch@...edance.com>
> ---
> Hi Muchun, since the code has changed, I dropped your Reviewed-by tag here.
>
> mm/memory.c | 18 +++++++++++++++++-
> 1 file changed, 17 insertions(+), 1 deletion(-)
>
> diff --git a/mm/memory.c b/mm/memory.c
> index 06674f94b7a4e..47974cc4bd7f2 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -1082,6 +1082,7 @@ 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 pmdval;
> pte_t ptent;
> spinlock_t *src_ptl, *dst_ptl;
> int progress, max_nr, ret = 0;
> @@ -1107,13 +1108,28 @@ 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);
> +
> + /*
> + * Since we may free the PTE page in retract_page_tables() without
> + * holding the read lock of mmap_lock, so we still need to do a
> + * pmd_same() check after holding the PTL.
> + */
> + src_pte = pte_offset_map_rw_nolock(src_mm, src_pmd, addr, &pmdval,
> + &src_ptl);
> if (!src_pte) {
> pte_unmap_unlock(dst_pte, dst_ptl);
> /* ret == 0 */
> goto out;
> }
> spin_lock_nested(src_ptl, SINGLE_DEPTH_NESTING);
> +
> + if (unlikely(!pmd_same(pmdval, pmdp_get_lockless(src_pmd)))) {
> + pte_unmap_unlock(src_pte, src_ptl);
> + pte_unmap_unlock(dst_pte, dst_ptl);
> + /* ret == 0 */
> + goto out;
> + }
> +
> orig_src_pte = src_pte;
> orig_dst_pte = dst_pte;
> arch_enter_lazy_mmu_mode();
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