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Message-ID: <1c114925-9206-42b1-b24b-bb123853a359@bytedance.com>
Date: Tue, 8 Oct 2024 11:53:23 +0800
From: Qi Zheng <zhengqi.arch@...edance.com>
To: Jann Horn <jannh@...gle.com>
Cc: akpm@...ux-foundation.org, david@...hat.com, linux-mm@...ck.org,
willy@...radead.org, hughd@...gle.com, lorenzo.stoakes@...cle.com,
joel@...lfernandes.org, linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH] mm/mremap: Fix move_normal_pmd/retract_page_tables race
Hi Jann,
On 2024/10/8 05:42, Jann Horn wrote:
[...]
>
> diff --git a/mm/mremap.c b/mm/mremap.c
> index 24712f8dbb6b..dda09e957a5d 100644
> --- a/mm/mremap.c
> +++ b/mm/mremap.c
> @@ -238,6 +238,7 @@ static bool move_normal_pmd(struct vm_area_struct *vma, unsigned long old_addr,
> {
> spinlock_t *old_ptl, *new_ptl;
> struct mm_struct *mm = vma->vm_mm;
> + bool res = false;
> pmd_t pmd;
>
> if (!arch_supports_page_table_move())
> @@ -277,19 +278,25 @@ static bool move_normal_pmd(struct vm_area_struct *vma, unsigned long old_addr,
> if (new_ptl != old_ptl)
> spin_lock_nested(new_ptl, SINGLE_DEPTH_NESTING);
>
> - /* Clear the pmd */
> pmd = *old_pmd;
> +
> + /* Racing with collapse? */
> + if (unlikely(!pmd_present(pmd) || pmd_leaf(pmd)))
Since we already hold the exclusive mmap lock, after a racing
with collapse occurs, the pmd entry cannot be refilled with
new content by page fault. So maybe we only need to recheck
pmd_none(pmd) here?
Thanks,
Qi
> + goto out_unlock;
> + /* Clear the pmd */
> pmd_clear(old_pmd);
> + res = true;
>
> VM_BUG_ON(!pmd_none(*new_pmd));
>
> pmd_populate(mm, new_pmd, pmd_pgtable(pmd));
> flush_tlb_range(vma, old_addr, old_addr + PMD_SIZE);
> +out_unlock:
> if (new_ptl != old_ptl)
> spin_unlock(new_ptl);
> spin_unlock(old_ptl);
>
> - return true;
> + return res;
> }
> #else
> static inline bool move_normal_pmd(struct vm_area_struct *vma,
>
> ---
> base-commit: 8cf0b93919e13d1e8d4466eb4080a4c4d9d66d7b
> change-id: 20241007-move_normal_pmd-vs-collapse-fix-2-387e9a68c7d6
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