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Message-ID: <1b03a7de-1278-4e36-8068-885dd1c29742@linux.dev>
Date: Thu, 5 Sep 2024 17:25:44 +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 09/14] mm: mremap: move_ptes() use
pte_offset_map_rw_nolock()
On 2024/9/4 16:40, Qi Zheng wrote:
> In move_ptes(), we may modify the new_pte after acquiring the new_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 do a pmd_same() check after holding the PTL.
retract_page_tables() and move_ptes() are synchronized with
i_mmap_lock, right?
Muchun,
Thanks.
>
> Signed-off-by: Qi Zheng <zhengqi.arch@...edance.com>
> ---
> mm/mremap.c | 20 ++++++++++++++++++--
> 1 file changed, 18 insertions(+), 2 deletions(-)
>
> diff --git a/mm/mremap.c b/mm/mremap.c
> index 24712f8dbb6b5..16e54151395ad 100644
> --- a/mm/mremap.c
> +++ b/mm/mremap.c
> @@ -143,6 +143,7 @@ static int move_ptes(struct vm_area_struct *vma, pmd_t *old_pmd,
> spinlock_t *old_ptl, *new_ptl;
> bool force_flush = false;
> unsigned long len = old_end - old_addr;
> + pmd_t pmdval;
> int err = 0;
>
> /*
> @@ -175,14 +176,29 @@ static int move_ptes(struct vm_area_struct *vma, pmd_t *old_pmd,
> err = -EAGAIN;
> goto out;
> }
> - new_pte = pte_offset_map_nolock(mm, new_pmd, new_addr, &new_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.
> + */
> + new_pte = pte_offset_map_rw_nolock(mm, new_pmd, new_addr, &pmdval,
> + &new_ptl);
> if (!new_pte) {
> pte_unmap_unlock(old_pte, old_ptl);
> err = -EAGAIN;
> goto out;
> }
> - if (new_ptl != old_ptl)
> + if (new_ptl != old_ptl) {
> spin_lock_nested(new_ptl, SINGLE_DEPTH_NESTING);
> +
> + if (unlikely(!pmd_same(pmdval, pmdp_get_lockless(new_pmd)))) {
> + pte_unmap_unlock(new_pte, new_ptl);
> + pte_unmap_unlock(old_pte, old_ptl);
> + err = -EAGAIN;
> + goto out;
> + }
> + }
> +
> flush_tlb_batched_pending(vma->vm_mm);
> arch_enter_lazy_mmu_mode();
>
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