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Message-ID: <cccc16ad-21fb-4c99-8c49-91ee15c202cc@bytedance.com>
Date: Thu, 5 Sep 2024 18:56:14 +0800
From: Qi Zheng <zhengqi.arch@...edance.com>
To: Muchun Song <muchun.song@...ux.dev>
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 2024/9/5 17:25, Muchun Song wrote:
>
>
> 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?
Right, will remove the pmd_same() check in v4. Thanks!
>
> 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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