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Message-ID: <03ebc281-c8e1-461b-a716-be6162947d31@bytedance.com>
Date: Thu, 5 Sep 2024 18:55:11 +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 08/14] mm: copy_pte_range() use
pte_offset_map_rw_nolock()
On 2024/9/5 16:57, Muchun Song wrote:
>
>
> 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().
Indeed, will change it in v4. Thanks!
>
> 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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