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Message-ID: <b989a811-f764-4b3d-b536-be4e68c0638e@bytedance.com>
Date: Tue, 8 Oct 2024 15:58:00 +0800
From: Qi Zheng <zhengqi.arch@...edance.com>
To: David Hildenbrand <david@...hat.com>
Cc: Jann Horn <jannh@...gle.com>, akpm@...ux-foundation.org,
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
On 2024/10/8 15:52, David Hildenbrand wrote:
> On 08.10.24 05:53, Qi Zheng wrote:
>> 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?
>
> My thinking was that it is cheap and more future proof to check that we
> really still have a page table here. For example, what if collapse code
> is ever changed to replace the page table by the collapsed PMD?
Ah, make sense.
Acked-by: Qi Zheng <zhengqi.arch@...edance.com>
>
> So unless there is a good reason not to have this check here, I would
> keep it like that.
>
> Thanks!
>
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