linux-kernel - Re: [PATCH] mm: khugepaged: fix potential page state corruption

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Date:   Wed, 25 Mar 2020 11:42:05 -0700
From:   Yang Shi <yang.shi@...ux.alibaba.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     kirill.shutemov@...ux.intel.com, hughd@...gle.com,
        aarcange@...hat.com, akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: khugepaged: fix potential page state corruption



On 3/25/20 4:26 AM, Kirill A. Shutemov wrote:
> On Tue, Mar 24, 2020 at 10:17:13AM -0700, Yang Shi wrote:
>>
>> On 3/19/20 3:49 AM, Kirill A. Shutemov wrote:
>>> On Wed, Mar 18, 2020 at 10:39:21PM -0700, Yang Shi wrote:
>>>> On 3/18/20 5:55 PM, Yang Shi wrote:
>>>>> On 3/18/20 5:12 PM, Kirill A. Shutemov wrote:
>>>>>> On Thu, Mar 19, 2020 at 07:19:42AM +0800, Yang Shi wrote:
>>>>>>> When khugepaged collapses anonymous pages, the base pages would
>>>>>>> be freed
>>>>>>> via pagevec or free_page_and_swap_cache().  But, the anonymous page may
>>>>>>> be added back to LRU, then it might result in the below race:
>>>>>>>
>>>>>>>       CPU A                CPU B
>>>>>>> khugepaged:
>>>>>>>      unlock page
>>>>>>>      putback_lru_page
>>>>>>>        add to lru
>>>>>>>                   page reclaim:
>>>>>>>                     isolate this page
>>>>>>>                     try_to_unmap
>>>>>>>      page_remove_rmap <-- corrupt _mapcount
>>>>>>>
>>>>>>> It looks nothing would prevent the pages from isolating by reclaimer.
>>>>>> Hm. Why should it?
>>>>>>
>>>>>> try_to_unmap() doesn't exclude parallel page unmapping. _mapcount is
>>>>>> protected by ptl. And this particular _mapcount pin is reachable for
>>>>>> reclaim as it's not part of usual page table tree. Basically
>>>>>> try_to_unmap() will never succeeds until we give up the _mapcount on
>>>>>> khugepaged side.
>>>>> I don't quite get. What does "not part of usual page table tree" means?
>>>>>
>>>>> How's about try_to_unmap() acquires ptl before khugepaged?
>>> The page table we are dealing with was detached from the process' page
>>> table tree: see pmdp_collapse_flush(). try_to_unmap() will not see the
>>> pte.
>> A follow-up question here. pmdp_collapse_flush() clears pmd entry and does
>> TLB shootdown on x86. I'm supposed the main purpose is to serialize fast gup
>> since it doesn't acquire any lock (mmap_sem, ptl ,etc), but disable
>> interrupt so the TLB shootdown IPI would get blocked. This could guarantee
>> synchronization on x86, but it looks not all architectures do TLB shootdown
>> or implement it via IPI, so how they could serialize with fast gup?
> The main purpose of pmdp_collapse_flush() is to block access to pages
> under collapse, including access via GUP (and its variants).
>
> It's up to architecture to implement it correctly, including TLB flush vs.
> GUP_fast serialization. Genetic way works fine for most architectures.
> Notable exceptions are Power and S390.

Thanks. I was wondering how Power and S390 serialized it. It looks they 
didn't deal with it at all.

>> In addition it looks acquiring pmd lock is not necessary. Before both write
>> mmap_sem and write anon_vma lock are acquired which could serialize page
>> fault and rmap walk, so it looks fast gup is the only one which could run
>> concurrently, but fast gup doesn't acquire ptl at all. It seems the
>> pmd_lock/unlock could be removed.
> This is likely true. And we have a comment there. But taking uncontended
> lock is check, so why not.

Yes, it sounds not harmful.

>