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Message-ID: <a7dd4376-bce2-ee79-623f-fa11d301b80d@redhat.com>
Date: Mon, 5 Sep 2022 13:11:40 +0200
From: David Hildenbrand <david@...hat.com>
To: Baolin Wang <baolin.wang@...ux.alibaba.com>,
John Hubbard <jhubbard@...dia.com>,
Yang Shi <shy828301@...il.com>, peterx@...hat.com,
kirill.shutemov@...ux.intel.com, jgg@...dia.com, hughd@...gle.com,
akpm@...ux-foundation.org
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: gup: fix the fast GUP race against THP collapse
On 05.09.22 12:24, David Hildenbrand wrote:
> On 05.09.22 12:16, Baolin Wang wrote:
>>
>>
>> On 9/5/2022 3:59 PM, David Hildenbrand wrote:
>>> On 05.09.22 00:29, John Hubbard wrote:
>>>> On 9/1/22 15:27, Yang Shi wrote:
>>>>> Since general RCU GUP fast was introduced in commit 2667f50e8b81 ("mm:
>>>>> introduce a general RCU get_user_pages_fast()"), a TLB flush is no
>>>>> longer
>>>>> sufficient to handle concurrent GUP-fast in all cases, it only handles
>>>>> traditional IPI-based GUP-fast correctly. On architectures that send
>>>>> an IPI broadcast on TLB flush, it works as expected. But on the
>>>>> architectures that do not use IPI to broadcast TLB flush, it may have
>>>>> the below race:
>>>>>
>>>>> CPU A CPU B
>>>>> THP collapse fast GUP
>>>>> gup_pmd_range() <--
>>>>> see valid pmd
>>>>> gup_pte_range()
>>>>> <-- work on pte
>>>>> pmdp_collapse_flush() <-- clear pmd and flush
>>>>> __collapse_huge_page_isolate()
>>>>> check page pinned <-- before GUP bump refcount
>>>>> pin the page
>>>>> check PTE <--
>>>>> no change
>>>>> __collapse_huge_page_copy()
>>>>> copy data to huge page
>>>>> ptep_clear()
>>>>> install huge pmd for the huge page
>>>>> return the
>>>>> stale page
>>>>> discard the stale page
>>>>
>>>> Hi Yang,
>>>>
>>>> Thanks for taking the trouble to write down these notes. I always
>>>> forget which race we are dealing with, and this is a great help. :)
>>>>
>>>> More...
>>>>
>>>>>
>>>>> The race could be fixed by checking whether PMD is changed or not after
>>>>> taking the page pin in fast GUP, just like what it does for PTE. If the
>>>>> PMD is changed it means there may be parallel THP collapse, so GUP
>>>>> should back off.
>>>>>
>>>>> Also update the stale comment about serializing against fast GUP in
>>>>> khugepaged.
>>>>>
>>>>> Fixes: 2667f50e8b81 ("mm: introduce a general RCU
>>>>> get_user_pages_fast()")
>>>>> Signed-off-by: Yang Shi <shy828301@...il.com>
>>>>> ---
>>>>> mm/gup.c | 30 ++++++++++++++++++++++++------
>>>>> mm/khugepaged.c | 10 ++++++----
>>>>> 2 files changed, 30 insertions(+), 10 deletions(-)
>>>>>
>>>>> diff --git a/mm/gup.c b/mm/gup.c
>>>>> index f3fc1f08d90c..4365b2811269 100644
>>>>> --- a/mm/gup.c
>>>>> +++ b/mm/gup.c
>>>>> @@ -2380,8 +2380,9 @@ static void __maybe_unused undo_dev_pagemap(int
>>>>> *nr, int nr_start,
>>>>> }
>>>>> #ifdef CONFIG_ARCH_HAS_PTE_SPECIAL
>>>>> -static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned
>>>>> long end,
>>>>> - unsigned int flags, struct page **pages, int *nr)
>>>>> +static int gup_pte_range(pmd_t pmd, pmd_t *pmdp, unsigned long addr,
>>>>> + unsigned long end, unsigned int flags,
>>>>> + struct page **pages, int *nr)
>>>>> {
>>>>> struct dev_pagemap *pgmap = NULL;
>>>>> int nr_start = *nr, ret = 0;
>>>>> @@ -2423,7 +2424,23 @@ static int gup_pte_range(pmd_t pmd, unsigned
>>>>> long addr, unsigned long end,
>>>>> goto pte_unmap;
>>>>> }
>>>>> - if (unlikely(pte_val(pte) != pte_val(*ptep))) {
>>>>> + /*
>>>>> + * THP collapse conceptually does:
>>>>> + * 1. Clear and flush PMD
>>>>> + * 2. Check the base page refcount
>>>>> + * 3. Copy data to huge page
>>>>> + * 4. Clear PTE
>>>>> + * 5. Discard the base page
>>>>> + *
>>>>> + * So fast GUP may race with THP collapse then pin and
>>>>> + * return an old page since TLB flush is no longer sufficient
>>>>> + * to serialize against fast GUP.
>>>>> + *
>>>>> + * Check PMD, if it is changed just back off since it
>>>>> + * means there may be parallel THP collapse.
>>>>> + */
>>>>
>>>> As I mentioned in the other thread, it would be a nice touch to move
>>>> such discussion into the comment header.
>>>>
>>>>> + if (unlikely(pmd_val(pmd) != pmd_val(*pmdp)) ||
>>>>> + unlikely(pte_val(pte) != pte_val(*ptep))) {
>>>>
>>>>
>>>> That should be READ_ONCE() for the *pmdp and *ptep reads. Because this
>>>> whole lockless house of cards may fall apart if we try reading the
>>>> page table values without READ_ONCE().
>>>
>>> I came to the conclusion that the implicit memory barrier when grabbing
>>> a reference on the page is sufficient such that we don't need READ_ONCE
>>> here.
>>
>> IMHO the compiler may optimize the code 'pte_val(*ptep)' to be always
>> get from a register, then we can get an old value if other thread did
>> set_pte(). I am not sure how the implicit memory barrier can pervent the
>> compiler optimization? Please correct me if I missed something.
>
> IIUC, an memory barrier always implies a compiler barrier.
>
To clarify what I mean, Documentation/atomic_t.txt documents
NOTE: when the atomic RmW ops are fully ordered, they should also imply
a compiler barrier.
--
Thanks,
David / dhildenb
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