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Message-ID: <7b7b80b5-ef13-314c-d739-8a75223f08a4@linux.alibaba.com>
Date: Mon, 5 Sep 2022 17:03:08 +0800
From: Baolin Wang <baolin.wang@...ux.alibaba.com>
To: John Hubbard <jhubbard@...dia.com>, Yang Shi <shy828301@...il.com>,
david@...hat.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 9/5/2022 6:29 AM, 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().
>
> That's a rather vague statement, and in fact, the READ_ONCE() should
> be paired with a page table write somewhere else, to make that claim
> more precise.
Agree. We also talked about using READ_ONCE() for *ptep (or using
ptep_get_lockless()) before in a concurrent scenario of GUP-fast and
migration [1].
>>> CPU 0: CPU 1:
>>> get_user_pages_fast()
>>> lockless_pages_from_mm()
>>> local_irq_save()
>>> gup_pgd_range()
>>> gup_p4d_range()
>>> gup_pud_range()
>>> gup_pmd_range()
>>> gup_pte_range()
>>> pte_t pte = ptep_get_lockless(ptep);
>>> migrate_vma_collect_pmd()
>>> ptep = pte_offset_map_lock(mm, pmdp, addr, &ptl)
>>> ptep_get_and_clear(mm, addr, ptep);
>>> page = pte_page(pte);
>>> set_pte_at(mm, addr, ptep, swp_pte);
>>> migrate_page_move_mapping()
>>> head = try_grab_compound_head(page, 1, flags);
[1]
https://lore.kernel.org/all/7ec1d098-0021-ae82-8d73-dd9c2eb80dab@linux.alibaba.com/
>> gup_put_folio(folio, 1, flags);
>> goto pte_unmap;
>> }
>> @@ -2470,8 +2487,9 @@ static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end,
>> * get_user_pages_fast_only implementation that can pin pages. Thus it's still
>> * useful to have gup_huge_pmd even if we can't operate on ptes.
>> */
>> -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)
>> {
>> return 0;
>> }
>> @@ -2791,7 +2809,7 @@ static int gup_pmd_range(pud_t *pudp, pud_t pud, unsigned long addr, unsigned lo
>> if (!gup_huge_pd(__hugepd(pmd_val(pmd)), addr,
>> PMD_SHIFT, next, flags, pages, nr))
>> return 0;
>> - } else if (!gup_pte_range(pmd, addr, next, flags, pages, nr))
>> + } else if (!gup_pte_range(pmd, pmdp, addr, next, flags, pages, nr))
>> return 0;
>> } while (pmdp++, addr = next, addr != end);
>>
>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>> index 2d74cf01f694..518b49095db3 100644
>> --- a/mm/khugepaged.c
>> +++ b/mm/khugepaged.c
>> @@ -1049,10 +1049,12 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
>>
>> pmd_ptl = pmd_lock(mm, pmd); /* probably unnecessary */
>> /*
>> - * After this gup_fast can't run anymore. This also removes
>> - * any huge TLB entry from the CPU so we won't allow
>> - * huge and small TLB entries for the same virtual address
>> - * to avoid the risk of CPU bugs in that area.
>> + * This removes any huge TLB entry from the CPU so we won't allow
>> + * huge and small TLB entries for the same virtual address to
>> + * avoid the risk of CPU bugs in that area.
>> + *
>> + * Parallel fast GUP is fine since fast GUP will back off when
>> + * it detects PMD is changed.
>> */
>> _pmd = pmdp_collapse_flush(vma, address, pmd);
>
> To follow up on David Hildenbrand's note about this in the nearby thread...
> I'm also not sure if pmdp_collapse_flush() implies a memory barrier on
> all arches. It definitely does do an atomic op with a return value on x86,
> but that's just one arch.
>
>
> thanks,
>
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