Messages in this thread Patch in this message | | | Date | Mon, 5 Sep 2022 22:35:13 +0800 | Subject | Re: [PATCH] mm: gup: fix the fast GUP race against THP collapse | From | Baolin Wang <> |
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On 9/5/2022 7:11 PM, David Hildenbrand wrote: > 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@gmail.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.
Right, I agree. That means the complier can not optimize the order of the 'pte_val(*ptep)', however what I am confusing is that the complier can still save the value of *ptep into a register or stack instead of reloading from memory?
A similar issue in commit d6c1f098f2a7 ("mm/swap_state: fix a data race in swapin_nr_pages").
--- a/mm/swap_state.c +++ b/mm/swap_state.c @@ -509,10 +509,11 @@ static unsigned long swapin_nr_pages(unsigned long offset) return 1;
hits = atomic_xchg(&swapin_readahead_hits, 0); - pages = __swapin_nr_pages(prev_offset, offset, hits, max_pages, + pages = __swapin_nr_pages(READ_ONCE(prev_offset), offset, hits, + max_pages, atomic_read(&last_readahead_pages)); if (!hits) - prev_offset = offset; + WRITE_ONCE(prev_offset, offset); atomic_set(&last_readahead_pages, pages);
return pages;
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