commit 70cbc3cc78a997d8247b50389d37c4e1736019da upstream

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

The race can 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.

Link: https://lkml.kernel.org/r/20220907180144.555485-1-shy828301@gmail.com Fixes: 2667f50e8b81 ("mm: introduce a general RCU get_user_pages_fast()") Acked-by: David Hildenbrand david@redhat.com Acked-by: Peter Xu peterx@redhat.com Signed-off-by: Yang Shi shy828301@gmail.com Reviewed-by: John Hubbard jhubbard@nvidia.com Cc: "Aneesh Kumar K.V" aneesh.kumar@linux.ibm.com Cc: Hugh Dickins hughd@google.com Cc: Jason Gunthorpe jgg@nvidia.com Cc: "Kirill A. Shutemov" kirill.shutemov@linux.intel.com Cc: Michael Ellerman mpe@ellerman.id.au Cc: Nicholas Piggin npiggin@gmail.com Cc: Christophe Leroy christophe.leroy@csgroup.eu Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org --- mm/gup.c | 34 ++++++++++++++++++++++++++++------ mm/khugepaged.c | 10 ++++++---- 2 files changed, 34 insertions(+), 10 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c index 05068d3d2557..1a23cd0b4fba 100644 --- a/mm/gup.c +++ b/mm/gup.c @@ -2266,8 +2266,28 @@ 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) +/* + * Fast-gup relies on pte change detection to avoid concurrent pgtable + * operations. + * + * To pin the page, fast-gup needs to do below in order: + * (1) pin the page (by prefetching pte), then (2) check pte not changed. + * + * For the rest of pgtable operations where pgtable updates can be racy + * with fast-gup, we need to do (1) clear pte, then (2) check whether page + * is pinned. + * + * Above will work for all pte-level operations, including THP split. + * + * For THP collapse, it's a bit more complicated because fast-gup may be + * walking a pgtable page that is being freed (pte is still valid but pmd + * can be cleared already). To avoid race in such condition, we need to + * also check pmd here to make sure pmd doesn't change (corresponds to + * pmdp_collapse_flush() in the THP collapse code path). + */ +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; @@ -2312,7 +2332,8 @@ 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))) { + if (unlikely(pmd_val(pmd) != pmd_val(*pmdp)) || + unlikely(pte_val(pte) != pte_val(*ptep))) { put_compound_head(head, 1, flags); goto pte_unmap; } @@ -2357,8 +2378,9 @@ 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; } @@ -2667,7 +2689,7 @@ 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 8a8b3aa92937..dd069afd9cb9 100644 --- a/mm/khugepaged.c +++ b/mm/khugepaged.c @@ -1146,10 +1146,12 @@ static void collapse_huge_page(struct mm_struct *mm,

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); spin_unlock(pmd_ptl);