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When lazy page table copy for hugetlb is disabled, that is, revert commit bcd51a3c679d ("hugetlb: lazy page table copies in fork()") fork()'ing with hugetlb PMD sharing quickly lockup - [ 239.446559] watchdog: BUG: soft lockup - CPU#75 stuck for 27s! [ 239.446611] RIP: 0010:native_queued_spin_lock_slowpath+0x7e/0x2e0 [ 239.446631] Call Trace: [ 239.446633] [ 239.446636] _raw_spin_lock+0x3f/0x60 [ 239.446639] copy_hugetlb_page_range+0x258/0xb50 [ 239.446645] copy_page_range+0x22b/0x2c0 [ 239.446651] dup_mmap+0x3e2/0x770 [ 239.446654] dup_mm.constprop.0+0x5e/0x230 [ 239.446657] copy_process+0xd17/0x1760 [ 239.446660] kernel_clone+0xc0/0x3e0 [ 239.446661] __do_sys_clone+0x65/0xa0 [ 239.446664] do_syscall_64+0x82/0x930 [ 239.446668] ? count_memcg_events+0xd2/0x190 [ 239.446671] ? syscall_trace_enter+0x14e/0x1f0 [ 239.446676] ? syscall_exit_work+0x118/0x150 [ 239.446677] ? arch_exit_to_user_mode_prepare.constprop.0+0x9/0xb0 [ 239.446681] ? clear_bhb_loop+0x30/0x80 [ 239.446684] ? clear_bhb_loop+0x30/0x80 [ 239.446686] entry_SYSCALL_64_after_hwframe+0x76/0x7e There are two options to resolve the potential latent issue: 1. warn against PMD sharing in copy_hugetlb_page_range(), 2. fix it. This patch opts for the second option. While at it, simplify the comment, the details are not actually relevant anymore. Fixes: 59d9094df3d79 ("mm: hugetlb: independent PMD page table shared count") Signed-off-by: Jane Chu Reviewed-by: Harry Yoo --- v2 -> v3: Comments from David: 1. provide wrapper ptdesc_pmd_is_shared() wrapper for ptdesc_pmd_pts_count(), 2. mention comment simplification. v1 -> v2: Comments from Harry: explain why backport to stable isn't needed. Fix the sha lines. --- include/linux/mm_types.h | 5 +++++ mm/hugetlb.c | 15 +++++---------- 2 files changed, 10 insertions(+), 10 deletions(-) diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h index 08bc2442db93..a643fae8a349 100644 --- a/include/linux/mm_types.h +++ b/include/linux/mm_types.h @@ -631,6 +631,11 @@ static inline int ptdesc_pmd_pts_count(struct ptdesc *ptdesc) { return atomic_read(&ptdesc->pt_share_count); } + +static inline bool ptdesc_pmd_is_shared(struct ptdesc *ptdesc) +{ + return !!ptdesc_pmd_pts_count(ptdesc); +} #else static inline void ptdesc_pmd_pts_init(struct ptdesc *ptdesc) { diff --git a/mm/hugetlb.c b/mm/hugetlb.c index eed59cfb5d21..6cfe0b43ab8f 100644 --- a/mm/hugetlb.c +++ b/mm/hugetlb.c @@ -5594,18 +5594,13 @@ int copy_hugetlb_page_range(struct mm_struct *dst, struct mm_struct *src, break; } - /* - * If the pagetables are shared don't copy or take references. - * - * dst_pte == src_pte is the common case of src/dest sharing. - * However, src could have 'unshared' and dst shares with - * another vma. So page_count of ptep page is checked instead - * to reliably determine whether pte is shared. - */ - if (page_count(virt_to_page(dst_pte)) > 1) { +#ifdef CONFIG_HUGETLB_PMD_PAGE_TABLE_SHARING + /* If the pagetables are shared, there is nothing to do */ + if (ptdesc_pmd_is_shared(virt_to_ptdesc(dst_pte))) { addr |= last_addr_mask; continue; } +#endif dst_ptl = huge_pte_lock(h, dst, dst_pte); src_ptl = huge_pte_lockptr(h, src, src_pte); @@ -7602,7 +7597,7 @@ int huge_pmd_unshare(struct mm_struct *mm, struct vm_area_struct *vma, hugetlb_vma_assert_locked(vma); if (sz != PMD_SIZE) return 0; - if (!ptdesc_pmd_pts_count(virt_to_ptdesc(ptep))) + if (!ptdesc_pmd_is_shared(virt_to_ptdesc(ptep))) return 0; pud_clear(pud); -- 2.43.5