mm/khugepaged: fix GUP-fast interaction by sending IPI

Since commit 70cbc3cc78a99 ("mm: gup: fix the fast GUP race against THP
collapse"), the lockless_pages_from_mm() fastpath rechecks the pmd_t to
ensure that the page table was not removed by khugepaged in between.

However, lockless_pages_from_mm() still requires that the page table is
not concurrently freed. Fix it by sending IPIs (if the architecture uses
semi-RCU-style page table freeing) before freeing/reusing page tables.

Link: https://lkml.kernel.org/r/[e-mail]
Link: https://lkml.kernel.org/r/[e-mail]
Link: https://lkml.kernel.org/r/[e-mail]
Fixes: ba76149f47d8 ("thp: khugepaged")
Signed-off-by: Jann Horn <[e-mail]>
Reviewed-by: Yang Shi <[e-mail]>
Acked-by: David Hildenbrand <[e-mail]>
Cc: John Hubbard <[e-mail]>
Cc: Peter Xu <[e-mail]>
Cc: <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

include/asm-generic/tlb.h

···
 #define tlb_needs_table_invalidate() (true)
 #endif
 
+void tlb_remove_table_sync_one(void);
+
 #else
 
 #ifdef tlb_needs_table_invalidate
 #error tlb_needs_table_invalidate() requires MMU_GATHER_RCU_TABLE_FREE
 #endif
+
+static inline void tlb_remove_table_sync_one(void) { }
 
 #endif /* CONFIG_MMU_GATHER_RCU_TABLE_FREE */
 

mm/khugepaged.c

···
 	_pmd = pmdp_collapse_flush(vma, address, pmd);
 	spin_unlock(pmd_ptl);
 	mmu_notifier_invalidate_range_end(&range);
+	tlb_remove_table_sync_one();
 
 	spin_lock(pte_ptl);
 	result =  __collapse_huge_page_isolate(vma, address, pte, cc,
···
 		lockdep_assert_held_write(&vma->anon_vma->root->rwsem);
 
 	pmd = pmdp_collapse_flush(vma, addr, pmdp);
+	tlb_remove_table_sync_one();
 	mm_dec_nr_ptes(mm);
 	page_table_check_pte_clear_range(mm, addr, pmd);
 	pte_free(mm, pmd_pgtable(pmd));

mm/mmu_gather.c

···
 	/* Simply deliver the interrupt */
 }
 
-static void tlb_remove_table_sync_one(void)
+void tlb_remove_table_sync_one(void)
 {
 	/*
 	 * This isn't an RCU grace period and hence the page-tables cannot be
···
 }
 
 #else /* !CONFIG_MMU_GATHER_RCU_TABLE_FREE */
-
-static void tlb_remove_table_sync_one(void) { }
 
 static void tlb_remove_table_free(struct mmu_table_batch *batch)
 {