mm/gup: Provide gup_get_pte() more generic

In order to write another lockless page-table walker, we need
gup_get_pte() exposed. While doing that, rename it to
ptep_get_lockless() to match the existing ptep_get() naming.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20201126121121.036370527@infradead.org

1 files changed, 1 insertions, 57 deletions

diff --git a/mm/gup.c b/mm/gup.c
index 98eb8e6d2609..44b0c6b89602 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -2085,62 +2085,6 @@ static void put_compound_head(struct page *page, int refs, unsigned int flags)
 	put_page(page);
 }
 
-#ifdef CONFIG_GUP_GET_PTE_LOW_HIGH
-
-/*
- * WARNING: only to be used in the get_user_pages_fast() implementation.
- *
- * With get_user_pages_fast(), we walk down the pagetables without taking any
- * locks.  For this we would like to load the pointers atomically, but sometimes
- * that is not possible (e.g. without expensive cmpxchg8b on x86_32 PAE).  What
- * we do have is the guarantee that a PTE will only either go from not present
- * to present, or present to not present or both -- it will not switch to a
- * completely different present page without a TLB flush in between; something
- * that we are blocking by holding interrupts off.
- *
- * Setting ptes from not present to present goes:
- *
- *   ptep->pte_high = h;
- *   smp_wmb();
- *   ptep->pte_low = l;
- *
- * And present to not present goes:
- *
- *   ptep->pte_low = 0;
- *   smp_wmb();
- *   ptep->pte_high = 0;
- *
- * We must ensure here that the load of pte_low sees 'l' IFF pte_high sees 'h'.
- * We load pte_high *after* loading pte_low, which ensures we don't see an older
- * value of pte_high.  *Then* we recheck pte_low, which ensures that we haven't
- * picked up a changed pte high. We might have gotten rubbish values from
- * pte_low and pte_high, but we are guaranteed that pte_low will not have the
- * present bit set *unless* it is 'l'. Because get_user_pages_fast() only
- * operates on present ptes we're safe.
- */
-static inline pte_t gup_get_pte(pte_t *ptep)
-{
-	pte_t pte;
-
-	do {
-		pte.pte_low = ptep->pte_low;
-		smp_rmb();
-		pte.pte_high = ptep->pte_high;
-		smp_rmb();
-	} while (unlikely(pte.pte_low != ptep->pte_low));
-
-	return pte;
-}
-#else /* CONFIG_GUP_GET_PTE_LOW_HIGH */
-/*
- * We require that the PTE can be read atomically.
- */
-static inline pte_t gup_get_pte(pte_t *ptep)
-{
-	return ptep_get(ptep);
-}
-#endif /* CONFIG_GUP_GET_PTE_LOW_HIGH */
-
 static void __maybe_unused undo_dev_pagemap(int *nr, int nr_start,
 					    unsigned int flags,
 					    struct page **pages)
@@ -2166,7 +2110,7 @@ static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end,
 
 	ptem = ptep = pte_offset_map(&pmd, addr);
 	do {
-		pte_t pte = gup_get_pte(ptep);
+		pte_t pte = ptep_get_lockless(ptep);
 		struct page *head, *page;
 
 		/*