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Date: Thu, 03 Dec 2020 09:24:33 -0000
From: "tip-bot2 for Peter Zijlstra" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: "Peter Zijlstra (Intel)" <peterz@...radead.org>, x86@...nel.org,
linux-kernel@...r.kernel.org
Subject: [tip: perf/core] mm/gup: Provide gup_get_pte() more generic
The following commit has been merged into the perf/core branch of tip:
Commit-ID: 2a4a06da8a4b93dd189171eed7a99fffd38f42f3
Gitweb: https://git.kernel.org/tip/2a4a06da8a4b93dd189171eed7a99fffd38f42f3
Author: Peter Zijlstra <peterz@...radead.org>
AuthorDate: Fri, 13 Nov 2020 11:41:40 +01:00
Committer: Peter Zijlstra <peterz@...radead.org>
CommitterDate: Thu, 03 Dec 2020 10:14:50 +01:00
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@...radead.org>
Link: https://lkml.kernel.org/r/20201126121121.036370527@infradead.org
---
include/linux/pgtable.h | 55 ++++++++++++++++++++++++++++++++++++++-
mm/gup.c | 58 +----------------------------------------
2 files changed, 56 insertions(+), 57 deletions(-)
diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
index 71125a4..ed9266c 100644
--- a/include/linux/pgtable.h
+++ b/include/linux/pgtable.h
@@ -258,6 +258,61 @@ static inline pte_t ptep_get(pte_t *ptep)
}
#endif
+#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 ptep_get_lockless(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 ptep_get_lockless(pte_t *ptep)
+{
+ return ptep_get(ptep);
+}
+#endif /* CONFIG_GUP_GET_PTE_LOW_HIGH */
+
#ifdef CONFIG_TRANSPARENT_HUGEPAGE
#ifndef __HAVE_ARCH_PMDP_HUGE_GET_AND_CLEAR
static inline pmd_t pmdp_huge_get_and_clear(struct mm_struct *mm,
diff --git a/mm/gup.c b/mm/gup.c
index 98eb8e6..44b0c6b 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;
/*
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