[PATCH v4 9/9] asm-generic/tlb: Provide MMU_GATHER_TABLE_FREE

Aneesh Kumar K.V
2020-01-16T06:46:35Z

As described in the comment, the correct order for freeing pages is:

1) unhook page
2) TLB invalidate page
3) free page

This order equally applies to page directories.

Currently there are two correct options:

- use tlb_remove_page(), when all page directores are full pages and
there are no futher contraints placed by things like software
walkers (HAVE_FAST_GUP).

- use MMU_GATHER_RCU_TABLE_FREE and tlb_remove_table() when the
architecture does not do IPI based TLB invalidate and has
HAVE_FAST_GUP (or software TLB fill).

This however leaves architectures that don't have page based
directories but don't need RCU in a bind. For those, provide
MMU_GATHER_TABLE_FREE, which provides the independent batching for
directories without the additional RCU freeing.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
---
arch/Kconfig | 5 ++
arch/arm/include/asm/tlb.h | 4 --
include/asm-generic/tlb.h | 72 +++++++++++-----------
mm/mmu_gather.c | 120 +++++++++++++++++++++++++++----------
4 files changed, 130 insertions(+), 71 deletions(-)

diff --git a/arch/Kconfig b/arch/Kconfig
index c35668fbf4d4..98de654b79b3 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -393,8 +393,12 @@ config HAVE_ARCH_JUMP_LABEL
config HAVE_ARCH_JUMP_LABEL_RELATIVE
bool

+config MMU_GATHER_TABLE_FREE
+ bool
+
config MMU_GATHER_RCU_TABLE_FREE
bool
+ select MMU_GATHER_TABLE_FREE

config MMU_GATHER_PAGE_SIZE
bool
@@ -404,6 +408,7 @@ config MMU_GATHER_NO_RANGE

config MMU_GATHER_NO_GATHER
bool
+ depends on MMU_GATHER_TABLE_FREE

config ARCH_HAVE_NMI_SAFE_CMPXCHG
bool
diff --git a/arch/arm/include/asm/tlb.h b/arch/arm/include/asm/tlb.h
index 46a21cee3442..4d4e7b6aabff 100644
--- a/arch/arm/include/asm/tlb.h
+++ b/arch/arm/include/asm/tlb.h
@@ -37,10 +37,6 @@ static inline void __tlb_remove_table(void *_table)

#include <asm-generic/tlb.h>

-#ifndef CONFIG_MMU_GATHER_RCU_TABLE_FREE
-#define tlb_remove_table(tlb, entry) tlb_remove_page(tlb, entry)
-#endif
-
static inline void
__pte_free_tlb(struct mmu_gather *tlb, pgtable_t pte, unsigned long addr)
{
diff --git a/include/asm-generic/tlb.h b/include/asm-generic/tlb.h
index ca0fe75b5355..f391f6b500b4 100644
--- a/include/asm-generic/tlb.h
+++ b/include/asm-generic/tlb.h
@@ -56,6 +56,15 @@
* Defaults to flushing at tlb_end_vma() to reset the range; helps when
* there's large holes between the VMAs.
*
+ * - tlb_remove_table()
+ *
+ * tlb_remove_table() is the basic primitive to free page-table directories
+ * (__p*_free_tlb()). In it's most primitive form it is an alias for
+ * tlb_remove_page() below, for when page directories are pages and have no
+ * additional constraints.
+ *
+ * See also MMU_GATHER_TABLE_FREE and MMU_GATHER_RCU_TABLE_FREE.
+ *
* - tlb_remove_page() / __tlb_remove_page()
* - tlb_remove_page_size() / __tlb_remove_page_size()
*
@@ -129,17 +138,24 @@
* This might be useful if your architecture has size specific TLB
* invalidation instructions.
*
- * MMU_GATHER_RCU_TABLE_FREE
+ * MMU_GATHER_TABLE_FREE
*
* This provides tlb_remove_table(), to be used instead of tlb_remove_page()
- * for page directores (__p*_free_tlb()). This provides separate freeing of
- * the page-table pages themselves in a semi-RCU fashion (see comment below).
- * Useful if your architecture doesn't use IPIs for remote TLB invalidates
- * and therefore doesn't naturally serialize with software page-table walkers.
+ * for page directores (__p*_free_tlb()).
+ *
+ * Useful if your architecture has non-page page directories.
*
* When used, an architecture is expected to provide __tlb_remove_table()
* which does the actual freeing of these pages.
*
+ * MMU_GATHER_RCU_TABLE_FREE
+ *
+ * Like MMU_GATHER_TABLE_FREE, and adds semi-RCU semantics to the free (see
+ * comment below).
+ *
+ * Useful if your architecture doesn't use IPIs for remote TLB invalidates
+ * and therefore doesn't naturally serialize with software page-table walkers.
+ *
* MMU_GATHER_NO_RANGE
*
* Use this if your architecture lacks an efficient flush_tlb_range().
@@ -155,37 +171,12 @@
* various ptep_get_and_clear() functions.
*/

-#ifdef CONFIG_MMU_GATHER_RCU_TABLE_FREE
-/*
- * Semi RCU freeing of the page directories.
- *
- * This is needed by some architectures to implement software pagetable walkers.
- *
- * gup_fast() and other software pagetable walkers do a lockless page-table
- * walk and therefore needs some synchronization with the freeing of the page
- * directories. The chosen means to accomplish that is by disabling IRQs over
- * the walk.
- *
- * Architectures that use IPIs to flush TLBs will then automagically DTRT,
- * since we unlink the page, flush TLBs, free the page. Since the disabling of
- * IRQs delays the completion of the TLB flush we can never observe an already
- * freed page.
- *
- * Architectures that do not have this (PPC) need to delay the freeing by some
- * other means, this is that means.
- *
- * What we do is batch the freed directory pages (tables) and RCU free them.
- * We use the sched RCU variant, as that guarantees that IRQ/preempt disabling
- * holds off grace periods.
- *
- * However, in order to batch these pages we need to allocate storage, this
- * allocation is deep inside the MM code and can thus easily fail on memory
- * pressure. To guarantee progress we fall back to single table freeing, see
- * the implementation of tlb_remove_table_one().
- *
- */
+#ifdef CONFIG_MMU_GATHER_TABLE_FREE
+
struct mmu_table_batch {
+#ifdef CONFIG_MMU_GATHER_RCU_TABLE_FREE
struct rcu_head rcu;
+#endif
unsigned int nr;
void *tables[0];
};
@@ -195,6 +186,17 @@ struct mmu_table_batch {

extern void tlb_remove_table(struct mmu_gather *tlb, void *table);

+#else /* !CONFIG_MMU_GATHER_HAVE_TABLE_FREE */
+
+/*
+ * Without MMU_GATHER_TABLE_FREE the architecture is assumed to have page based
+ * page directories and we can use the normal page batching to free them.
+ */
+#define tlb_remove_table(tlb, page) tlb_remove_page((tlb), (page))
+
+#endif /* CONFIG_MMU_GATHER_TABLE_FREE */
+
+#ifdef CONFIG_MMU_GATHER_RCU_TABLE_FREE
/*
* This allows an architecture that does not use the linux page-tables for
* hardware to skip the TLBI when freeing page tables.
@@ -248,7 +250,7 @@ extern bool __tlb_remove_page_size(struct mmu_gather *tlb, struct page *page,
struct mmu_gather {
struct mm_struct *mm;

-#ifdef CONFIG_MMU_GATHER_RCU_TABLE_FREE
+#ifdef CONFIG_MMU_GATHER_TABLE_FREE
struct mmu_table_batch *batch;
#endif

diff --git a/mm/mmu_gather.c b/mm/mmu_gather.c
index a28c74328085..a3538cb2bcbe 100644
--- a/mm/mmu_gather.c
+++ b/mm/mmu_gather.c
@@ -91,56 +91,106 @@ bool __tlb_remove_page_size(struct mmu_gather *tlb, struct page *page, int page_

#endif /* MMU_GATHER_NO_GATHER */

-#ifdef CONFIG_MMU_GATHER_RCU_TABLE_FREE
+#ifdef CONFIG_MMU_GATHER_TABLE_FREE

-/*
- * See the comment near struct mmu_table_batch.
- */
+static void __tlb_remove_table_free(struct mmu_table_batch *batch)
+{
+ int i;
+
+ for (i = 0; i < batch->nr; i++)
+ __tlb_remove_table(batch->tables[i]);
+
+ free_page((unsigned long)batch);
+}
+
+#ifdef CONFIG_MMU_GATHER_RCU_TABLE_FREE

/*
- * If we want tlb_remove_table() to imply TLB invalidates.
+ * Semi RCU freeing of the page directories.
+ *
+ * This is needed by some architectures to implement software pagetable walkers.
+ *
+ * gup_fast() and other software pagetable walkers do a lockless page-table
+ * walk and therefore needs some synchronization with the freeing of the page
+ * directories. The chosen means to accomplish that is by disabling IRQs over
+ * the walk.
+ *
+ * Architectures that use IPIs to flush TLBs will then automagically DTRT,
+ * since we unlink the page, flush TLBs, free the page. Since the disabling of
+ * IRQs delays the completion of the TLB flush we can never observe an already
+ * freed page.
+ *
+ * Architectures that do not have this (PPC) need to delay the freeing by some
+ * other means, this is that means.
+ *
+ * What we do is batch the freed directory pages (tables) and RCU free them.
+ * We use the sched RCU variant, as that guarantees that IRQ/preempt disabling
+ * holds off grace periods.
+ *
+ * However, in order to batch these pages we need to allocate storage, this
+ * allocation is deep inside the MM code and can thus easily fail on memory
+ * pressure. To guarantee progress we fall back to single table freeing, see
+ * the implementation of tlb_remove_table_one().
+ *
*/
-static inline void tlb_table_invalidate(struct mmu_gather *tlb)
-{
- if (tlb_needs_table_invalidate()) {
- /*
- * Invalidate page-table caches used by hardware walkers. Then
- * we still need to RCU-sched wait while freeing the pages
- * because software walkers can still be in-flight.
- */
- tlb_flush_mmu_tlbonly(tlb);
- }
-}

static void tlb_remove_table_smp_sync(void *arg)
{
/* Simply deliver the interrupt */
}

-static void tlb_remove_table_one(void *table)
+static void tlb_remove_table_sync_one(void)
{
/*
* This isn't an RCU grace period and hence the page-tables cannot be
* assumed to be actually RCU-freed.
*
* It is however sufficient for software page-table walkers that rely on
- * IRQ disabling. See the comment near struct mmu_table_batch.
+ * IRQ disabling.
*/
smp_call_function(tlb_remove_table_smp_sync, NULL, 1);
- __tlb_remove_table(table);
}

static void tlb_remove_table_rcu(struct rcu_head *head)
{
- struct mmu_table_batch *batch;
- int i;
+ __tlb_remove_table_free(container_of(head, struct mmu_table_batch, rcu));
+}

- batch = container_of(head, struct mmu_table_batch, rcu);
+static void tlb_remove_table_free(struct mmu_table_batch *batch)
+{
+ call_rcu(&batch->rcu, tlb_remove_table_rcu);
+}

- for (i = 0; i < batch->nr; i++)
- __tlb_remove_table(batch->tables[i]);
+#else /* !CONFIG_MMU_GATHER_RCU_TABLE_FREE */

- free_page((unsigned long)batch);
+static void tlb_remove_table_sync_one(void) { }
+
+static void tlb_remove_table_free(struct mmu_table_batch *batch)
+{
+ __tlb_remove_table_free(batch);
+}
+
+#endif /* CONFIG_MMU_GATHER_RCU_TABLE_FREE */
+
+/*
+ * If we want tlb_remove_table() to imply TLB invalidates.
+ */
+static inline void tlb_table_invalidate(struct mmu_gather *tlb)
+{
+ if (tlb_needs_table_invalidate()) {
+ /*
+ * Invalidate page-table caches used by hardware walkers. Then
+ * we still need to RCU-sched wait while freeing the pages
+ * because software walkers can still be in-flight.
+ */
+ tlb_flush_mmu_tlbonly(tlb);
+ }
+}
+
+static void tlb_remove_table_one(void *table)
+{
+ tlb_remove_table_sync_one();
+ __tlb_remove_table(table);
}

static void tlb_table_flush(struct mmu_gather *tlb)
@@ -149,7 +199,7 @@ static void tlb_table_flush(struct mmu_gather *tlb)

if (*batch) {
tlb_table_invalidate(tlb);
- call_rcu(&(*batch)->rcu, tlb_remove_table_rcu);
+ tlb_remove_table_free(*batch);
*batch = NULL;
}
}
@@ -173,13 +223,21 @@ void tlb_remove_table(struct mmu_gather *tlb, void *table)
tlb_table_flush(tlb);
}

-#endif /* CONFIG_MMU_GATHER_RCU_TABLE_FREE */
+static inline void tlb_table_init(struct mmu_gather *tlb)
+{
+ tlb->batch = NULL;
+}
+
+#else /* !CONFIG_MMU_GATHER_TABLE_FREE */
+
+static inline void tlb_table_flush(struct mmu_gather *tlb) { }
+static inline void tlb_table_init(struct mmu_gather *tlb) { }
+
+#endif /* CONFIG_MMU_GATHER_TABLE_FREE */

static void tlb_flush_mmu_free(struct mmu_gather *tlb)
{
-#ifdef CONFIG_MMU_GATHER_RCU_TABLE_FREE
tlb_table_flush(tlb);
-#endif
#ifndef CONFIG_MMU_GATHER_NO_GATHER
tlb_batch_pages_flush(tlb);
#endif
@@ -220,9 +278,7 @@ void tlb_gather_mmu(struct mmu_gather *tlb, struct mm_struct *mm,
tlb->batch_count = 0;
#endif

-#ifdef CONFIG_MMU_GATHER_RCU_TABLE_FREE
- tlb->batch = NULL;
-#endif
+ tlb_table_init(tlb);
#ifdef CONFIG_MMU_GATHER_PAGE_SIZE
tlb->page_size = 0;
#endif
--
2.24.1