mm: mmu_gather rework

Rework the existing mmu_gather infrastructure.

The direct purpose of these patches was to allow preemptible mmu_gather,
but even without that I think these patches provide an improvement to the
status quo.

The first 9 patches rework the mmu_gather infrastructure.  For review
purpose I've split them into generic and per-arch patches with the last of
those a generic cleanup.

The next patch provides generic RCU page-table freeing, and the followup
is a patch converting s390 to use this.  I've also got 4 patches from
DaveM lined up (not included in this series) that uses this to implement
gup_fast() for sparc64.

Then there is one patch that extends the generic mmu_gather batching.

After that follow the mm preemptibility patches, these make part of the mm
a lot more preemptible.  It converts i_mmap_lock and anon_vma->lock to
mutexes which together with the mmu_gather rework makes mmu_gather
preemptible as well.

Making i_mmap_lock a mutex also enables a clean-up of the truncate code.

This also allows for preemptible mmu_notifiers, something that XPMEM I
think wants.

Furthermore, it removes the new and universially detested unmap_mutex.

This patch:

Remove the first obstacle towards a fully preemptible mmu_gather.

The current scheme assumes mmu_gather is always done with preemption
disabled and uses per-cpu storage for the page batches.  Change this to
try and allocate a page for batching and in case of failure, use a small
on-stack array to make some progress.

Preemptible mmu_gather is desired in general and usable once i_mmap_lock
becomes a mutex.  Doing it before the mutex conversion saves us from
having to rework the code by moving the mmu_gather bits inside the
pte_lock.

Also avoid flushing the tlb batches from under the pte lock, this is
useful even without the i_mmap_lock conversion as it significantly reduces
pte lock hold times.

[akpm@linux-foundation.org: fix comment tpyo]
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: David Miller <davem@davemloft.net>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: Russell King <rmk@arm.linux.org.uk>
Cc: Paul Mundt <lethal@linux-sh.org>
Cc: Jeff Dike <jdike@addtoit.com>
Cc: Richard Weinberger <richard@nod.at>
Cc: Tony Luck <tony.luck@intel.com>
Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Acked-by: Hugh Dickins <hughd@google.com>
Acked-by: Mel Gorman <mel@csn.ul.ie>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Nick Piggin <npiggin@kernel.dk>
Cc: Namhyung Kim <namhyung@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 69 insertions, 27 deletions

diff --git a/include/asm-generic/tlb.h b/include/asm-generic/tlb.h
index e43f9766259f..2d3547c84235 100644
--- a/include/asm-generic/tlb.h
+++ b/include/asm-generic/tlb.h
@@ -5,6 +5,8 @@
  * Copyright 2001 Red Hat, Inc.
  * Based on code from mm/memory.c Copyright Linus Torvalds and others.
  *
+ * Copyright 2011 Red Hat, Inc., Peter Zijlstra <pzijlstr@redhat.com>
+ *
  * This program is free software; you can redistribute it and/or
  * modify it under the terms of the GNU General Public License
  * as published by the Free Software Foundation; either version
@@ -22,51 +24,71 @@
  * and page free order so much..
  */
 #ifdef CONFIG_SMP
-  #ifdef ARCH_FREE_PTR_NR
-    #define FREE_PTR_NR   ARCH_FREE_PTR_NR
-  #else
-    #define FREE_PTE_NR 506
-  #endif
   #define tlb_fast_mode(tlb) ((tlb)->nr == ~0U)
 #else
-  #define FREE_PTE_NR   1
   #define tlb_fast_mode(tlb) 1
 #endif
 
+/*
+ * If we can't allocate a page to make a big batch of page pointers
+ * to work on, then just handle a few from the on-stack structure.
+ */
+#define MMU_GATHER_BUNDLE       8
+
 /* struct mmu_gather is an opaque type used by the mm code for passing around
  * any data needed by arch specific code for tlb_remove_page.
  */
 struct mmu_gather {
         struct mm_struct        *mm;
         unsigned int            nr;     /* set to ~0U means fast mode */
+        unsigned int            max;    /* nr < max */
         unsigned int            need_flush;/* Really unmapped some ptes? */
         unsigned int            fullmm; /* non-zero means full mm flush */
-        struct page *           pages[FREE_PTE_NR];
+#ifdef HAVE_ARCH_MMU_GATHER
+        struct arch_mmu_gather  arch;
+#endif
+        struct page             **pages;
+        struct page             *local[MMU_GATHER_BUNDLE];
 };
 
-/* Users of the generic TLB shootdown code must declare this storage space. */
-DECLARE_PER_CPU(struct mmu_gather, mmu_gathers);
+static inline void __tlb_alloc_page(struct mmu_gather *tlb)
+{
+        unsigned long addr = __get_free_pages(GFP_NOWAIT | __GFP_NOWARN, 0);
+
+        if (addr) {
+                tlb->pages = (void *)addr;
+                tlb->max = PAGE_SIZE / sizeof(struct page *);
+        }
+}
 
 /* tlb_gather_mmu
- *      Return a pointer to an initialized struct mmu_gather.
+ *      Called to initialize an (on-stack) mmu_gather structure for page-table
+ *      tear-down from @mm. The @fullmm argument is used when @mm is without
+ *      users and we're going to destroy the full address space (exit/execve).
  */
-static inline struct mmu_gather *
-tlb_gather_mmu(struct mm_struct *mm, unsigned int full_mm_flush)
+static inline void
+tlb_gather_mmu(struct mmu_gather *tlb, struct mm_struct *mm, bool fullmm)
 {
-        struct mmu_gather *tlb = &get_cpu_var(mmu_gathers);
-
         tlb->mm = mm;
 
-        /* Use fast mode if only one CPU is online */
-        tlb->nr = num_online_cpus() > 1 ? 0U : ~0U;
+        tlb->max = ARRAY_SIZE(tlb->local);
+        tlb->pages = tlb->local;
+
+        if (num_online_cpus() > 1) {
+                tlb->nr = 0;
+                __tlb_alloc_page(tlb);
+        } else /* Use fast mode if only one CPU is online */
+                tlb->nr = ~0U;
 
-        tlb->fullmm = full_mm_flush;
+        tlb->fullmm = fullmm;
 
-        return tlb;
+#ifdef HAVE_ARCH_MMU_GATHER
+        tlb->arch = ARCH_MMU_GATHER_INIT;
+#endif
 }
 
 static inline void
-tlb_flush_mmu(struct mmu_gather *tlb, unsigned long start, unsigned long end)
+tlb_flush_mmu(struct mmu_gather *tlb)
 {
         if (!tlb->need_flush)
                 return;
@@ -75,6 +97,13 @@ tlb_flush_mmu(struct mmu_gather *tlb, unsigned long start, unsigned long end)
         if (!tlb_fast_mode(tlb)) {
                 free_pages_and_swap_cache(tlb->pages, tlb->nr);
                 tlb->nr = 0;
+                /*
+                 * If we are using the local on-stack array of pages for MMU
+                 * gather, try allocating an off-stack array again as we have
+                 * recently freed pages.
+                 */
+                if (tlb->pages == tlb->local)
+                        __tlb_alloc_page(tlb);
         }
 }
 
@@ -85,29 +114,42 @@ tlb_flush_mmu(struct mmu_gather *tlb, unsigned long start, unsigned long end)
 static inline void
 tlb_finish_mmu(struct mmu_gather *tlb, unsigned long start, unsigned long end)
 {
-        tlb_flush_mmu(tlb, start, end);
+        tlb_flush_mmu(tlb);
 
         /* keep the page table cache within bounds */
         check_pgt_cache();
 
-        put_cpu_var(mmu_gathers);
+        if (tlb->pages != tlb->local)
+                free_pages((unsigned long)tlb->pages, 0);
 }
 
-/* tlb_remove_page
+/* __tlb_remove_page
  *      Must perform the equivalent to __free_pte(pte_get_and_clear(ptep)), while
  *      handling the additional races in SMP caused by other CPUs caching valid
- *      mappings in their TLBs.
+ *      mappings in their TLBs. Returns the number of free page slots left.
+ *      When out of page slots we must call tlb_flush_mmu().
  */
-static inline void tlb_remove_page(struct mmu_gather *tlb, struct page *page)
+static inline int __tlb_remove_page(struct mmu_gather *tlb, struct page *page)
 {
         tlb->need_flush = 1;
         if (tlb_fast_mode(tlb)) {
                 free_page_and_swap_cache(page);
-                return;
+                return 1; /* avoid calling tlb_flush_mmu() */
         }
         tlb->pages[tlb->nr++] = page;
-        if (tlb->nr >= FREE_PTE_NR)
-                tlb_flush_mmu(tlb, 0, 0);
+        VM_BUG_ON(tlb->nr > tlb->max);
+
+        return tlb->max - tlb->nr;
+}
+
+/* tlb_remove_page
+ *      Similar to __tlb_remove_page but will call tlb_flush_mmu() itself when
+ *      required.
+ */
+static inline void tlb_remove_page(struct mmu_gather *tlb, struct page *page)
+{
+        if (!__tlb_remove_page(tlb, page))
+                tlb_flush_mmu(tlb);
 }
 
 /**