[PATCH] freepgt: free_pgtables use vma list

Recent woes with some arches needing their own pgd_addr_end macro; and 4-level
clear_page_range regression since 2.6.10's clear_page_tables; and its
long-standing well-known inefficiency in searching throughout the higher-level
page tables for those few entries to clear and free: all can be blamed on
ignoring the list of vmas when we free page tables.

Replace exit_mmap's clear_page_range of the total user address space by
free_pgtables operating on the mm's vma list; unmap_region use it in the same
way, giving floor and ceiling beyond which it may not free tables.  This
brings lmbench fork/exec/sh numbers back to 2.6.10 (unless preempt is enabled,
in which case latency fixes spoil unmap_vmas throughput).

Beware: the do_mmap_pgoff driver failure case must now use unmap_region
instead of zap_page_range, since a page table might have been allocated, and
can only be freed while it is touched by some vma.

Move free_pgtables from mmap.c to memory.c, where its lower levels are adapted
from the clear_page_range levels.  (Most of free_pgtables' old code was
actually for a non-existent case, prev not properly set up, dating from before
hch gave us split_vma.) Pass mmu_gather** in the public interfaces, since we
might want to add latency lockdrops later; but no attempt to do so yet, going
by vma should itself reduce latency.

But what if is_hugepage_only_range?  Those ia64 and ppc64 cases need careful
examination: put that off until a later patch of the series.

What of x86_64's 32bit vdso page __map_syscall32 maps outside any vma?

And the range to sparc64's flush_tlb_pgtables?  It's less clear to me now that
we need to do more than is done here - every PMD_SIZE ever occupied will be
flushed, do we really have to flush every PGDIR_SIZE ever partially occupied? 
A shame to complicate it unnecessarily.

Special thanks to David Miller for time spent repairing my ceilings.

Signed-off-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 115 insertions, 37 deletions

diff --git a/mm/memory.c b/mm/memory.c
index fb6e5deb873a..fee5dc8fc36c 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -110,87 +110,165 @@ void pmd_clear_bad(pmd_t *pmd)
  * Note: this doesn't free the actual pages themselves. That
  * has been handled earlier when unmapping all the memory regions.
  */
-static inline void clear_pte_range(struct mmu_gather *tlb, pmd_t *pmd,
-                                unsigned long addr, unsigned long end)
+static void free_pte_range(struct mmu_gather *tlb, pmd_t *pmd)
 {
-        if (!((addr | end) & ~PMD_MASK)) {
-                /* Only free fully aligned ranges */
-                struct page *page = pmd_page(*pmd);
-                pmd_clear(pmd);
-                dec_page_state(nr_page_table_pages);
-                tlb->mm->nr_ptes--;
-                pte_free_tlb(tlb, page);
-        }
+        struct page *page = pmd_page(*pmd);
+        pmd_clear(pmd);
+        pte_free_tlb(tlb, page);
+        dec_page_state(nr_page_table_pages);
+        tlb->mm->nr_ptes--;
 }
 
-static inline void clear_pmd_range(struct mmu_gather *tlb, pud_t *pud,
-                                unsigned long addr, unsigned long end)
+static inline void free_pmd_range(struct mmu_gather *tlb, pud_t *pud,
+                                unsigned long addr, unsigned long end,
+                                unsigned long floor, unsigned long ceiling)
 {
         pmd_t *pmd;
         unsigned long next;
-        pmd_t *empty_pmd = NULL;
+        unsigned long start;
 
+        start = addr;
         pmd = pmd_offset(pud, addr);
-
-        /* Only free fully aligned ranges */
-        if (!((addr | end) & ~PUD_MASK))
-                empty_pmd = pmd;
         do {
                 next = pmd_addr_end(addr, end);
                 if (pmd_none_or_clear_bad(pmd))
                         continue;
-                clear_pte_range(tlb, pmd, addr, next);
+                free_pte_range(tlb, pmd);
         } while (pmd++, addr = next, addr != end);
 
-        if (empty_pmd) {
-                pud_clear(pud);
-                pmd_free_tlb(tlb, empty_pmd);
+        start &= PUD_MASK;
+        if (start < floor)
+                return;
+        if (ceiling) {
+                ceiling &= PUD_MASK;
+                if (!ceiling)
+                        return;
         }
+        if (end - 1 > ceiling - 1)
+                return;
+
+        pmd = pmd_offset(pud, start);
+        pud_clear(pud);
+        pmd_free_tlb(tlb, pmd);
 }
 
-static inline void clear_pud_range(struct mmu_gather *tlb, pgd_t *pgd,
-                                unsigned long addr, unsigned long end)
+static inline void free_pud_range(struct mmu_gather *tlb, pgd_t *pgd,
+                                unsigned long addr, unsigned long end,
+                                unsigned long floor, unsigned long ceiling)
 {
         pud_t *pud;
         unsigned long next;
-        pud_t *empty_pud = NULL;
+        unsigned long start;
 
+        start = addr;
         pud = pud_offset(pgd, addr);
-
-        /* Only free fully aligned ranges */
-        if (!((addr | end) & ~PGDIR_MASK))
-                empty_pud = pud;
         do {
                 next = pud_addr_end(addr, end);
                 if (pud_none_or_clear_bad(pud))
                         continue;
-                clear_pmd_range(tlb, pud, addr, next);
+                free_pmd_range(tlb, pud, addr, next, floor, ceiling);
         } while (pud++, addr = next, addr != end);
 
-        if (empty_pud) {
-                pgd_clear(pgd);
-                pud_free_tlb(tlb, empty_pud);
+        start &= PGDIR_MASK;
+        if (start < floor)
+                return;
+        if (ceiling) {
+                ceiling &= PGDIR_MASK;
+                if (!ceiling)
+                        return;
         }
+        if (end - 1 > ceiling - 1)
+                return;
+
+        pud = pud_offset(pgd, start);
+        pgd_clear(pgd);
+        pud_free_tlb(tlb, pud);
 }
 
 /*
- * This function clears user-level page tables of a process.
- * Unlike other pagetable walks, some memory layouts might give end 0.
+ * This function frees user-level page tables of a process.
+ *
  * Must be called with pagetable lock held.
  */
-void clear_page_range(struct mmu_gather *tlb,
-                                unsigned long addr, unsigned long end)
+static inline void free_pgd_range(struct mmu_gather *tlb,
+                        unsigned long addr, unsigned long end,
+                        unsigned long floor, unsigned long ceiling)
 {
         pgd_t *pgd;
         unsigned long next;
+        unsigned long start;
 
+        /*
+         * The next few lines have given us lots of grief...
+         *
+         * Why are we testing PMD* at this top level?  Because often
+         * there will be no work to do at all, and we'd prefer not to
+         * go all the way down to the bottom just to discover that.
+         *
+         * Why all these "- 1"s?  Because 0 represents both the bottom
+         * of the address space and the top of it (using -1 for the
+         * top wouldn't help much: the masks would do the wrong thing).
+         * The rule is that addr 0 and floor 0 refer to the bottom of
+         * the address space, but end 0 and ceiling 0 refer to the top
+         * Comparisons need to use "end - 1" and "ceiling - 1" (though
+         * that end 0 case should be mythical).
+         *
+         * Wherever addr is brought up or ceiling brought down, we must
+         * be careful to reject "the opposite 0" before it confuses the
+         * subsequent tests.  But what about where end is brought down
+         * by PMD_SIZE below? no, end can't go down to 0 there.
+         *
+         * Whereas we round start (addr) and ceiling down, by different
+         * masks at different levels, in order to test whether a table
+         * now has no other vmas using it, so can be freed, we don't
+         * bother to round floor or end up - the tests don't need that.
+         */
+
+        addr &= PMD_MASK;
+        if (addr < floor) {
+                addr += PMD_SIZE;
+                if (!addr)
+                        return;
+        }
+        if (ceiling) {
+                ceiling &= PMD_MASK;
+                if (!ceiling)
+                        return;
+        }
+        if (end - 1 > ceiling - 1)
+                end -= PMD_SIZE;
+        if (addr > end - 1)
+                return;
+
+        start = addr;
         pgd = pgd_offset(tlb->mm, addr);
         do {
                 next = pgd_addr_end(addr, end);
                 if (pgd_none_or_clear_bad(pgd))
                         continue;
-                clear_pud_range(tlb, pgd, addr, next);
+                free_pud_range(tlb, pgd, addr, next, floor, ceiling);
         } while (pgd++, addr = next, addr != end);
+
+        if (!tlb_is_full_mm(tlb))
+                flush_tlb_pgtables(tlb->mm, start, end);
+}
+
+void free_pgtables(struct mmu_gather **tlb, struct vm_area_struct *vma,
+                                unsigned long floor, unsigned long ceiling)
+{
+        while (vma) {
+                struct vm_area_struct *next = vma->vm_next;
+                unsigned long addr = vma->vm_start;
+
+                /* Optimization: gather nearby vmas into a single call down */
+                while (next && next->vm_start <= vma->vm_end + PMD_SIZE) {
+                        vma = next;
+                        next = vma->vm_next;
+                }
+                free_pgd_range(*tlb, addr, vma->vm_end,
+                                floor, next? next->vm_start: ceiling);
+                vma = next;
+        }
 }
 
 pte_t fastcall * pte_alloc_map(struct mm_struct *mm, pmd_t *pmd, unsigned long address)