[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>

2 files changed, 136 insertions, 118 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)
diff --git a/mm/mmap.c b/mm/mmap.c
index a95ebda27446..926d03015471 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -29,6 +29,10 @@
 #include <asm/cacheflush.h>
 #include <asm/tlb.h>
 
+static void unmap_region(struct mm_struct *mm,
+                struct vm_area_struct *vma, struct vm_area_struct *prev,
+                unsigned long start, unsigned long end);
+
 /*
  * WARNING: the debugging will use recursive algorithms so never enable this
  * unless you know what you are doing.
@@ -1129,7 +1133,8 @@ unmap_and_free_vma:
         fput(file);
 
         /* Undo any partial mapping done by a device driver. */
-        zap_page_range(vma, vma->vm_start, vma->vm_end - vma->vm_start, NULL);
+        unmap_region(mm, vma, prev, vma->vm_start, vma->vm_end);
+        charged = 0;
 free_vma:
         kmem_cache_free(vm_area_cachep, vma);
 unacct_error:
@@ -1572,66 +1577,6 @@ find_extend_vma(struct mm_struct * mm, unsigned long addr)
 }
 #endif
 
-/*
- * Try to free as many page directory entries as we can,
- * without having to work very hard at actually scanning
- * the page tables themselves.
- *
- * Right now we try to free page tables if we have a nice
- * PGDIR-aligned area that got free'd up. We could be more
- * granular if we want to, but this is fast and simple,
- * and covers the bad cases.
- *
- * "prev", if it exists, points to a vma before the one
- * we just free'd - but there's no telling how much before.
- */
-static void free_pgtables(struct mmu_gather *tlb, struct vm_area_struct *prev,
-        unsigned long start, unsigned long end)
-{
-        unsigned long first = start & PGDIR_MASK;
-        unsigned long last = end + PGDIR_SIZE - 1;
-        struct mm_struct *mm = tlb->mm;
-
-        if (last > MM_VM_SIZE(mm) || last < end)
-                last = MM_VM_SIZE(mm);
-
-        if (!prev) {
-                prev = mm->mmap;
-                if (!prev)
-                        goto no_mmaps;
-                if (prev->vm_end > start) {
-                        if (last > prev->vm_start)
-                                last = prev->vm_start;
-                        goto no_mmaps;
-                }
-        }
-        for (;;) {
-                struct vm_area_struct *next = prev->vm_next;
-
-                if (next) {
-                        if (next->vm_start < start) {
-                                prev = next;
-                                continue;
-                        }
-                        if (last > next->vm_start)
-                                last = next->vm_start;
-                }
-                if (prev->vm_end > first)
-                        first = prev->vm_end;
-                break;
-        }
-no_mmaps:
-        if (last < first)       /* for arches with discontiguous pgd indices */
-                return;
-        if (first < FIRST_USER_PGD_NR * PGDIR_SIZE)
-                first = FIRST_USER_PGD_NR * PGDIR_SIZE;
-        /* No point trying to free anything if we're in the same pte page */
-        if ((first & PMD_MASK) < (last & PMD_MASK)) {
-                clear_page_range(tlb, first, last);
-                flush_tlb_pgtables(mm, first, last);
-        }
-}
-
 /* Normal function to fix up a mapping
  * This function is the default for when an area has no specific
  * function.  This may be used as part of a more specific routine.
@@ -1674,24 +1619,22 @@ static void unmap_vma_list(struct mm_struct *mm,
  * Called with the page table lock held.
  */
 static void unmap_region(struct mm_struct *mm,
-        struct vm_area_struct *vma,
-        struct vm_area_struct *prev,
-        unsigned long start,
-        unsigned long end)
+                struct vm_area_struct *vma, struct vm_area_struct *prev,
+                unsigned long start, unsigned long end)
 {
+        struct vm_area_struct *next = prev? prev->vm_next: mm->mmap;
         struct mmu_gather *tlb;
         unsigned long nr_accounted = 0;
 
         lru_add_drain();
+        spin_lock(&mm->page_table_lock);
         tlb = tlb_gather_mmu(mm, 0);
         unmap_vmas(&tlb, mm, vma, start, end, &nr_accounted, NULL);
         vm_unacct_memory(nr_accounted);
-
-        if (is_hugepage_only_range(mm, start, end - start))
-                hugetlb_free_pgtables(tlb, prev, start, end);
-        else
-                free_pgtables(tlb, prev, start, end);
+        free_pgtables(&tlb, vma, prev? prev->vm_end: 0,
+                                 next? next->vm_start: 0);
         tlb_finish_mmu(tlb, start, end);
+        spin_unlock(&mm->page_table_lock);
 }
 
 /*
@@ -1823,9 +1766,7 @@ int do_munmap(struct mm_struct *mm, unsigned long start, size_t len)
          * Remove the vma's, and unmap the actual pages
          */
         detach_vmas_to_be_unmapped(mm, mpnt, prev, end);
-        spin_lock(&mm->page_table_lock);
         unmap_region(mm, mpnt, prev, start, end);
-        spin_unlock(&mm->page_table_lock);
 
         /* Fix up all other VM information */
         unmap_vma_list(mm, mpnt);
@@ -1957,25 +1898,21 @@ EXPORT_SYMBOL(do_brk);
 void exit_mmap(struct mm_struct *mm)
 {
         struct mmu_gather *tlb;
-        struct vm_area_struct *vma;
+        struct vm_area_struct *vma = mm->mmap;
         unsigned long nr_accounted = 0;
 
         lru_add_drain();
 
         spin_lock(&mm->page_table_lock);
 
-        tlb = tlb_gather_mmu(mm, 1);
         flush_cache_mm(mm);
-        /* Use ~0UL here to ensure all VMAs in the mm are unmapped */
-        mm->map_count -= unmap_vmas(&tlb, mm, mm->mmap, 0,
-                                        ~0UL, &nr_accounted, NULL);
+        tlb = tlb_gather_mmu(mm, 1);
+        /* Use -1 here to ensure all VMAs in the mm are unmapped */
+        mm->map_count -= unmap_vmas(&tlb, mm, vma, 0, -1, &nr_accounted, NULL);
         vm_unacct_memory(nr_accounted);
-        BUG_ON(mm->map_count);  /* This is just debugging */
-        clear_page_range(tlb, FIRST_USER_PGD_NR * PGDIR_SIZE, MM_VM_SIZE(mm));
-        
+        free_pgtables(&tlb, vma, 0, 0);
         tlb_finish_mmu(tlb, 0, MM_VM_SIZE(mm));
 
-        vma = mm->mmap;
         mm->mmap = mm->mmap_cache = NULL;
         mm->mm_rb = RB_ROOT;
         set_mm_counter(mm, rss, 0);
@@ -1993,6 +1930,9 @@ void exit_mmap(struct mm_struct *mm)
                 remove_vm_struct(vma);
                 vma = next;
         }
+
+        BUG_ON(mm->map_count);  /* This is just debugging */
+        BUG_ON(mm->nr_ptes);    /* This is just debugging */
 }
 
 /* Insert vm structure into process list sorted by address