[PATCH] mm: move_page_tables by extents

Speeding up mremap's moving of ptes has never been a priority, but the locking
will get more complicated shortly, and is already too baroque.

Scrap the current one-by-one moving, do an extent at a time: curtailed by end
of src and dst pmds (have to use PMD_SIZE: the way pmd_addr_end gets elided
doesn't match this usage), and by latency considerations.

One nice property of the old method is lost: it never allocated a page table
unless absolutely necessary, so you could free empty page tables by mremapping
to and fro.  Whereas this way, it allocates a dst table wherever there was a
src table.  I keep diving in to reinstate the old behaviour, then come out
preferring not to clutter how it now is.

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, 72 insertions, 96 deletions

diff --git a/mm/mremap.c b/mm/mremap.c
index 55df8f53e84d..f4e562098500 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -22,40 +22,15 @@
 #include <asm/cacheflush.h>
 #include <asm/tlbflush.h>
 
-static pte_t *get_one_pte_map_nested(struct mm_struct *mm, unsigned long addr)
-{
-        pgd_t *pgd;
-        pud_t *pud;
-        pmd_t *pmd;
-        pte_t *pte = NULL;
-
-        pgd = pgd_offset(mm, addr);
-        if (pgd_none_or_clear_bad(pgd))
-                goto end;
-
-        pud = pud_offset(pgd, addr);
-        if (pud_none_or_clear_bad(pud))
-                goto end;
-
-        pmd = pmd_offset(pud, addr);
-        if (pmd_none_or_clear_bad(pmd))
-                goto end;
-
-        pte = pte_offset_map_nested(pmd, addr);
-        if (pte_none(*pte)) {
-                pte_unmap_nested(pte);
-                pte = NULL;
-        }
-end:
-        return pte;
-}
-
-static pte_t *get_one_pte_map(struct mm_struct *mm, unsigned long addr)
+static pmd_t *get_old_pmd(struct mm_struct *mm, unsigned long addr)
 {
         pgd_t *pgd;
         pud_t *pud;
         pmd_t *pmd;
 
+        /*
+         * We don't need page_table_lock: we have mmap_sem exclusively.
+         */
         pgd = pgd_offset(mm, addr);
         if (pgd_none_or_clear_bad(pgd))
                 return NULL;
@@ -68,35 +43,48 @@ static pte_t *get_one_pte_map(struct mm_struct *mm, unsigned long addr)
         if (pmd_none_or_clear_bad(pmd))
                 return NULL;
 
-        return pte_offset_map(pmd, addr);
+        return pmd;
 }
 
-static inline pte_t *alloc_one_pte_map(struct mm_struct *mm, unsigned long addr)
+static pmd_t *alloc_new_pmd(struct mm_struct *mm, unsigned long addr)
 {
         pgd_t *pgd;
         pud_t *pud;
-        pmd_t *pmd;
-        pte_t *pte = NULL;
+        pmd_t *pmd = NULL;
+        pte_t *pte;
 
+        /*
+         * We do need page_table_lock: because allocators expect that.
+         */
+        spin_lock(&mm->page_table_lock);
         pgd = pgd_offset(mm, addr);
-
         pud = pud_alloc(mm, pgd, addr);
         if (!pud)
-                return NULL;
+                goto out;
+
         pmd = pmd_alloc(mm, pud, addr);
-        if (pmd)
-                pte = pte_alloc_map(mm, pmd, addr);
-        return pte;
+        if (!pmd)
+                goto out;
+
+        pte = pte_alloc_map(mm, pmd, addr);
+        if (!pte) {
+                pmd = NULL;
+                goto out;
+        }
+        pte_unmap(pte);
+out:
+        spin_unlock(&mm->page_table_lock);
+        return pmd;
 }
 
-static int
-move_one_page(struct vm_area_struct *vma, unsigned long old_addr,
-                struct vm_area_struct *new_vma, unsigned long new_addr)
+static void move_ptes(struct vm_area_struct *vma, pmd_t *old_pmd,
+                unsigned long old_addr, unsigned long old_end,
+                struct vm_area_struct *new_vma, pmd_t *new_pmd,
+                unsigned long new_addr)
 {
         struct address_space *mapping = NULL;
         struct mm_struct *mm = vma->vm_mm;
-        int error = 0;
-        pte_t *src, *dst;
+        pte_t *old_pte, *new_pte, pte;
 
         if (vma->vm_file) {
                 /*
@@ -111,74 +99,62 @@ move_one_page(struct vm_area_struct *vma, unsigned long old_addr,
                     new_vma->vm_truncate_count != vma->vm_truncate_count)
                         new_vma->vm_truncate_count = 0;
         }
-        spin_lock(&mm->page_table_lock);
 
-        src = get_one_pte_map_nested(mm, old_addr);
-        if (src) {
-                /*
-                 * Look to see whether alloc_one_pte_map needs to perform a
-                 * memory allocation.  If it does then we need to drop the
-                 * atomic kmap
-                 */
-                dst = get_one_pte_map(mm, new_addr);
-                if (unlikely(!dst)) {
-                        pte_unmap_nested(src);
-                        if (mapping)
-                                spin_unlock(&mapping->i_mmap_lock);
-                        dst = alloc_one_pte_map(mm, new_addr);
-                        if (mapping && !spin_trylock(&mapping->i_mmap_lock)) {
-                                spin_unlock(&mm->page_table_lock);
-                                spin_lock(&mapping->i_mmap_lock);
-                                spin_lock(&mm->page_table_lock);
-                        }
-                        src = get_one_pte_map_nested(mm, old_addr);
-                }
-                /*
-                 * Since alloc_one_pte_map can drop and re-acquire
-                 * page_table_lock, we should re-check the src entry...
-                 */
-                if (src) {
-                        if (dst) {
-                                pte_t pte;
-                                pte = ptep_clear_flush(vma, old_addr, src);
-
-                                /* ZERO_PAGE can be dependant on virtual addr */
-                                pte = move_pte(pte, new_vma->vm_page_prot,
-                                                        old_addr, new_addr);
-                                set_pte_at(mm, new_addr, dst, pte);
-                        } else
-                                error = -ENOMEM;
-                        pte_unmap_nested(src);
-                }
-                if (dst)
-                        pte_unmap(dst);
+        spin_lock(&mm->page_table_lock);
+        old_pte = pte_offset_map(old_pmd, old_addr);
+        new_pte = pte_offset_map_nested(new_pmd, new_addr);
+
+        for (; old_addr < old_end; old_pte++, old_addr += PAGE_SIZE,
+                                   new_pte++, new_addr += PAGE_SIZE) {
+                if (pte_none(*old_pte))
+                        continue;
+                pte = ptep_clear_flush(vma, old_addr, old_pte);
+                /* ZERO_PAGE can be dependant on virtual addr */
+                pte = move_pte(pte, new_vma->vm_page_prot, old_addr, new_addr);
+                set_pte_at(mm, new_addr, new_pte, pte);
         }
+
+        pte_unmap_nested(new_pte - 1);
+        pte_unmap(old_pte - 1);
         spin_unlock(&mm->page_table_lock);
         if (mapping)
                 spin_unlock(&mapping->i_mmap_lock);
-        return error;
 }
 
+#define LATENCY_LIMIT   (64 * PAGE_SIZE)
+
 static unsigned long move_page_tables(struct vm_area_struct *vma,
                 unsigned long old_addr, struct vm_area_struct *new_vma,
                 unsigned long new_addr, unsigned long len)
 {
-        unsigned long offset;
+        unsigned long extent, next, old_end;
+        pmd_t *old_pmd, *new_pmd;
 
-        flush_cache_range(vma, old_addr, old_addr + len);
+        old_end = old_addr + len;
+        flush_cache_range(vma, old_addr, old_end);
 
-        /*
-         * This is not the clever way to do this, but we're taking the
-         * easy way out on the assumption that most remappings will be
-         * only a few pages.. This also makes error recovery easier.
-         */
-        for (offset = 0; offset < len; offset += PAGE_SIZE) {
-                if (move_one_page(vma, old_addr + offset,
-                                new_vma, new_addr + offset) < 0)
-                        break;
+        for (; old_addr < old_end; old_addr += extent, new_addr += extent) {
                 cond_resched();
+                next = (old_addr + PMD_SIZE) & PMD_MASK;
+                if (next - 1 > old_end)
+                        next = old_end;
+                extent = next - old_addr;
+                old_pmd = get_old_pmd(vma->vm_mm, old_addr);
+                if (!old_pmd)
+                        continue;
+                new_pmd = alloc_new_pmd(vma->vm_mm, new_addr);
+                if (!new_pmd)
+                        break;
+                next = (new_addr + PMD_SIZE) & PMD_MASK;
+                if (extent > next - new_addr)
+                        extent = next - new_addr;
+                if (extent > LATENCY_LIMIT)
+                        extent = LATENCY_LIMIT;
+                move_ptes(vma, old_pmd, old_addr, old_addr + extent,
+                                new_vma, new_pmd, new_addr);
         }
-        return offset;
+
+        return len + old_addr - old_end;        /* how much done */
 }
 
 static unsigned long move_vma(struct vm_area_struct *vma,