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Subject[PATCH 11/21] 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>
---
mm/mremap.c | 166 +++++++++++++++++++++++++-----------------------------------
1 files changed, 71 insertions(+), 95 deletions(-)
--- mm10/mm/mremap.c 2005-09-24 19:27:33.000000000 +0100
+++ mm11/mm/mremap.c 2005-09-24 19:28:42.000000000 +0100
@@ -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_
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
new_vma->vm_truncate_count != vma->vm_truncate_count)
new_vma->vm_truncate_count = 0;
}
+
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);

- 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);
+ 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,
-
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