[PATCH] mm: follow_page with inner ptlock

Final step in pushing down common core's page_table_lock.  follow_page no
longer wants caller to hold page_table_lock, uses pte_offset_map_lock itself;
and so no page_table_lock is taken in get_user_pages itself.

But get_user_pages (and get_futex_key) do then need follow_page to pin the
page for them: take Daniel's suggestion of bitflags to follow_page.

Need one for WRITE, another for TOUCH (it was the accessed flag before:
vanished along with check_user_page_readable, but surely get_numa_maps is
wrong to mark every page it finds as accessed), another for GET.

And another, ANON to dispose of untouched_anonymous_page: it seems silly for
that to descend a second time, let follow_page observe if there was no page
table and return ZERO_PAGE if so.  Fix minor bug in that: check VM_LOCKED -
make_pages_present ought to make readonly anonymous present.

Give get_numa_maps a cond_resched while we're there.

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, 71 insertions, 81 deletions

diff --git a/mm/memory.c b/mm/memory.c
index 51f7c0a220d4..8461e2dd91d7 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -807,86 +807,82 @@ unsigned long zap_page_range(struct vm_area_struct *vma, unsigned long address,
 
 /*
  * Do a quick page-table lookup for a single page.
- * mm->page_table_lock must be held.
  */
-struct page *follow_page(struct mm_struct *mm, unsigned long address, int write)
+struct page *follow_page(struct mm_struct *mm, unsigned long address,
+                        unsigned int flags)
 {
         pgd_t *pgd;
         pud_t *pud;
         pmd_t *pmd;
         pte_t *ptep, pte;
+        spinlock_t *ptl;
         unsigned long pfn;
         struct page *page;
 
-        page = follow_huge_addr(mm, address, write);
-        if (! IS_ERR(page))
-                return page;
+        page = follow_huge_addr(mm, address, flags & FOLL_WRITE);
+        if (!IS_ERR(page)) {
+                BUG_ON(flags & FOLL_GET);
+                goto out;
+        }
 
+        page = NULL;
         pgd = pgd_offset(mm, address);
         if (pgd_none(*pgd) || unlikely(pgd_bad(*pgd)))
-                goto out;
+                goto no_page_table;
 
         pud = pud_offset(pgd, address);
         if (pud_none(*pud) || unlikely(pud_bad(*pud)))
-                goto out;
+                goto no_page_table;
         
         pmd = pmd_offset(pud, address);
         if (pmd_none(*pmd) || unlikely(pmd_bad(*pmd)))
+                goto no_page_table;
+
+        if (pmd_huge(*pmd)) {
+                BUG_ON(flags & FOLL_GET);
+                page = follow_huge_pmd(mm, address, pmd, flags & FOLL_WRITE);
                 goto out;
-        if (pmd_huge(*pmd))
-                return follow_huge_pmd(mm, address, pmd, write);
+        }
 
-        ptep = pte_offset_map(pmd, address);
+        ptep = pte_offset_map_lock(mm, pmd, address, &ptl);
         if (!ptep)
                 goto out;
 
         pte = *ptep;
-        pte_unmap(ptep);
-        if (pte_present(pte)) {
-                if (write && !pte_write(pte))
-                        goto out;
-                pfn = pte_pfn(pte);
-                if (pfn_valid(pfn)) {
-                        page = pfn_to_page(pfn);
-                        if (write && !pte_dirty(pte) &&!PageDirty(page))
-                                set_page_dirty(page);
-                        mark_page_accessed(page);
-                        return page;
-                }
-        }
+        if (!pte_present(pte))
+                goto unlock;
+        if ((flags & FOLL_WRITE) && !pte_write(pte))
+                goto unlock;
+        pfn = pte_pfn(pte);
+        if (!pfn_valid(pfn))
+                goto unlock;
 
+        page = pfn_to_page(pfn);
+        if (flags & FOLL_GET)
+                get_page(page);
+        if (flags & FOLL_TOUCH) {
+                if ((flags & FOLL_WRITE) &&
+                    !pte_dirty(pte) && !PageDirty(page))
+                        set_page_dirty(page);
+                mark_page_accessed(page);
+        }
+unlock:
+        pte_unmap_unlock(ptep, ptl);
 out:
-        return NULL;
-}
-
-static inline int
-untouched_anonymous_page(struct mm_struct* mm, struct vm_area_struct *vma,
-                         unsigned long address)
-{
-        pgd_t *pgd;
-        pud_t *pud;
-        pmd_t *pmd;
-
-        /* Check if the vma is for an anonymous mapping. */
-        if (vma->vm_ops && vma->vm_ops->nopage)
-                return 0;
-
-        /* Check if page directory entry exists. */
-        pgd = pgd_offset(mm, address);
-        if (pgd_none(*pgd) || unlikely(pgd_bad(*pgd)))
-                return 1;
-
-        pud = pud_offset(pgd, address);
-        if (pud_none(*pud) || unlikely(pud_bad(*pud)))
-                return 1;
-
-        /* Check if page middle directory entry exists. */
-        pmd = pmd_offset(pud, address);
-        if (pmd_none(*pmd) || unlikely(pmd_bad(*pmd)))
-                return 1;
+        return page;
 
-        /* There is a pte slot for 'address' in 'mm'. */
-        return 0;
+no_page_table:
+        /*
+         * When core dumping an enormous anonymous area that nobody
+         * has touched so far, we don't want to allocate page tables.
+         */
+        if (flags & FOLL_ANON) {
+                page = ZERO_PAGE(address);
+                if (flags & FOLL_GET)
+                        get_page(page);
+                BUG_ON(flags & FOLL_WRITE);
+        }
+        return page;
 }
 
 int get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
@@ -894,18 +890,19 @@ int get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
                 struct page **pages, struct vm_area_struct **vmas)
 {
         int i;
-        unsigned int flags;
+        unsigned int vm_flags;
 
         /* 
          * Require read or write permissions.
          * If 'force' is set, we only require the "MAY" flags.
          */
-        flags = write ? (VM_WRITE | VM_MAYWRITE) : (VM_READ | VM_MAYREAD);
-        flags &= force ? (VM_MAYREAD | VM_MAYWRITE) : (VM_READ | VM_WRITE);
+        vm_flags  = write ? (VM_WRITE | VM_MAYWRITE) : (VM_READ | VM_MAYREAD);
+        vm_flags &= force ? (VM_MAYREAD | VM_MAYWRITE) : (VM_READ | VM_WRITE);
         i = 0;
 
         do {
-                struct vm_area_struct * vma;
+                struct vm_area_struct *vma;
+                unsigned int foll_flags;
 
                 vma = find_extend_vma(mm, start);
                 if (!vma && in_gate_area(tsk, start)) {
@@ -946,7 +943,7 @@ int get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
                 }
 
                 if (!vma || (vma->vm_flags & (VM_IO | VM_RESERVED))
-                                || !(flags & vma->vm_flags))
+                                || !(vm_flags & vma->vm_flags))
                         return i ? : -EFAULT;
 
                 if (is_vm_hugetlb_page(vma)) {
@@ -954,29 +951,25 @@ int get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
                                                 &start, &len, i);
                         continue;
                 }
-                spin_lock(&mm->page_table_lock);
+
+                foll_flags = FOLL_TOUCH;
+                if (pages)
+                        foll_flags |= FOLL_GET;
+                if (!write && !(vma->vm_flags & VM_LOCKED) &&
+                    (!vma->vm_ops || !vma->vm_ops->nopage))
+                        foll_flags |= FOLL_ANON;
+
                 do {
-                        int write_access = write;
                         struct page *page;
 
-                        cond_resched_lock(&mm->page_table_lock);
-                        while (!(page = follow_page(mm, start, write_access))) {
-                                int ret;
-
-                                /*
-                                 * Shortcut for anonymous pages. We don't want
-                                 * to force the creation of pages tables for
-                                 * insanely big anonymously mapped areas that
-                                 * nobody touched so far. This is important
-                                 * for doing a core dump for these mappings.
-                                 */
-                                if (!write && untouched_anonymous_page(mm,vma,start)) {
-                                        page = ZERO_PAGE(start);
-                                        break;
-                                }
-                                spin_unlock(&mm->page_table_lock);
-                                ret = __handle_mm_fault(mm, vma, start, write_access);
+                        if (write)
+                                foll_flags |= FOLL_WRITE;
 
+                        cond_resched();
+                        while (!(page = follow_page(mm, start, foll_flags))) {
+                                int ret;
+                                ret = __handle_mm_fault(mm, vma, start,
+                                                foll_flags & FOLL_WRITE);
                                 /*
                                  * The VM_FAULT_WRITE bit tells us that do_wp_page has
                                  * broken COW when necessary, even if maybe_mkwrite
@@ -984,7 +977,7 @@ int get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
                                  * subsequent page lookups as if they were reads.
                                  */
                                 if (ret & VM_FAULT_WRITE)
-                                        write_access = 0;
+                                        foll_flags &= ~FOLL_WRITE;
                                 
                                 switch (ret & ~VM_FAULT_WRITE) {
                                 case VM_FAULT_MINOR:
@@ -1000,12 +993,10 @@ int get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
                                 default:
                                         BUG();
                                 }
-                                spin_lock(&mm->page_table_lock);
                         }
                         if (pages) {
                                 pages[i] = page;
                                 flush_dcache_page(page);
-                                page_cache_get(page);
                         }
                         if (vmas)
                                 vmas[i] = vma;
@@ -1013,7 +1004,6 @@ int get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
                         start += PAGE_SIZE;
                         len--;
                 } while (len && start < vma->vm_end);
-                spin_unlock(&mm->page_table_lock);
         } while (len);
         return i;
 }