mm: change anon_vma linking to fix multi-process server scalability issue

The old anon_vma code can lead to scalability issues with heavily forking
workloads.  Specifically, each anon_vma will be shared between the parent
process and all its child processes.

In a workload with 1000 child processes and a VMA with 1000 anonymous
pages per process that get COWed, this leads to a system with a million
anonymous pages in the same anon_vma, each of which is mapped in just one
of the 1000 processes.  However, the current rmap code needs to walk them
all, leading to O(N) scanning complexity for each page.

This can result in systems where one CPU is walking the page tables of
1000 processes in page_referenced_one, while all other CPUs are stuck on
the anon_vma lock.  This leads to catastrophic failure for a benchmark
like AIM7, where the total number of processes can reach in the tens of
thousands.  Real workloads are still a factor 10 less process intensive
than AIM7, but they are catching up.

This patch changes the way anon_vmas and VMAs are linked, which allows us
to associate multiple anon_vmas with a VMA.  At fork time, each child
process gets its own anon_vmas, in which its COWed pages will be
instantiated.  The parents' anon_vma is also linked to the VMA, because
non-COWed pages could be present in any of the children.

This reduces rmap scanning complexity to O(1) for the pages of the 1000
child processes, with O(N) complexity for at most 1/N pages in the system.
 This reduces the average scanning cost in heavily forking workloads from
O(N) to 2.

The only real complexity in this patch stems from the fact that linking a
VMA to anon_vmas now involves memory allocations.  This means vma_adjust
can fail, if it needs to attach a VMA to anon_vma structures.  This in
turn means error handling needs to be added to the calling functions.

A second source of complexity is that, because there can be multiple
anon_vmas, the anon_vma linking in vma_adjust can no longer be done under
"the" anon_vma lock.  To prevent the rmap code from walking up an
incomplete VMA, this patch introduces the VM_LOCK_RMAP VMA flag.  This bit
flag uses the same slot as the NOMMU VM_MAPPED_COPY, with an ifdef in mm.h
to make sure it is impossible to compile a kernel that needs both symbolic
values for the same bitflag.

Some test results:

Without the anon_vma changes, when AIM7 hits around 9.7k users (on a test
box with 16GB RAM and not quite enough IO), the system ends up running
>99% in system time, with every CPU on the same anon_vma lock in the
pageout code.

With these changes, AIM7 hits the cross-over point around 29.7k users.
This happens with ~99% IO wait time, there never seems to be any spike in
system time.  The anon_vma lock contention appears to be resolved.

[akpm@linux-foundation.org: cleanups]
Signed-off-by: Rik van Riel <riel@redhat.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Larry Woodman <lwoodman@redhat.com>
Cc: Lee Schermerhorn <Lee.Schermerhorn@hp.com>
Cc: Minchan Kim <minchan.kim@gmail.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Hugh Dickins <hugh.dickins@tiscali.co.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 97 insertions, 41 deletions

diff --git a/mm/mmap.c b/mm/mmap.c
index 31656147128e..6a0c15db7f60 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -437,7 +437,6 @@ __vma_link(struct mm_struct *mm, struct vm_area_struct *vma,
 {
         __vma_link_list(mm, vma, prev, rb_parent);
         __vma_link_rb(mm, vma, rb_link, rb_parent);
-        __anon_vma_link(vma);
 }
 
 static void vma_link(struct mm_struct *mm, struct vm_area_struct *vma,
@@ -499,7 +498,7 @@ __vma_unlink(struct mm_struct *mm, struct vm_area_struct *vma,
  * are necessary.  The "insert" vma (if any) is to be inserted
  * before we drop the necessary locks.
  */
-void vma_adjust(struct vm_area_struct *vma, unsigned long start,
+int vma_adjust(struct vm_area_struct *vma, unsigned long start,
         unsigned long end, pgoff_t pgoff, struct vm_area_struct *insert)
 {
         struct mm_struct *mm = vma->vm_mm;
@@ -542,6 +541,28 @@ again:			remove_next = 1 + (end > next->vm_end);
                 }
         }
 
+        /*
+         * When changing only vma->vm_end, we don't really need anon_vma lock.
+         */
+        if (vma->anon_vma && (insert || importer || start != vma->vm_start))
+                anon_vma = vma->anon_vma;
+        if (anon_vma) {
+                /*
+                 * Easily overlooked: when mprotect shifts the boundary,
+                 * make sure the expanding vma has anon_vma set if the
+                 * shrinking vma had, to cover any anon pages imported.
+                 */
+                if (importer && !importer->anon_vma) {
+                        /* Block reverse map lookups until things are set up. */
+                        importer->vm_flags |= VM_LOCK_RMAP;
+                        if (anon_vma_clone(importer, vma)) {
+                                importer->vm_flags &= ~VM_LOCK_RMAP;
+                                return -ENOMEM;
+                        }
+                        importer->anon_vma = anon_vma;
+                }
+        }
+
         if (file) {
                 mapping = file->f_mapping;
                 if (!(vma->vm_flags & VM_NONLINEAR))
@@ -567,25 +588,6 @@ again:			remove_next = 1 + (end > next->vm_end);
                 }
         }
 
-        /*
-         * When changing only vma->vm_end, we don't really need
-         * anon_vma lock.
-         */
-        if (vma->anon_vma && (insert || importer || start != vma->vm_start))
-                anon_vma = vma->anon_vma;
-        if (anon_vma) {
-                spin_lock(&anon_vma->lock);
-                /*
-                 * Easily overlooked: when mprotect shifts the boundary,
-                 * make sure the expanding vma has anon_vma set if the
-                 * shrinking vma had, to cover any anon pages imported.
-                 */
-                if (importer && !importer->anon_vma) {
-                        importer->anon_vma = anon_vma;
-                        __anon_vma_link(importer);
-                }
-        }
-
         if (root) {
                 flush_dcache_mmap_lock(mapping);
                 vma_prio_tree_remove(vma, root);
@@ -616,8 +618,11 @@ again:			remove_next = 1 + (end > next->vm_end);
                 __vma_unlink(mm, next, vma);
                 if (file)
                         __remove_shared_vm_struct(next, file, mapping);
-                if (next->anon_vma)
-                        __anon_vma_merge(vma, next);
+                /*
+                 * This VMA is now dead, no need for rmap to follow it.
+                 * Call anon_vma_merge below, outside of i_mmap_lock.
+                 */
+                next->vm_flags |= VM_LOCK_RMAP;
         } else if (insert) {
                 /*
                  * split_vma has split insert from vma, and needs
@@ -627,17 +632,25 @@ again:			remove_next = 1 + (end > next->vm_end);
                 __insert_vm_struct(mm, insert);
         }
 
-        if (anon_vma)
-                spin_unlock(&anon_vma->lock);
         if (mapping)
                 spin_unlock(&mapping->i_mmap_lock);
 
+        /*
+         * The current VMA has been set up. It is now safe for the
+         * rmap code to get from the pages to the ptes.
+         */
+        if (anon_vma && importer)
+                importer->vm_flags &= ~VM_LOCK_RMAP;
+
         if (remove_next) {
                 if (file) {
                         fput(file);
                         if (next->vm_flags & VM_EXECUTABLE)
                                 removed_exe_file_vma(mm);
                 }
+                /* Protected by mmap_sem and VM_LOCK_RMAP. */
+                if (next->anon_vma)
+                        anon_vma_merge(vma, next);
                 mm->map_count--;
                 mpol_put(vma_policy(next));
                 kmem_cache_free(vm_area_cachep, next);
@@ -653,6 +666,8 @@ again:			remove_next = 1 + (end > next->vm_end);
         }
 
         validate_mm(mm);
+
+        return 0;
 }
 
 /*
@@ -759,6 +774,7 @@ struct vm_area_struct *vma_merge(struct mm_struct *mm,
 {
         pgoff_t pglen = (end - addr) >> PAGE_SHIFT;
         struct vm_area_struct *area, *next;
+        int err;
 
         /*
          * We later require that vma->vm_flags == vm_flags,
@@ -792,11 +808,13 @@ struct vm_area_struct *vma_merge(struct mm_struct *mm,
                                 is_mergeable_anon_vma(prev->anon_vma,
                                                       next->anon_vma)) {
                                                         /* cases 1, 6 */
-                        vma_adjust(prev, prev->vm_start,
+                        err = vma_adjust(prev, prev->vm_start,
                                 next->vm_end, prev->vm_pgoff, NULL);
                 } else                                  /* cases 2, 5, 7 */
-                        vma_adjust(prev, prev->vm_start,
+                        err = vma_adjust(prev, prev->vm_start,
                                 end, prev->vm_pgoff, NULL);
+                if (err)
+                        return NULL;
                 return prev;
         }
 
@@ -808,11 +826,13 @@ struct vm_area_struct *vma_merge(struct mm_struct *mm,
                         can_vma_merge_before(next, vm_flags,
                                         anon_vma, file, pgoff+pglen)) {
                 if (prev && addr < prev->vm_end)        /* case 4 */
-                        vma_adjust(prev, prev->vm_start,
+                        err = vma_adjust(prev, prev->vm_start,
                                 addr, prev->vm_pgoff, NULL);
                 else                                    /* cases 3, 8 */
-                        vma_adjust(area, addr, next->vm_end,
+                        err = vma_adjust(area, addr, next->vm_end,
                                 next->vm_pgoff - pglen, NULL);
+                if (err)
+                        return NULL;
                 return area;
         }
 
@@ -1205,6 +1225,7 @@ munmap_back:
         vma->vm_flags = vm_flags;
         vma->vm_page_prot = vm_get_page_prot(vm_flags);
         vma->vm_pgoff = pgoff;
+        INIT_LIST_HEAD(&vma->anon_vma_chain);
 
         if (file) {
                 error = -EINVAL;
@@ -1865,6 +1886,7 @@ static int __split_vma(struct mm_struct * mm, struct vm_area_struct * vma,
 {
         struct mempolicy *pol;
         struct vm_area_struct *new;
+        int err = -ENOMEM;
 
         if (is_vm_hugetlb_page(vma) && (addr &
                                         ~(huge_page_mask(hstate_vma(vma)))))
@@ -1872,11 +1894,13 @@ static int __split_vma(struct mm_struct * mm, struct vm_area_struct * vma,
 
         new = kmem_cache_alloc(vm_area_cachep, GFP_KERNEL);
         if (!new)
-                return -ENOMEM;
+                goto out_err;
 
         /* most fields are the same, copy all, and then fixup */
         *new = *vma;
 
+        INIT_LIST_HEAD(&new->anon_vma_chain);
+
         if (new_below)
                 new->vm_end = addr;
         else {
@@ -1886,11 +1910,14 @@ static int __split_vma(struct mm_struct * mm, struct vm_area_struct * vma,
 
         pol = mpol_dup(vma_policy(vma));
         if (IS_ERR(pol)) {
-                kmem_cache_free(vm_area_cachep, new);
-                return PTR_ERR(pol);
+                err = PTR_ERR(pol);
+                goto out_free_vma;
         }
         vma_set_policy(new, pol);
 
+        if (anon_vma_clone(new, vma))
+                goto out_free_mpol;
+
         if (new->vm_file) {
                 get_file(new->vm_file);
                 if (vma->vm_flags & VM_EXECUTABLE)
@@ -1901,12 +1928,28 @@ static int __split_vma(struct mm_struct * mm, struct vm_area_struct * vma,
                 new->vm_ops->open(new);
 
         if (new_below)
-                vma_adjust(vma, addr, vma->vm_end, vma->vm_pgoff +
+                err = vma_adjust(vma, addr, vma->vm_end, vma->vm_pgoff +
                         ((addr - new->vm_start) >> PAGE_SHIFT), new);
         else
-                vma_adjust(vma, vma->vm_start, addr, vma->vm_pgoff, new);
+                err = vma_adjust(vma, vma->vm_start, addr, vma->vm_pgoff, new);
 
-        return 0;
+        /* Success. */
+        if (!err)
+                return 0;
+
+        /* Clean everything up if vma_adjust failed. */
+        new->vm_ops->close(new);
+        if (new->vm_file) {
+                if (vma->vm_flags & VM_EXECUTABLE)
+                        removed_exe_file_vma(mm);
+                fput(new->vm_file);
+        }
+ out_free_mpol:
+        mpol_put(pol);
+ out_free_vma:
+        kmem_cache_free(vm_area_cachep, new);
+ out_err:
+        return err;
 }
 
 /*
@@ -2116,6 +2159,7 @@ unsigned long do_brk(unsigned long addr, unsigned long len)
                 return -ENOMEM;
         }
 
+        INIT_LIST_HEAD(&vma->anon_vma_chain);
         vma->vm_mm = mm;
         vma->vm_start = addr;
         vma->vm_end = addr + len;
@@ -2252,10 +2296,11 @@ struct vm_area_struct *copy_vma(struct vm_area_struct **vmap,
                 if (new_vma) {
                         *new_vma = *vma;
                         pol = mpol_dup(vma_policy(vma));
-                        if (IS_ERR(pol)) {
-                                kmem_cache_free(vm_area_cachep, new_vma);
-                                return NULL;
-                        }
+                        if (IS_ERR(pol))
+                                goto out_free_vma;
+                        INIT_LIST_HEAD(&new_vma->anon_vma_chain);
+                        if (anon_vma_clone(new_vma, vma))
+                                goto out_free_mempol;
                         vma_set_policy(new_vma, pol);
                         new_vma->vm_start = addr;
                         new_vma->vm_end = addr + len;
@@ -2271,6 +2316,12 @@ struct vm_area_struct *copy_vma(struct vm_area_struct **vmap,
                 }
         }
         return new_vma;
+
+ out_free_mempol:
+        mpol_put(pol);
+ out_free_vma:
+        kmem_cache_free(vm_area_cachep, new_vma);
+        return NULL;
 }
 
 /*
@@ -2348,6 +2399,7 @@ int install_special_mapping(struct mm_struct *mm,
         if (unlikely(vma == NULL))
                 return -ENOMEM;
 
+        INIT_LIST_HEAD(&vma->anon_vma_chain);
         vma->vm_mm = mm;
         vma->vm_start = addr;
         vma->vm_end = addr + len;
@@ -2448,6 +2500,7 @@ static void vm_lock_mapping(struct mm_struct *mm, struct address_space *mapping)
 int mm_take_all_locks(struct mm_struct *mm)
 {
         struct vm_area_struct *vma;
+        struct anon_vma_chain *avc;
         int ret = -EINTR;
 
         BUG_ON(down_read_trylock(&mm->mmap_sem));
@@ -2465,7 +2518,8 @@ int mm_take_all_locks(struct mm_struct *mm)
                 if (signal_pending(current))
                         goto out_unlock;
                 if (vma->anon_vma)
-                        vm_lock_anon_vma(mm, vma->anon_vma);
+                        list_for_each_entry(avc, &vma->anon_vma_chain, same_vma)
+                                vm_lock_anon_vma(mm, avc->anon_vma);
         }
 
         ret = 0;
@@ -2520,13 +2574,15 @@ static void vm_unlock_mapping(struct address_space *mapping)
 void mm_drop_all_locks(struct mm_struct *mm)
 {
         struct vm_area_struct *vma;
+        struct anon_vma_chain *avc;
 
         BUG_ON(down_read_trylock(&mm->mmap_sem));
         BUG_ON(!mutex_is_locked(&mm_all_locks_mutex));
 
         for (vma = mm->mmap; vma; vma = vma->vm_next) {
                 if (vma->anon_vma)
-                        vm_unlock_anon_vma(vma->anon_vma);
+                        list_for_each_entry(avc, &vma->anon_vma_chain, same_vma)
+                                vm_unlock_anon_vma(avc->anon_vma);
                 if (vma->vm_file && vma->vm_file->f_mapping)
                         vm_unlock_mapping(vma->vm_file->f_mapping);
         }