mm: madvise avoid exclusive mmap_sem

Avoid down_write of the mmap_sem in madvise when we can help it.

Acked-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Nick Piggin <npiggin@suse.de>
Cc: Rik van Riel <riel@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/madvise.c b/mm/madvise.c
index 603c525..e75096b 100644 (file)
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -11,6 +11,24 @@
 #include <linux/mempolicy.h>
 #include <linux/hugetlb.h>
 
+/*
+ * Any behaviour which results in changes to the vma->vm_flags needs to
+ * take mmap_sem for writing. Others, which simply traverse vmas, need
+ * to only take it for reading.
+ */
+static int madvise_need_mmap_write(int behavior)
+{
+       switch (behavior) {
+       case MADV_REMOVE:
+       case MADV_WILLNEED:
+       case MADV_DONTNEED:
+               return 0;
+       default:
+               /* be safe, default to 1. list exceptions explicitly */
+               return 1;
+       }
+}
+
 /*
  * We can potentially split a vm area into separate
  * areas, each area with its own behavior.
@@ -183,9 +201,9 @@ static long madvise_remove(struct vm_area_struct *vma,
                        + ((loff_t)vma->vm_pgoff << PAGE_SHIFT);
 
        /* vmtruncate_range needs to take i_mutex and i_alloc_sem */
-       up_write(&current->mm->mmap_sem);
+       up_read(&current->mm->mmap_sem);
        error = vmtruncate_range(mapping->host, offset, endoff);
-       down_write(&current->mm->mmap_sem);
+       down_read(&current->mm->mmap_sem);
        return error;
 }
 
@@ -270,7 +288,10 @@ asmlinkage long sys_madvise(unsigned long start, size_t len_in, int behavior)
        int error = -EINVAL;
        size_t len;
 
-       down_write(&current->mm->mmap_sem);
+       if (madvise_need_mmap_write(behavior))
+               down_write(&current->mm->mmap_sem);
+       else
+               down_read(&current->mm->mmap_sem);
 
        if (start & ~PAGE_MASK)
                goto out;
@@ -332,6 +353,10 @@ asmlinkage long sys_madvise(unsigned long start, size_t len_in, int behavior)
                        vma = find_vma(current->mm, start);
        }
 out:
-       up_write(&current->mm->mmap_sem);
+       if (madvise_need_mmap_write(behavior))
+               up_write(&current->mm->mmap_sem);
+       else
+               up_read(&current->mm->mmap_sem);
+
        return error;
 }