mm: madvise avoid exclusive mmap_sem

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Gitweb:     
http://git.kernel.org/git/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=0a27a14a62921b438bb6f33772690d345a089be6
Commit:     0a27a14a62921b438bb6f33772690d345a089be6
Parent:     b4169525bc2336ea6581c6ff2aa88b2671e3b9f9
Author:     Nick Piggin <[EMAIL PROTECTED]>
AuthorDate: Sun May 6 14:49:53 2007 -0700
Committer:  Linus Torvalds <[EMAIL PROTECTED]>
CommitDate: Mon May 7 12:12:54 2007 -0700

    mm: madvise avoid exclusive mmap_sem
    
    Avoid down_write of the mmap_sem in madvise when we can help it.
    
    Acked-by: Hugh Dickins <[EMAIL PROTECTED]>
    Signed-off-by: Nick Piggin <[EMAIL PROTECTED]>
    Cc: Rik van Riel <[EMAIL PROTECTED]>
    Signed-off-by: Andrew Morton <[EMAIL PROTECTED]>
    Signed-off-by: Linus Torvalds <[EMAIL PROTECTED]>
---
 mm/madvise.c |   33 +++++++++++++++++++++++++++++----
 1 files changed, 29 insertions(+), 4 deletions(-)

diff --git a/mm/madvise.c b/mm/madvise.c
index 603c525..e75096b 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -12,6 +12,24 @@
 #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;
 }
-
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