hugetlbfs: avoid taking i_mutex from hugetlbfs_read()

Taking i_mutex in hugetlbfs_read() can result in deadlock with mmap as
explained below

 Thread A:
  read() on hugetlbfs
   hugetlbfs_read() called
    i_mutex grabbed
     hugetlbfs_read_actor() called
      __copy_to_user() called
       page fault is triggered
 Thread B, sharing address space with A:
  mmap() the same file
   ->mmap_sem is grabbed on task_B->mm->mmap_sem
    hugetlbfs_file_mmap() is called
     attempt to grab ->i_mutex and block waiting for A to give it up
 Thread A:
  pagefault handled blocked on attempt to grab task_A->mm->mmap_sem,
 which happens to be the same thing as task_B->mm->mmap_sem.  Block waiting
 for B to give it up.

AFAIU the i_mutex locking was added to hugetlbfs_read() as per
http://lkml.indiana.edu/hypermail/linux/kernel/0707.2/3066.html to take
care of the race between truncate and read.  This patch fixes this by
looking at page->mapping under lock_page() (find_lock_page()) to ensure
that the inode didn't get truncated in the range during a parallel read.

Ideally we can extend the patch to make sure we don't increase i_size in
mmap.  But that will break userspace, because applications will now have
to use truncate(2) to increase i_size in hugetlbfs.

Based on the original patch from Hillf Danton.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Cc: Hillf Danton <dhillf@gmail.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Hugh Dickins <hughd@google.com>
Cc: <stable@kernel.org>		[everything after 2007 :)]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 9 insertions, 16 deletions

diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
index b7bc7868c7b5..19654cfe780b 100644
--- a/fs/hugetlbfs/inode.c
+++ b/fs/hugetlbfs/inode.c
@@ -245,17 +245,10 @@ static ssize_t hugetlbfs_read(struct file *filp, char __user *buf,
         loff_t isize;
         ssize_t retval = 0;
 
-        mutex_lock(&inode->i_mutex);
-
         /* validate length */
         if (len == 0)
                 goto out;
 
-        isize = i_size_read(inode);
-        if (!isize)
-                goto out;
-
-        end_index = (isize - 1) >> huge_page_shift(h);
         for (;;) {
                 struct page *page;
                 unsigned long nr, ret;
@@ -263,18 +256,21 @@ static ssize_t hugetlbfs_read(struct file *filp, char __user *buf,
 
                 /* nr is the maximum number of bytes to copy from this page */
                 nr = huge_page_size(h);
+                isize = i_size_read(inode);
+                if (!isize)
+                        goto out;
+                end_index = (isize - 1) >> huge_page_shift(h);
                 if (index >= end_index) {
                         if (index > end_index)
                                 goto out;
                         nr = ((isize - 1) & ~huge_page_mask(h)) + 1;
-                        if (nr <= offset) {
+                        if (nr <= offset)
                                 goto out;
-                        }
                 }
                 nr = nr - offset;
 
                 /* Find the page */
-                page = find_get_page(mapping, index);
+                page = find_lock_page(mapping, index);
                 if (unlikely(page == NULL)) {
                         /*
                          * We have a HOLE, zero out the user-buffer for the
@@ -286,17 +282,18 @@ static ssize_t hugetlbfs_read(struct file *filp, char __user *buf,
                         else
                                 ra = 0;
                 } else {
+                        unlock_page(page);
+
                         /*
                          * We have the page, copy it to user space buffer.
                          */
                         ra = hugetlbfs_read_actor(page, offset, buf, len, nr);
                         ret = ra;
+                        page_cache_release(page);
                 }
                 if (ra < 0) {
                         if (retval == 0)
                                 retval = ra;
-                        if (page)
-                                page_cache_release(page);
                         goto out;
                 }
 
@@ -306,16 +303,12 @@ static ssize_t hugetlbfs_read(struct file *filp, char __user *buf,
                 index += offset >> huge_page_shift(h);
                 offset &= ~huge_page_mask(h);
 
-                if (page)
-                        page_cache_release(page);
-
                 /* short read or no more work */
                 if ((ret != nr) || (len == 0))
                         break;
         }
 out:
         *ppos = ((loff_t)index << huge_page_shift(h)) + offset;
-        mutex_unlock(&inode->i_mutex);
         return retval;
 }