Re: [PATCH] hugetlbfs: Add new rw_semaphore to fix truncate/read race

Hillf Danton
2012-02-28T12:17:30Z

On Tue, Feb 28, 2012 at 6:15 PM, Aneesh Kumar K.V
<aneesh.kumar@linux.vnet.ibm.com> wrote:
>
> Will update the patch with these details
>

A scratch is cooked, based on the -next tree, for accelerating your redelivery,
if you like it, in which i_mutex is eliminated directly and page lock is used.

-hd

--- a/fs/hugetlbfs/inode.c Tue Feb 28 19:43:32 2012
+++ b/fs/hugetlbfs/inode.c Tue Feb 28 19:56:50 2012
@@ -245,17 +245,10 @@ static ssize_t hugetlbfs_read(struct fil
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;
@@ -263,6 +256,8 @@ static ssize_t hugetlbfs_read(struct fil

/* nr is the maximum number of bytes to copy from this page */
nr = huge_page_size(h);
+ isize = i_size_read(inode);
+ end_index = isize >> huge_page_shift(h);
if (index >= end_index) {
if (index > end_index)
goto out;
@@ -274,7 +269,7 @@ static ssize_t hugetlbfs_read(struct fil
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 +281,30 @@ static ssize_t hugetlbfs_read(struct fil
else
ra = 0;
} else {
+ unlock_page(page);
+
+ /* Without i_mutex held, check isize again */
+ nr = huge_page_size(h);
+ isize = i_size_read(inode);
+ end_index = isize >> huge_page_shift(h);
+ if (index == end_index) {
+ nr = isize & ~huge_page_mask(h);
+ if (nr <= offset) {
+ page_cache_release(page);
+ goto out;
+ }
+ }
+ nr -= offset;
/*
* 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 +314,12 @@ static ssize_t hugetlbfs_read(struct fil
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;
}

--