[PATCH] mm: fix cpu hangs on truncating last page of a 16t sparse file

angelo
2015-09-26T23:02:21Z

Hi all,

running xfstests, generic 308 on whatever 32bit arch is possible
to observe cpu to hang near 100% on unlink.
The test removes a sparse file of length 16tera where only the last
4096 bytes block is mapped.
At line 265 of truncate.c there is a
if (index >= end)
break;
But if index is, as in this case, a 4294967295, it match -1 used as
eof. Hence the cpu loops 100% just after.

-------------------

On 32bit archs, with CONFIG_LBDAF=y, if truncating last page
of a 16tera file, "index" variable is set to 4294967295, and hence
matches with -1 used as EOF value. This result in an inifite loop
when unlink is executed on this file.

Signed-off-by: Angelo Dureghello <angelo@sysam.it>
---
mm/truncate.c | 11 ++++++-----
1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/mm/truncate.c b/mm/truncate.c
index 76e35ad..3751034 100644
--- a/mm/truncate.c
+++ b/mm/truncate.c
@@ -283,14 +283,15 @@ void truncate_inode_pages_range(struct
address_space *mapping,
pagevec_remove_exceptionals(&pvec);
pagevec_release(&pvec);
cond_resched();
- index++;
+ if (index < end)
+ index++;
}

if (partial_start) {
struct page *page = find_lock_page(mapping, start - 1);
if (page) {
unsigned int top = PAGE_CACHE_SIZE;
- if (start > end) {
+ if (start > end && end != -1) {
/* Truncation within a single page */
top = partial_end;
partial_end = 0;
@@ -322,7 +323,7 @@ void truncate_inode_pages_range(struct address_space
*mapping,
* If the truncation happened within a single page no pages
* will be released, just zeroed, so we can bail out now.
*/
- if (start >= end)
+ if (start >= end && end != -1)
return;

index = start;
@@ -337,7 +338,7 @@ void truncate_inode_pages_range(struct address_space
*mapping,
index = start;
continue;
}
- if (index == start && indices[0] >= end) {
+ if (index == start && (indices[0] >= end && end != -1)) {
/* All gone out of hole to be punched, we're
done */
pagevec_remove_exceptionals(&pvec);
pagevec_release(&pvec);
@@ -348,7 +349,7 @@ void truncate_inode_pages_range(struct address_space
*mapping,

/* We rely upon deletion not changing
page->index */
index = indices[i];
- if (index >= end) {
+ if (index >= end && (end != -1)) {
/* Restart punch to make sure all gone */
index = start - 1;
break;
--
2.5.3