Hi Hugh,

thanks for the fast reply..

Looks like the XFS file system can support files until 16 Tera
when CONFIG_LBDAF is enabled.

On XFS, 32 bit arch, s_maxbytes is actually set (CONFIG_LBDAF=y) as
17592186044415.

But if s_maxbytes doesn't have to be greater than MAX_LFS_FILESIZE,
i agree the issue should be fixed in layers above.

The fact is that everything still works correct until an index as
17592186044415 - 4096, and there can be users that could already
have so big files in use in their setup.

What do you think ?

Best regards
Angelo Dureghello

On 27/09/2015 03:36, Hugh Dickins wrote:

Let's Cc linux-fsdevel, who will be more knowledgable.

On Sun, 27 Sep 2015, angelo wrote:

quoted

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.

That's odd.  I've not checked your patch, because I think the problem
would go beyond truncate, and the root cause lie elsewhere.

My understanding is that the 32-bit
#define MAX_LFS_FILESIZE (((loff_t)PAGE_CACHE_SIZE << (BITS_PER_LONG-1))-1)
makes a page->index of -1 (or any "negative") impossible to reach.

I don't know offhand the rules for mounting a filesystem populated with
a 64-bit kernel on a 32-bit kernel, what's to happen when a too-large
file is encountered; but assume that's not the case here - you're
just running xfstests/tests/generic/308.

Is pwrite missing a check for offset beyond s_maxbytes?

Or is this filesystem-dependent?  Which filesystem?

Hugh

quoted

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

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 <redacted>
---
  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

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