Excerpts from Zhong, Xin's message of 2011-03-02 05:58:49 -0500:
> I downloaded openmotif and run the command as Mitch mentioned and was able to recreate the problem locally. And I managed to simplify the command into a very simple program which can capture the problem easily. See below code:
>
> #include <sys/types.h>
> #include <sys/stat.h>
> #include <fcntl.h>
> static char a[4096*3];
> int main()
> {
> int fd = open("out", O_WRONLY|O_CREAT|O_TRUNC, 0666);
> write(fd,a+1, 4096*2);
> exit(0);
> }
>
> It seems that if we give an unaligned address to btrfs write and the buffer reside on more than 2 pages. It will trigger this bug.
> If we give an aligned address to btrfs write, it works well no matter how many pages are given.
>
> I use ftrace to observe it. It seems iov_iter_fault_in_readable do not trigger pagefault handling when the address is not aligned. I do not quite understand the reason behind it. But the solution should be to process the page one by one. And that's also what generic file write routine does.
>
> Any suggestion are welcomed. Thanks!
Great job guys. I'm using this on top of my debugging patch. It passes
the unaligned test but I'll give it a real run tonight and look for
other problems.
(This is almost entirely untested, please don't use it quite yet)
-chris
diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
index 89a6a26..6a44add 100644
--- a/fs/btrfs/file.c
+++ b/fs/btrfs/file.c
@@ -1039,6 +1038,14 @@ static ssize_t btrfs_file_aio_write(struct kiocb *iocb,
copied = btrfs_copy_from_user(pos, num_pages,
write_bytes, pages, &i);
+
+ /*
+ * if we have trouble faulting in the pages, fall
+ * back to one page at a time
+ */
+ if (copied < write_bytes)
+ nrptrs = 1;
+
if (copied == 0)
dirty_pages = 0;
else
--
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