RE: [PATCH] btrfs file write debugging patch

[Chris Mason <chris.mason@oracle.com>]

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