From mboxrd@z Thu Jan 1 00:00:00 1970 From: Chris Mason Subject: RE: [PATCH] btrfs file write debugging patch Date: Thu, 03 Mar 2011 20:51:55 -0500 Message-ID: <1299203447-sup-9359@think> References: <1865303E0DED764181A9D882DEF65FB68662CD02C8@shsmsx502.ccr.corp.intel.com> Content-Type: text/plain; charset=UTF-8 Cc: Mitch Harder , Xin Zhong , "linux-btrfs@vger.kernel.org" To: "Zhong, Xin" Return-path: In-reply-to: <1865303E0DED764181A9D882DEF65FB68662CD02C8@shsmsx502.ccr.corp.intel.com> List-ID: 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 > #include > #include > 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