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Date: Wed, 4 Apr 2018 20:52:00 -0700
From: Matthew Wilcox <willy@infradead.org>
To: Mike Kravetz <mike.kravetz@oracle.com>
Cc: linux-mm@kvack.org, linux-fsdevel@vger.kernel.org,
	Matthew Wilcox <mawilcox@microsoft.com>, Jan Kara <jack@suse.cz>,
	Jeff Layton <jlayton@redhat.com>,
	Lukas Czerner <lczerner@redhat.com>,
	Ross Zwisler <ross.zwisler@linux.intel.com>,
	Christoph Hellwig <hch@lst.de>,
	Goldwyn Rodrigues <rgoldwyn@suse.com>,
	Nicholas Piggin <npiggin@gmail.com>,
	Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>,
	linux-nilfs@vger.kernel.org, Jaegeuk Kim <jaegeuk@kernel.org>,
	Chao Yu <yuchao0@huawei.com>,
	linux-f2fs-devel@lists.sourceforge.net,
	Oleg Drokin <oleg.drokin@intel.com>,
	Andreas Dilger <andreas.dilger@intel.com>,
	James Simmons <jsimmons@infradead.org>
Subject: Re: [PATCH v10 00/62] Convert page cache to XArray
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On Wed, Apr 04, 2018 at 09:35:46AM -0700, Mike Kravetz wrote:
> Running with this XArray series on top of next-20180329 consistently 'hangs'
> on shutdown looping (?forever?) in tag_pages_for_writeback/xas_for_each_tag.
> All I have to do is make sure there is some activity on the ext4 fs before
> shutdown.  Not sure if this is a 'next-20180329' issue or XArray issue.
> But the fact that we are looping in xas_for_each_tag looks suspicious.

Thanks for your help debugging this!  Particularly collecting the xa_dump.
I got bit by the undefined behaviour of shifting by BITS_PER_LONG,
but of course it was subtle.

The userspace testing framework wasn't catching this for a couple of
reasons; I'll work on making sure it catches this kind of thing in
the future.

I'll fold this in and post a v11 later this week or early next week.

diff --git a/include/linux/xarray.h b/include/linux/xarray.h
index eac04922eba2..f5b7e507a86f 100644
--- a/include/linux/xarray.h
+++ b/include/linux/xarray.h
@@ -904,9 +929,12 @@ static inline unsigned int xas_find_chunk(struct xa_state *xas, bool advance,
 	if (advance)
 		offset++;
 	if (XA_CHUNK_SIZE == BITS_PER_LONG) {
-		unsigned long data = *addr & (~0UL << offset);
-		if (data)
-			return __ffs(data);
+		if (offset < XA_CHUNK_SIZE) {
+			unsigned long data = *addr & (~0UL << offset);
+
+			if (data)
+				return __ffs(data);
+		}
 		return XA_CHUNK_SIZE;
 	}