From mboxrd@z Thu Jan  1 00:00:00 1970
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Subject: Re: [PATCH v10 00/62] Convert page cache to XArray
To: Matthew Wilcox <willy@infradead.org>
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>
References: <20180330034245.10462-1-willy@infradead.org>
 <ff6a317f-920b-62c7-9a7a-9bf235371d41@oracle.com>
 <20180405035200.GE9301@bombadil.infradead.org>
From: Mike Kravetz <mike.kravetz@oracle.com>
Message-ID: <e2b37818-53bf-8e38-82c5-d05ac2804420@oracle.com>
Date: Thu, 5 Apr 2018 11:33:29 -0700
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On 04/04/2018 08:52 PM, Matthew Wilcox wrote:
> 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.

Nice!  Added patch and it solves the hangs I observed.

-- 
Mike Kravetz

> 
> 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;
>  	}
>  
>