linux-kernel - Re: kernel BUGs when removing largish files with the SLOB allocator

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Date:	Wed, 13 Sep 2006 12:16:02 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	David Howells <dhowells@...hat.com>
CC:	Matt Mackall <mpm@...enic.com>, Aubrey <aubreylee@...il.com>,
	linux-kernel@...r.kernel.org, davidm@...pgear.com,
	gerg@...pgear.com
Subject: Re: kernel BUGs when removing largish files with the SLOB allocator

David Howells wrote:
> Matt Mackall <mpm@...enic.com> wrote:
> 
> 
>>>+		for (i = 0; i < (1 << bb->order); i++) {
>>>+			SetPageSlab(page);
>>>+			page++;
>>>+		}
>>
>>for ( ; page < page + (1 << bb->order), page++)
>>      SetPageSlab(page);
> 
> 
> Ugh.  No.  You can't do that.  "page < page + X" will be true until "page + X"
> wraps the end of memory.
> 
> 
>>>+				for (i = 0; i < (1 << bb->order); i++) {
>>>+					if (!TestClearPageSlab(page))
>>>+						BUG();
>>>+					page++;
>>>+				}
>>
>>Please drop the BUG. We've already established it's on our lists by
>>this point.
> 
> 
> I disagree.  Let's catch accidental reuse of pages.  It should, however, be
> marked unlikely().

If you do this, the biggest problem with those ops is that they are atomic,
and the latter also requires strong memory barriers. Don't use RMW variants,
and use __ prepended iff you are the only user of the page at this point.

-- 
SUSE Labs, Novell Inc.
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