linux-kernel - Re: [patch 2/4 -mm] flex_array: add flex_array

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Date:	Mon, 24 Aug 2009 13:50:27 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Dave Hansen <dave@...ux.vnet.ibm.com>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [patch 2/4 -mm] flex_array: add flex_array_clear function

On Mon, 24 Aug 2009, Dave Hansen wrote:

> > Sure, if you never increase FLEX_ARRAY_PART_SIZE.  Otherwise, doing
> > 
> > static char zero_part[FLEX_ARRAY_PART_SIZE] = {
> > 	[0 ... FLEX_ARRAY_PART_SIZE - 1] = 0
> > };
> > 
> > and using flex_array_put(fa, element_nr, &zero_part) would work although 
> > you're trading off cleaner, yet not more efficient, code at the cost of 
> > FLEX_ARRAY_PART_SIZE wasted memory and memcpy() being slower than 
> > memset().
> 
> Yeah, that's true.  How about using the get() function?
> 
> int flex_array_clear(struct flex_array *fa, unsigned int element_nr)
> {
> 	void *element = flex_array_get(fa, element_nr);
> 	memset(element, FLEX_ARRAY_FREE, fa->element_size);
> }
> 

The idea was to eventually be able to distinguish between 
use-uninitialized and use-after-free and flex_array_clear() was a 
convenient way of providing an interface to identify the later.  So when 
an array is fully initialized (or fully cleared after a previous use where 
all elements we're used), you couldn't do flex_array_clear() on an element 
before flex_array_put() if its part isn't allocated yet with this 
implementation.

> It'll keep us from having to keep around a zero'd element. 
> 
> But, I guess we could also do:
> 
> 	struct flex_array_part *zero_part = empty_zero_page;
> 
> And use a BUILD_BUG_ON(FLEX_ARRAY_PART_SIZE > PAGE_SIZE).  But the whole
> point of this was to have elements that are smaller than PAGE_SIZE.
> Having that as a constraint doesn't seem too bad. :)
> 

Hmm, I think being able to increase FLEX_ARRAY_PART_SIZE is eventually 
going to become an integral part of the entire library so that it supports 
larger number of entries (and order-1 allocations aren't as difficult with 
anti-fragmentation), especially for those with larger element sizes.  
Otherwise, there's no need for FLEX_ARRAY_PART_SIZE in the first place.
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