Messages in this thread | | | Date | Mon, 24 Aug 2009 13:50:27 -0700 (PDT) | From | David Rientjes <> | Subject | Re: [patch 2/4 -mm] flex_array: add flex_array_clear function |
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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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