Messages in this thread | | | Date | Tue, 26 Jun 2012 21:15:13 +0100 | From | Mel Gorman <> | Subject | Re: [PATCH 1/4] mm: introduce compaction and migration for virtio ballooned pages |
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On Tue, Jun 26, 2012 at 06:52:58PM +0200, Andi Kleen wrote: > > > > What shocked me actually is that VM_BUG_ON code is executed on > > !CONFIG_DEBUG_VM builds and has been since 2.6.36 due to commit [4e60c86bd: > > gcc-4.6: mm: fix unused but set warnings]. I thought the whole point of > > VM_BUG_ON was to avoid expensive and usually unnecessary checks. Andi, > > was this deliberate? > > The idea was that the compiler optimizes it away anyways. > > I'm not fully sure what putback_balloon_page does, but if it just tests > a bit (without non variable test_bit) it should be ok. >
This was the definition before
#ifdef CONFIG_DEBUG_VM #define VM_BUG_ON(cond) BUG_ON(cond) #else #define VM_BUG_ON(cond) do { } while (0) #endif
and now it's
#ifdef CONFIG_DEBUG_VM #define VM_BUG_ON(cond) BUG_ON(cond) #else #define VM_BUG_ON(cond) do { (void)(cond); } while (0) #endif
How is the compiler meant to optimise away "cond" if it's a function call?
In the old definition VM_BUG_ON did nothing and the intention was that the "cond" should never had any side-effects. It was to be used for potentially expensive tests to catch additional issues in DEBUG_VM kernels. My concern is that after commit 4e60c86bd that the VM doing these additional checks unnecesarily with a performance hit. In most cases the checks are small but in others such as free_pages we are calling virt_addr_valid() which is heavier.
What did I miss? If nothing, then I will revert this particular change and Rafael will need to be sure his patch is not using VM_BUG_ON to call a function with side-effects.
-- Mel Gorman SUSE Labs
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