Messages in this thread | | | Date | Wed, 22 Apr 2009 18:11:51 +0100 | From | Mel Gorman <> | Subject | Re: [PATCH 02/22] Do not sanity check order in the fast path |
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On Wed, Apr 22, 2009 at 09:13:11AM -0700, Dave Hansen wrote: > On Wed, 2009-04-22 at 14:53 +0100, Mel Gorman wrote: > > No user of the allocator API should be passing in an order >= MAX_ORDER > > but we check for it on each and every allocation. Delete this check and > > make it a VM_BUG_ON check further down the call path. > > Should we get the check re-added to some of the upper-level functions, > then? Perhaps __get_free_pages() or things like alloc_pages_exact()? >
I don't think so, no. It just moves the source of the text bloat and for the few callers that are asking for something that will never succeed.
> I'm selfishly thinking of what I did in profile_init(). Can I slab > alloc it? Nope. Page allocator? Nope. Oh, well, try vmalloc(): > > prof_buffer = kzalloc(buffer_bytes, GFP_KERNEL); > if (prof_buffer) > return 0; > > prof_buffer = alloc_pages_exact(buffer_bytes, GFP_KERNEL|__GFP_ZERO); > if (prof_buffer) > return 0; > > prof_buffer = vmalloc(buffer_bytes); > if (prof_buffer) > return 0; > > free_cpumask_var(prof_cpu_mask); > return -ENOMEM; >
Can this ever actually be asking for an order larger than MAX_ORDER though? If so, you're condemning it to always behave poorly.
> Same thing in __kmalloc_section_memmap(): > > page = alloc_pages(GFP_KERNEL|__GFP_NOWARN, get_order(memmap_size)); > if (page) > goto got_map_page; > > ret = vmalloc(memmap_size); > if (ret) > goto got_map_ptr; >
If I'm reading that right, the order will never be a stupid order. It can fail for higher orders in which case it falls back to vmalloc() . For example, to hit that limit, the section size for a 4K kernel, maximum usable order of 10, the section size would need to be 256MB (assuming struct page size of 64 bytes). I don't think it's ever that size and if so, it'll always be sub-optimal which is a poor choice to make.
> I depend on the allocator to tell me when I've fed it too high of an > order. If we really need this, perhaps we should do an audit and then > add a WARN_ON() for a few releases to catch the stragglers. >
I consider it buggy to ask for something so large that you always end up with the worst option - vmalloc(). How about leaving it as a VM_BUG_ON to get as many reports as possible on who is depending on this odd behaviour?
If there are users with good reasons, then we could convert this to WARN_ON to fix up the callers. I suspect that the allocator can already cope with recieving a stupid order silently but slowly. It should go all the way to the bottom and just never find anything useful and return NULL. zone_watermark_ok is the most dangerous looking part but even it should never get to MAX_ORDER because it should always find there are not enough free pages and return before it overruns.
-- Mel Gorman Part-time Phd Student Linux Technology Center University of Limerick IBM Dublin Software Lab
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