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    SubjectRe: [PATCH 02/22] Do not sanity check order in the fast path
    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

    > 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

    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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      Last update: 2009-04-22 19:15    [W:0.022 / U:12.244 seconds]
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