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    SubjectRe: [PATCH] mm: Warn about costly page allocation
    On Mon, Jul 09, 2012 at 05:46:57PM +0900, Minchan Kim wrote:
    > > > <SNIP>
    > > > +#if defined(CONFIG_DEBUG_VM) && !defined(CONFIG_COMPACTION)
    > > > +static inline void check_page_alloc_costly_order(unsigned int order)
    > > > +{
    > > > + if (unlikely(order > PAGE_ALLOC_COSTLY_ORDER)) {
    > > > + printk_once("WARNING: You are tring to allocate %d-order page."
    > > > + " You might need to turn on CONFIG_COMPACTION\n", order);
    > > > + }
    > >
    > > WARN_ON_ONCE would tell you what is trying to satisfy the allocation.
    >
    > Do you mean that it would be better to use WARN_ON_ONCE rather than raw printk?

    Yes.

    > If so, I would like to insist raw printk because WARN_ON_ONCE could be disabled
    > by !CONFIG_BUG.
    > If I miss something, could you elaborate it more?
    >

    Ok, but all this will tell you is that *something* tried a high-order
    allocation. It will not tell you who and because it's a printk_once, it
    will also not tell you how often it's happening. You could add a
    dump_stack to capture that information.

    > >
    > > It should further check if this is a GFP_MOVABLE allocation or not and if
    > > not, then it should either be documented that compaction may only delay
    > > allocation failures and that they may need to consider reserving the memory
    > > in advance or doing something like forcing MIGRATE_RESERVE to only be used
    > > for high-order allocations.
    >
    > Okay. but I got confused you want to add above description in code directly
    > like below or write it down in comment of check_page_alloc_costly_order?
    >

    You're aiming this at embedded QA people according to your changelog so
    do whatever you think is going to be the most effective. It's already
    "known" that high-order kernel allocations are meant to be unreliable and
    apparently this is being ignored. The in-code warning could look
    something like

    if (unlikely(order > PAGE_ALLOC_COSTLY_ORDER)) {
    printk_once("%s: page allocation high-order stupidity: order:%d, mode:0x%x\n",
    current->comm, order, gfp_mask);
    if (gfp_flags & __GFP_MOVABLE) {
    printk_once("Enable compaction or whatever\n");
    dump_stack();
    } else {
    printk_once("Regular high-order kernel allocations like this will eventually start failing.");
    dump_stack();
    }
    }

    There should be a comment above it giving more information if you think
    the embedded people will actually read it. Of course, if this warning
    triggers during driver initialisation then it might be a completely useless.
    You could rate limit the warning (printk_ratelimit()) instead to be more
    effective. As I don't know what sort of device drivers you are seeing this
    problem with I can't judge what the best style of warning would be.

    --
    Mel Gorman
    SUSE Labs


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