Why does “page allocation failure” occur whereas there are still “58*4096kB (C)” could be used?

[Valdis Kl=?utf-8?Q?=c4=93?=tnieks]

On Thu, 18 Jun 2020 14:21:05 +0800, 孙世龙 said:

> Why does "page allocation failure" occur whereas there are still "58*4096kB
> (C)"(*I think it indicates there are 58 order 10 memory could be used*)
> could be used?
>
> Here is the related log:
>
> [ 2161.623563] xxxx: page allocation failure: order:10,
> mode:0x2084020(GFP_ATOMIC|__GFP_COMP)

Most likely, the allocation wanted some other type of allocation.
The (C) on the order-10 says it's an CMA area.

        static const char types[MIGRATE_TYPES] = {
                [MIGRATE_UNMOVABLE]     = 'U',
                [MIGRATE_MOVABLE]       = 'M',
                [MIGRATE_RECLAIMABLE]   = 'E',
                [MIGRATE_HIGHATOMIC]    = 'H',
#ifdef CONFIG_CMA
                [MIGRATE_CMA]           = 'C',
#endif
#ifdef CONFIG_MEMORY_ISOLATION
                [MIGRATE_ISOLATE]       = 'I',
#endif

If the call was for an unmovable, movable, reclaimable, or highatomic
allocation, you lose.

If you look at the source for alloc_ap_req(), you find it wants GFP_ATOMIC, not
CMA.  And your box is fresh out of contiguous order-10 spaces that aren't CMA,
and you're down to your last 3 order-9 flagged as (UEC).

I admit I find it a tad suspicious that the USB gadget driver asks for a 4M
chunk of memory.  Does USB actually support single transfers that large? (I'm
not a USB expert)

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