Re: [PATCH v2 3/3] percpu: improve allocation success rate for non-GFP_KERNEL callers

[Tahsin Erdogan <tahsin@google.com>]

On Mon, Feb 27, 2017 at 1:52 AM, Michal Hocko <mhocko@kernel.org> wrote:
> On Sat 25-02-17 20:38:29, Tahsin Erdogan wrote:
>> When pcpu_alloc() is called with gfp != GFP_KERNEL, the likelihood of
>> a failure is higher than GFP_KERNEL case. This is mainly because
>> pcpu_alloc() relies on previously allocated reserves and does not make
>> an effort to add memory to its pools for non-GFP_KERNEL case.
>
> Who is going to use a different mask?

blkg_create() makes a call with a non-GFP_KERNEL mask:
   new_blkg = blkg_alloc(blkcg, q, GFP_NOWAIT | __GFP_NOWARN);

which turns into a call stack like below:

__vmalloc+0x45/0x50
pcpu_mem_zalloc+0x50/0x80
pcpu_populate_chunk+0x3b/0x380
pcpu_alloc+0x588/0x6e0
__alloc_percpu_gfp+0xd/0x10
__percpu_counter_init+0x55/0xc0
blkg_alloc+0x76/0x230
blkg_create+0x489/0x670
blkg_lookup_create+0x9a/0x230
generic_make_request_checks+0x7dd/0x890
generic_make_request+0x1f/0x180
submit_bio+0x61/0x120


> We already have __vmalloc_gfp, why this cannot be used? Also note that
> vmalloc dosn't really support arbitrary gfp flags. One have to be really
> careful because there are some internal allocations which are hardcoded
> GFP_KERNEL. Also this patch doesn't really add any new callers so it is
> hard to tell whether what you do actually makes sense and is correct.

Did you mean to say __vmalloc? If so, yes, I should use that.

By the way, I now noticed the might_sleep() in alloc_vmap_area() which makes
it unsafe to call vmalloc* in GFP_ATOMIC contexts. It was added recently:

commit 5803ed292e63 ("mm: mark all calls into the vmalloc subsystem as
potentially sleeping")

Any suggestions on how to deal with that? For instance, would it be
safe to replace it with:

might_sleep_if(gfpflags_allow_blocking(gfp_mask));

and then skip purge_vmap_area_lazy() if blocking is not allowed?

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