Re: [PATCH] crypto: scompress - eliminate percpu scratch buffers

On 21 July 2017 at 14:31, Herbert Xu [off-list ref] wrote:

On Fri, Jul 21, 2017 at 02:24:02PM +0100, Ard Biesheuvel wrote:

quoted

OK, but that doesn't really answer any of my questions:
- Should we enforce that CRYPTO_ACOMP_ALLOC_OUTPUT is mutually
exclusive with CRYPTO_TFM_REQ_MAY_SLEEP, or should
crypto_scomp_sg_alloc() always use GFP_ATOMIC? We need one or the
other, because the current situation is buggy.

Yeah I'm not surprised that it is broken, because there are currently
no users of the API.  We should fix the acomp API.

quoted

- Could we move the scratch buffers to the request structure instead?
Or may that be allocated from softirq context as well?

Yes the IPcomp requests would be allocated from softirq context.

__acomp_request_alloc() currently uses GFP_KERNEL explicitly, so that
needs to be fixed as well then.

quoted

- Would you mind a patch that defers the allocation of the per-CPU
buffers to the first invocation of crypto_scomp_init_tfm()?

Sure.  That's what we currently do for IPcomp too.

OK.

quoted

- Would you mind a patch that makes the code only use the per-CPU
buffers if we are running atomically to begin with?

That would mean dropping the first packet so no.

I think you misunderstood me: the idea is not to allocate the per-CPU
buffers at that time, but avoiding them and use the heap directly (as
my patch does now). This way, we don't unnecessarily disable
preemption for calls executing in process context.

I.e..

if (req->base.flags & CRYPTO_TFM_REQ_MAY_SLEEP) {
  cpu = get_cpu();
  scratch_src = *per_cpu_ptr(scomp_src_scratches, cpu);
  ...
} else {
  scratch_src = (u8 *)__get_free_pages(gfp, alloc_order);
}

and make the put_cpu() at the end conditional in the same way.

In any case, my main gripe with this code is the unconditional
allocation of the scratch buffers (taking up memory and vmalloc space)
so if we can defer that to crypto_scomp_init_tfm() instead, I already
have most of what I need.

-- 
Ard.