RE: [PATCH 2/6] x86: hv: hv_init.c: Replace alloc_page() with kmem_cache_alloc()

From: Vitaly Kuznetsov <vkuznets@redhat.com>
To: Michael Kelley <mikelley@microsoft.com>,
	"m.maya.nakamura" <m.maya.nakamura@gmail.com>
Cc: KY Srinivasan <kys@microsoft.com>,
	Haiyang Zhang <haiyangz@microsoft.com>,
	Stephen Hemminger <sthemmin@microsoft.com>,
	"sashal\@kernel.org" <sashal@kernel.org>,
	"x86\@kernel.org" <x86@kernel.org>,
	"linux-hyperv\@vger.kernel.org" <linux-hyperv@vger.kernel.org>,
	"linux-kernel\@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: RE: [PATCH 2/6] x86: hv: hv_init.c: Replace alloc_page() with kmem_cache_alloc()
Date: Fri, 10 May 2019 09:21:35 -0400	[thread overview]
Message-ID: <87r296qwbk.fsf@vitty.brq.redhat.com> (raw)
In-Reply-To: <MN2PR21MB1232C6ABA5DAC847C8A910E1D70C0@MN2PR21MB1232.namprd21.prod.outlook.com>

Michael Kelley <mikelley@microsoft.com> writes:

> From: Vitaly Kuznetsov <vkuznets@redhat.com>  Sent: Wednesday, May 8, 2019 7:55 AM
>> >>
>> >> Sorry, my bad: I meant to say "not cache-like" (these allocations are
>> >> not 'cache') but the typo made it completely incomprehensible.
>> >
>> > No worries! Thank you for sharing your thoughts with me, Vitaly.
>> >
>> > Do you know of any alternatives to kmem_cache that can allocate memory
>> > in a specified size (different than a guest page size) with alignment?
>> > Memory allocated by alloc_page() is aligned but limited to the guest
>> > page size, and kmalloc() can allocate a particular size but it seems
>> > that it does not guarantee alignment. I am asking this while considering
>> > the changes for architecture independent code.
>> >
>> 
>> I think we can consider these allocations being DMA-like (because
>> Hypervisor accesses this memory too) so you can probably take a look at
>> dma_pool_create()/dma_pool_alloc() and friends.
>> 
>
> I've taken a look at dma_pool_create(), and it takes a "struct device"
> argument with which the DMA pool will be associated.  That probably
> makes DMA pools a bad choice for this usage.  Pages need to be allocated
> pretty early during boot for Hyper-V communication, and even if the
> device subsystem is initialized early enough to create a fake device,
> such a dependency seems rather dubious.

We can probably use dma_pool_create()/dma_pool_alloc() from vmbus module
but these 'early' allocations may not have a device to bind to indeed.

>
> kmem_cache_create/alloc() seems like the only choice to get
> guaranteed alignment.  Do you see any actual problem with
> using kmem_cache_*, other than the naming?  It seems like these
> kmem_cache_*  functions really just act as a sub-allocator for
> known-size allocations, and "cache" is a common usage
> pattern, but not necessarily the only usage pattern.

Yes, it's basically the name - it makes it harder to read the code and
some future refactoring of kmem_cache_* may not take our use-case into
account (as we're misusing the API). We can try renaming it to something
generic of course and see what -mm people have to say :-)

-- 
Vitaly