linux-kernel - RE: [PATCH 2/6] x86: hv: hv_init.c: Replace alloc_page() with kmem_cache

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Message-ID: <8736lmqk3e.fsf@vitty.brq.redhat.com>
Date:   Fri, 10 May 2019 13:45:41 -0400
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Michael Kelley <mikelley@...rosoft.com>,
        "m.maya.nakamura" <m.maya.nakamura@...il.com>
Cc:     KY Srinivasan <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "sashal\@kernel.org" <sashal@...nel.org>,
        "x86\@kernel.org" <x86@...nel.org>,
        "linux-hyperv\@vger.kernel.org" <linux-hyperv@...r.kernel.org>,
        "linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH 2/6] x86: hv: hv_init.c: Replace alloc_page() with kmem_cache_alloc()

Michael Kelley <mikelley@...rosoft.com> writes:

> From: Vitaly Kuznetsov <vkuznets@...hat.com>  Sent: Friday, May 10, 2019 6:22 AM
>> >>
>> >> 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 :-)
>> 
>
> This makes me think of creating Hyper-V specific alloc/free functions
> that wrap whatever the backend allocator actually is.  So we have
> hv_alloc_hyperv_page() and hv_free_hyperv_page().  That makes the
> code very readable and the intent is super clear.
>
> As for the backend allocator, an alternative is to write our own simple
> allocator.  It maintains a single free list.  If hv_alloc_hyperv_page() is
> called, and the free list is empty, do alloc_page() and break it up into
> Hyper-V sized pages to replenish the free list.  (On x86, these end up
> being 1-for-1 operations.)  hv_free_hyperv_page() just puts the Hyper-V
> page back on the free list.  Don't worry trying to combine and do
> free_page() since there's very little free'ing done anyway.  And I'm
> assuming GPF_KERNEL is all we need.
>
> If in the future Linux provides an alternate general-purpose allocator
> that guarantees alignment, we can ditch the simple allocator and use
> the new mechanism with some simple code changes in one place.
>
> Thoughts?
>

+1 for adding wrappers and if the allocator turns out to be more-or-less
trivial I think we can live with that for the time being.

-- 
Vitaly