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Date:   Tue, 14 Apr 2020 22:13:44 +1000
From:   Nicholas Piggin <npiggin@gmail.com>
Subject: Re: [PATCH v2 4/4] mm/vmalloc: Hugepage vmalloc mappings
To:     Christoph Hellwig <hch@infradead.org>
Cc:     Borislav Petkov <bp@alien8.de>,
        Catalin Marinas <catalin.marinas@arm.com>,
        "H. Peter Anvin" <hpa@zytor.com>, linux-arch@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, linuxppc-dev@lists.ozlabs.org,
        Ingo Molnar <mingo@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Will Deacon <will@kernel.org>, x86@kernel.org
References: <20200413125303.423864-1-npiggin@gmail.com>
        <20200413125303.423864-5-npiggin@gmail.com>
        <20200414072316.GA5503@infradead.org>
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Excerpts from Christoph Hellwig's message of April 14, 2020 5:23 pm:
> On Mon, Apr 13, 2020 at 10:53:03PM +1000, Nicholas Piggin wrote:
>> For platforms that define HAVE_ARCH_HUGE_VMAP and support PMD vmap mappi=
ngs,
>> have vmalloc attempt to allocate PMD-sized pages first, before falling b=
ack
>> to small pages. Allocations which use something other than PAGE_KERNEL
>> protections are not permitted to use huge pages yet, not all callers exp=
ect
>> this (e.g., module allocations vs strict module rwx).
>>=20
>> This gives a 6x reduction in dTLB misses for a `git diff` (of linux), fr=
om
>> 45600 to 6500 and a 2.2% reduction in cycles on a 2-node POWER9.
>>=20
>> This can result in more internal fragmentation and memory overhead for a
>> given allocation. It can also cause greater NUMA unbalance on hashdist
>> allocations.
>>=20
>> There may be other callers that expect small pages under vmalloc but use
>> PAGE_KERNEL, I'm not sure if it's feasible to catch them all. An
>> alternative would be a new function or flag which enables large mappings=
,
>> and use that in callers.
>=20
> Why do we even use vmalloc in this case rather than just doing a huge
> page allocation?

Which case? Usually the answer would be because you don't want to use
contiguous physical memory and/or you don't want to use the linear=20
mapping.

> What callers are you intersted in?

The dentry and inode caches for this test, obviously.

Lots of other things could possibly benefit though, other system=20
hashes like networking, but lot of other vmalloc callers that might
benefit right away, some others could use some work to batch up
allocation sizes to benefit.

Thanks,
Nick