linux-kernel - Re: [PATCH v2 4/4] mm/vmalloc: Hugepage vmalloc mappings

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Message-ID: <20200414072316.GA5503@infradead.org>
Date:   Tue, 14 Apr 2020 00:23:16 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Nicholas Piggin <npiggin@...il.com>
Cc:     linux-mm@...ck.org, linux-arch@...r.kernel.org,
        Catalin Marinas <catalin.marinas@....com>, x86@...nel.org,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Will Deacon <will@...nel.org>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v2 4/4] mm/vmalloc: Hugepage vmalloc mappings

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 mappings,
> have vmalloc attempt to allocate PMD-sized pages first, before falling back
> to small pages. Allocations which use something other than PAGE_KERNEL
> protections are not permitted to use huge pages yet, not all callers expect
> this (e.g., module allocations vs strict module rwx).
> 
> This gives a 6x reduction in dTLB misses for a `git diff` (of linux), from
> 45600 to 6500 and a 2.2% reduction in cycles on a 2-node POWER9.
> 
> This can result in more internal fragmentation and memory overhead for a
> given allocation. It can also cause greater NUMA unbalance on hashdist
> allocations.
> 
> 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.

Why do we even use vmalloc in this case rather than just doing a huge
page allocation?  What callers are you intersted in?