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

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Message-Id: <1586863931.xb4yeowkao.astroid@bobo.none>
Date:   Tue, 14 Apr 2020 21:39:53 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Borislav Petkov <bp@...en8.de>,
        Catalin Marinas <catalin.marinas@....com>,
        "H. Peter Anvin" <hpa@...or.com>, linux-arch@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linuxppc-dev@...ts.ozlabs.org,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Will Deacon <will@...nel.org>, x86@...nel.org
Subject: Re: [PATCH v2 4/4] mm/vmalloc: Hugepage vmalloc mappings

Excerpts from Matthew Wilcox's message of April 13, 2020 11:41 pm:
> On Mon, Apr 13, 2020 at 10:53:03PM +1000, Nicholas Piggin wrote:
>> +static int vmap_pages_range_noflush(unsigned long start, unsigned long end,
>> +				    pgprot_t prot, struct page **pages,
>> +				    unsigned int page_shift)
>> +{
>> +	if (page_shift == PAGE_SIZE) {
> 
> ... I think you meant 'page_shift == PAGE_SHIFT'

Thanks, good catch. I obviously didn't test the fallback path (the
other path works for small pages, it just goes one at a time).

> Overall I like this series, although it's a bit biased towards CPUs
> which have page sizes which match PMD/PUD sizes.  It doesn't offer the
> possibility of using 64kB page sizes on ARM, for example.

No, it's just an incremental step on existing huge vmap stuff in
tree, so such a thing would be out of scope.

> But it's a
> step in the right direction.
> 

I don't know about moving kernel maps away from a generic Linux page
table format. I quite like moving to it and making it as generic as
possible.

On the other hand, I also would like to make some arch-specific
allowances for certain special cases that may not fit within the
standard page table format, but it might be a much more specific and
limited interface than the general vmalloc stuff.

Thanks,
Nick