linux-kernel - Re: [PATCH 2/6] mm: Add vm_insert_pfn

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Message-ID: <CALCETrUgS3Ex9-wMquYrFi823JChsDT1+3nVdv_ezGa3T55vyQ@mail.gmail.com>
Date:	Fri, 11 Dec 2015 14:44:38 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [PATCH 2/6] mm: Add vm_insert_pfn_prot

On Fri, Dec 11, 2015 at 2:33 PM, Andrew Morton
<akpm@...ux-foundation.org> wrote:
> On Thu, 10 Dec 2015 19:21:43 -0800 Andy Lutomirski <luto@...nel.org> wrote:
>
>> The x86 vvar mapping contains pages with differing cacheability
>> flags.  This is currently only supported using (io_)remap_pfn_range,
>> but those functions can't be used inside page faults.
>
> Foggy.  What does "support" mean here?

We currently have a hack in which every x86 mm has a "vvar" vma that
has a .fault handler that always fails (it's the vm_special_mapping
fault handler backed by an empty pages array).  To make everything
work, at mm startup, the vdso code uses remap_pfn_range and
io_remap_pfn_range to poke the pfns into the page tables.

I'd much rather implement this using the new .fault mechanism, and the
canonical way to implement .fault seems to be vm_insert_pfn, and
vm_insert_pfn doesn't allow setting per-page cacheability.
Unfortunately, one of the three x86 vvar pages needs to be uncacheable
because it's a genuine IO page, so I can't use vm_insert_pfn.

I suppose I could just call io_remap_pfn_range from .fault, but I
think that's frowned upon.  Admittedly, I wasn't really sure *why*
that's frowned upon.  This goes way back to 2007
(e0dc0d8f4a327d033bfb63d43f113d5f31d11b3c) when .fault got fancier.

>
>> Add vm_insert_pfn_prot to support varying cacheability within the
>> same non-COW VMA in a more sane manner.
>
> Here, "support" presumably means "insertion of pfns".  Can we spell all
> this out more completely please?

Yes, will fix.

>
>> x86 needs this to avoid a CRIU-breaking and memory-wasting explosion
>> of VMAs when supporting userspace access to the HPET.
>>
>
> OtherwiseAck.

--Andy

-- 
Andy Lutomirski
AMA Capital Management, LLC
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