linux-kernel - Re: [RFC PATCH 1/2] mm,drm/ttm: Block fast GUP to TTM huge pages

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Message-ID: <e5199438-9a0d-2801-f9f6-ceb13d7a9c61@shipmail.org>
Date:   Wed, 24 Mar 2021 21:22:16 +0100
From:   Thomas Hellström (Intel) 
        <thomas_os@...pmail.org>
To:     Dave Hansen <dave.hansen@...el.com>,
        "Williams, Dan J" <dan.j.williams@...el.com>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "christian.koenig@....com" <christian.koenig@....com>,
        "jgg@...dia.com" <jgg@...dia.com>,
        "airlied@...ux.ie" <airlied@...ux.ie>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>
Subject: Re: [RFC PATCH 1/2] mm,drm/ttm: Block fast GUP to TTM huge pages


On 3/24/21 5:34 PM, Dave Hansen wrote:
> On 3/24/21 3:05 AM, Thomas Hellström (Intel) wrote:
>> Yes, I agree. Seems like the special (SW1) is available also for huge
>> page table entries on x86 AFAICT, although just not implemented.
>> Otherwise the SW bits appear completely used up.
> Although the _PAGE_BIT_SOFTW* bits are used up, there's plenty of room
> in the hardware PTEs.  Bits 52->58 are software-available, and we're
> only using 58 at the moment.
>
> We also have not been careful at *all* about how _PAGE_BIT_SOFTW* are
> used.  It's quite possible we can encode another use even in the
> existing bits.
>
> Personally, I'd just try:
>
> #define _PAGE_BIT_SOFTW5        57      /* available for programmer */
>
OK, I'll follow your advise here. FWIW I grepped for SW1 and it seems 
used in a selftest, but only for PTEs AFAICT.

Oh, and we don't care about 32-bit much anymore?

/Thomas