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

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Date:   Thu, 25 Mar 2021 18:51:26 +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 9:25 PM, Dave Hansen wrote:
> On 3/24/21 1:22 PM, Thomas Hellström (Intel) wrote:
>>> 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?
> On x86, we have 64-bit PTEs when running 32-bit kernels if PAE is
> enabled.  IOW, we can handle the majority of 32-bit CPUs out there.
>
> But, yeah, we don't care about 32-bit. :)

Hmm,

Actually it makes some sense to use SW1, to make it end up in the same 
dword as the PSE bit, as from what I can tell, reading of a 64-bit pmd_t 
on 32-bit PAE is not atomic, so in theory a huge pmd could be modified 
while reading the pmd_t making the dwords inconsistent.... How does that 
work with fast gup anyway?

In any case, what would be the best cause of action here? Use SW1 or 
disable completely for 32-bit?

/Thomas