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

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Date:   Tue, 23 Mar 2021 16:05:08 +0100
From:   Thomas Hellström (Intel) 
        <thomas_os@...pmail.org>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     dri-devel@...ts.freedesktop.org,
        Christian Koenig <christian.koenig@....com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 1/2] mm,drm/ttm: Block fast GUP to TTM huge pages


On 3/23/21 2:52 PM, Jason Gunthorpe wrote:
> On Sun, Mar 21, 2021 at 07:45:28PM +0100, Thomas Hellström (Intel) wrote:
>> diff --git a/mm/gup.c b/mm/gup.c
>> index e40579624f10..1b6a127f0bdd 100644
>> +++ b/mm/gup.c
>> @@ -1993,6 +1993,17 @@ static void __maybe_unused undo_dev_pagemap(int *nr, int nr_start,
>>   }
>>   
>>   #ifdef CONFIG_ARCH_HAS_PTE_SPECIAL
>> +/*
>> + * If we can't determine whether or not a pte is special, then fail immediately
>> + * for ptes. Note, we can still pin HugeTLB as it is guaranteed not to be
>> + * special. For THP, special huge entries are indicated by xxx_devmap()
>> + * returning true, but a corresponding call to get_dev_pagemap() will
>> + * return NULL.
>> + *
>> + * For a futex to be placed on a THP tail page, get_futex_key requires a
>> + * get_user_pages_fast_only implementation that can pin pages. Thus it's still
>> + * useful to have gup_huge_pmd even if we can't operate on ptes.
>> + */
> Why move this comment? I think it was correct where it was

Yes, you're right. I misread it to refer to the actual code in the 
gup_pte_range function rather than to the empty version. I'll move it back.

/Thomas


>
> Jason