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From: "Thomas Hellström (Intel)" <thomas_os@shipmail.org>
To: Jason Gunthorpe <jgg@nvidia.com>
Cc: dri-devel@lists.freedesktop.org,
	Christian Koenig <christian.koenig@amd.com>,
	David Airlie <airlied@linux.ie>,
	Andrew Morton <akpm@linux-foundation.org>,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [RFC PATCH 1/2] mm,drm/ttm: Block fast GUP to TTM huge pages
Date: Tue, 23 Mar 2021 18:06:53 +0100	[thread overview]
Message-ID: <5824b731-ca6a-92fd-e314-d986b6a7b101@shipmail.org> (raw)
In-Reply-To: <20210323163715.GJ2356281@nvidia.com>


On 3/23/21 5:37 PM, Jason Gunthorpe wrote:
> On Tue, Mar 23, 2021 at 05:34:51PM +0100, Thomas Hellström (Intel) wrote:
>
>>>> @@ -210,6 +211,20 @@ static vm_fault_t ttm_bo_vm_insert_huge(struct vm_fault *vmf,
>>>>    	if ((pfn & (fault_page_size - 1)) != 0)
>>>>    		goto out_fallback;
>>>> +	/*
>>>> +	 * Huge entries must be special, that is marking them as devmap
>>>> +	 * with no backing device map range. If there is a backing
>>>> +	 * range, Don't insert a huge entry.
>>>> +	 * If this check turns out to be too much of a performance hit,
>>>> +	 * we can instead have drivers indicate whether they may have
>>>> +	 * backing device map ranges and if not, skip this lookup.
>>>> +	 */
>>> I think we can do this statically:
>>> - if it's system memory we know there's no devmap for it, and we do the
>>>     trick to block gup_fast
>> Yes, that should work.
>>> - if it's iomem, we know gup_fast wont work anyway if don't set PFN_DEV,
>>>     so might as well not do that
>> I think gup_fast will unfortunately mistake a huge iomem page for an
>> ordinary page and try to access a non-existant struct page for it, unless we
>> do the devmap trick.
>>
>> And the lookup would then be for the rare case where a driver would have
>> already registered a dev_pagemap for an iomem area which may also be mapped
>> through TTM (like the patch from Felix a couple of weeks ago). If a driver
>> can promise not to do that, then we can safely remove the lookup.
> Isn't the devmap PTE flag arch optional? Does this fall back to not
> using huge pages on arches that don't support it?

Good point. No, currently it's only conditioned on transhuge page support.
Need to condition it on also devmap support.

>
> Also, I feel like this code to install "pte_special" huge pages does
> not belong in the drm subsystem..

I could add helpers in huge_memory.c:

vmf_insert_pfn_pmd_prot_special() and
vmf_insert_pfn_pud_prot_special()

/Thomas

>
> Jason


  parent reply	other threads:[~2021-03-23 17:07 UTC|newest]

Thread overview: 63+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-03-21 18:45 [RFC PATCH 0/2] mm,drm/ttm: Always block GUP to TTM pages Thomas Hellström (Intel)
2021-03-21 18:45 ` [RFC PATCH 1/2] mm,drm/ttm: Block fast GUP to TTM huge pages Thomas Hellström (Intel)
2021-03-23 11:34   ` Daniel Vetter
2021-03-23 16:34     ` Thomas Hellström (Intel)
2021-03-23 16:37       ` Jason Gunthorpe
2021-03-23 16:59         ` Christoph Hellwig
2021-03-23 17:06         ` Thomas Hellström (Intel) [this message]
2021-03-24  9:56           ` Daniel Vetter
2021-03-24 12:24             ` Jason Gunthorpe
2021-03-24 12:35               ` Thomas Hellström (Intel)
2021-03-24 12:41                 ` Jason Gunthorpe
2021-03-24 13:35                   ` Thomas Hellström (Intel)
2021-03-24 13:48                     ` Jason Gunthorpe
2021-03-24 15:50                       ` Thomas Hellström (Intel)
2021-03-24 16:38                         ` Jason Gunthorpe
2021-03-24 18:31                           ` Christian König
2021-03-24 20:07                             ` Thomas Hellström (Intel)
2021-03-24 23:14                               ` Jason Gunthorpe
2021-03-25  7:48                                 ` Thomas Hellström (Intel)
2021-03-25  8:27                                   ` Christian König
2021-03-25  9:51                                     ` Thomas Hellström (Intel)
2021-03-25 11:30                                       ` Jason Gunthorpe
2021-03-25 11:53                                         ` Thomas Hellström (Intel)
2021-03-25 12:01                                           ` Jason Gunthorpe
2021-03-25 12:09                                             ` Christian König
2021-03-25 12:36                                               ` Thomas Hellström (Intel)
2021-03-25 13:02                                                 ` Christian König
2021-03-25 13:31                                                   ` Thomas Hellström (Intel)
2021-03-25 12:42                                               ` Jason Gunthorpe
2021-03-25 13:05                                                 ` Christian König
2021-03-25 13:17                                                   ` Jason Gunthorpe
2021-03-25 13:26                                                     ` Christian König
2021-03-25 13:33                                                       ` Jason Gunthorpe
2021-03-25 13:54                                                         ` Christian König
2021-03-25 13:56                                                           ` Jason Gunthorpe
2021-03-25  7:49                                 ` Christian König
2021-03-25  9:41                                   ` Daniel Vetter
2021-03-23 13:52   ` Jason Gunthorpe
2021-03-23 15:05     ` Thomas Hellström (Intel)
2021-03-23 19:52   ` Williams, Dan J
2021-03-23 20:42     ` Thomas Hellström (Intel)
2021-03-24  9:58       ` Daniel Vetter
2021-03-24 10:05         ` Thomas Hellström (Intel)
     [not found]           ` <75423f64-adef-a2c4-8e7d-2cb814127b18@intel.com>
2021-03-24 20:22             ` Thomas Hellström (Intel)
2021-03-24 20:25               ` Dave Hansen
2021-03-25 17:51                 ` Thomas Hellström (Intel)
2021-03-25 17:55                   ` Jason Gunthorpe
2021-03-25 18:13                     ` Thomas Hellström (Intel)
2021-03-25 18:24                       ` Jason Gunthorpe
2021-03-25 18:42                         ` Thomas Hellström (Intel)
2021-03-26  9:08                         ` Thomas Hellström (Intel)
2021-03-26 11:46                           ` Jason Gunthorpe
2021-03-26 12:33                             ` Thomas Hellström (Intel)
2021-03-21 18:45 ` [RFC PATCH 2/2] mm,drm/ttm: Use VM_PFNMAP for TTM vmas Thomas Hellström (Intel)
2021-03-22  7:47   ` Christian König
2021-03-22  8:13     ` Thomas Hellström (Intel)
2021-03-23 11:57       ` Christian König
2021-03-23 11:47   ` Daniel Vetter
2021-03-23 14:04     ` Jason Gunthorpe
2021-03-23 15:51       ` Thomas Hellström (Intel)
2021-03-23 14:00   ` Jason Gunthorpe
2021-03-23 15:46     ` Thomas Hellström (Intel)
2021-03-23 16:06       ` Jason Gunthorpe

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