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From: "Thomas Hellström (Intel)" <thomas_os@shipmail.org>
To: dri-devel@lists.freedesktop.org,
	Christian Koenig <christian.koenig@amd.com>,
	David Airlie <airlied@linux.ie>,
	Andrew Morton <akpm@linux-foundation.org>,
	Jason Gunthorpe <jgg@nvidia.com>,
	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 17:34:51 +0100	[thread overview]
Message-ID: <314fc020-d243-dbf0-acb3-ecfcc9c2443c@shipmail.org> (raw)
In-Reply-To: <YFnST5VLcEgv9q+s@phenom.ffwll.local>

Hi,

On 3/23/21 12:34 PM, Daniel Vetter wrote:
> On Sun, Mar 21, 2021 at 07:45:28PM +0100, Thomas Hellström (Intel) wrote:
>> TTM sets up huge page-table-entries both to system- and device memory,
>> and we don't want gup to assume there are always valid backing struct
>> pages for these. For PTEs this is handled by setting the pte_special bit,
>> but for the huge PUDs and PMDs, we have neither pmd_special nor
>> pud_special. Normally, huge TTM entries are identified by looking at
>> vma_is_special_huge(), but fast gup can't do that, so as an alternative
>> define _devmap entries for which there are no backing dev_pagemap as
>> special, update documentation and make huge TTM entries _devmap, after
>> verifying that there is no backing dev_pagemap.
>>
>> One other alternative would be to block TTM huge page-table-entries
>> completely, and while currently only vmwgfx use them, they would be
>> beneficial to other graphis drivers moving forward as well.
>>
>> Cc: Christian Koenig <christian.koenig@amd.com>
>> Cc: David Airlie <airlied@linux.ie>
>> Cc: Daniel Vetter <daniel@ffwll.ch>
>> Cc: Andrew Morton <akpm@linux-foundation.org>
>> Cc: Jason Gunthorpe <jgg@nvidia.com>
>> Cc: linux-mm@kvack.org
>> Cc: dri-devel@lists.freedesktop.org
>> Cc: linux-kernel@vger.kernel.org
>> Signed-off-by: Thomas Hellström (Intel) <thomas_os@shipmail.org>
>> ---
>>   drivers/gpu/drm/ttm/ttm_bo_vm.c | 17 ++++++++++++++++-
>>   mm/gup.c                        | 21 +++++++++++----------
>>   mm/memremap.c                   |  5 +++++
>>   3 files changed, 32 insertions(+), 11 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/ttm/ttm_bo_vm.c b/drivers/gpu/drm/ttm/ttm_bo_vm.c
>> index 6dc96cf66744..1c34983480e5 100644
>> --- a/drivers/gpu/drm/ttm/ttm_bo_vm.c
>> +++ b/drivers/gpu/drm/ttm/ttm_bo_vm.c
>> @@ -195,6 +195,7 @@ static vm_fault_t ttm_bo_vm_insert_huge(struct vm_fault *vmf,
>>   	pfn_t pfnt;
>>   	struct ttm_tt *ttm = bo->ttm;
>>   	bool write = vmf->flags & FAULT_FLAG_WRITE;
>> +	struct dev_pagemap *pagemap;
>>   
>>   	/* Fault should not cross bo boundary. */
>>   	page_offset &= ~(fault_page_size - 1);
>> @@ -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.

/Thomas




  reply	other threads:[~2021-03-23 16:35 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) [this message]
2021-03-23 16:37       ` Jason Gunthorpe
2021-03-23 16:59         ` Christoph Hellwig
2021-03-23 17:06         ` Thomas Hellström (Intel)
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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