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From: Jason Gunthorpe <jgg@nvidia.com>
To: "Thomas Hellström (Intel)" <thomas_os@shipmail.org>,
	"David Airlie" <airlied@linux.ie>,
	linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org,
	linux-mm@kvack.org, "Andrew Morton" <akpm@linux-foundation.org>,
	"Christian Koenig" <christian.koenig@amd.com>
Subject: Re: [RFC PATCH 1/2] mm,drm/ttm: Block fast GUP to TTM huge pages
Date: Wed, 24 Mar 2021 09:24:30 -0300	[thread overview]
Message-ID: <20210324122430.GW2356281@nvidia.com> (raw)
In-Reply-To: <YFsM23t2niJwhpM/@phenom.ffwll.local>

On Wed, Mar 24, 2021 at 10:56:43AM +0100, Daniel Vetter wrote:
> On Tue, Mar 23, 2021 at 06:06:53PM +0100, Thomas Hellström (Intel) wrote:
> > 
> > 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()
> 
> The somewhat annoying thing is that we'd need an error code so we fall
> back to pte fault handling. That's at least my understanding of how
> pud/pmd fault handling works. Not sure how awkward that is going to be
> with the overall fault handling flow.
> 
> But aside from that I think this makes tons of sense.

Why should the driver be so specific?

vmf_insert_pfn_range_XXX()

And it will figure out the optimal way to build the page tables.

Driver should provide the largest physically contiguous range it can

Jason


  reply	other threads:[~2021-03-24 12:24 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)
2021-03-24  9:56           ` Daniel Vetter
2021-03-24 12:24             ` Jason Gunthorpe [this message]
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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