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

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Date:   Wed, 24 Mar 2021 10:56:43 +0100
From:   Daniel Vetter <daniel@...ll.ch>
To:     Thomas Hellström (Intel) 
        <thomas_os@...pmail.org>
Cc:     Jason Gunthorpe <jgg@...dia.com>, David Airlie <airlied@...ux.ie>,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        Christian Koenig <christian.koenig@....com>
Subject: Re: [RFC PATCH 1/2] mm,drm/ttm: Block fast GUP to TTM huge pages

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.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch