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From: Jason Gunthorpe <jgg@nvidia.com>
To: "Thomas Hellström (Intel)" <thomas_os@shipmail.org>
Cc: "Christian König" <christian.koenig@amd.com>,
	"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>
Subject: Re: [RFC PATCH 1/2] mm,drm/ttm: Block fast GUP to TTM huge pages
Date: Thu, 25 Mar 2021 08:30:23 -0300	[thread overview]
Message-ID: <20210325113023.GT2356281@nvidia.com> (raw)
In-Reply-To: <15da5784-96ca-25e5-1485-3ce387ee6695@shipmail.org>

On Thu, Mar 25, 2021 at 10:51:35AM +0100, Thomas Hellström (Intel) wrote:

> > Please explain that further. Why do we need the mmap lock to insert PMDs
> > but not when insert PTEs?
> 
> We don't. But once you've inserted a PMD directory you can't remove it
> unless you have the mmap lock (and probably also the i_mmap_lock in write
> mode). That for example means that if you have a VRAM region mapped with
> huge PMDs, and then it gets evicted, and you happen to read a byte from it
> when it's evicted and therefore populate the full region with PTEs pointing
> to system pages, you can't go back to huge PMDs again without a munmap() in
> between.

This is all basically magic to me still, but THP does this
transformation and I think what it does could work here too. We
probably wouldn't be able to upgrade while handling fault, but at the
same time, this should be quite rare as it would require the driver to
have supplied a small page for this VMA at some point.

> > Apart from that I still don't fully get why we need this in the first
> > place.
> 
> Because virtual huge page address boundaries need to be aligned with
> physical huge page address boundaries, and mmap can happen before bos are
> populated so you have no way of knowing how physical huge page
> address

But this is a mmap-time problem, fault can't fix mmap using the wrong VA.

> > I really don't see that either. When a buffer is accessed by the CPU it
> > is in > 90% of all cases completely accessed. Not faulting in full
> > ranges is just optimizing for a really unlikely case here.
> 
> It might be that you're right, but are all drivers wanting to use this like
> drm in this respect? Using the interface to fault in a 1G range in the hope
> it could map it to a huge pud may unexpectedly consume and populate some 16+
> MB of page tables.

If the underlying device block size is so big then sure, why not? The
"unexpectedly" should be quite rare/non existant anyhow.

Jason
 


  reply	other threads:[~2021-03-25 11:30 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
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 [this message]
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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