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

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Message-ID: <20210325124206.GA599656@nvidia.com>
Date:   Thu, 25 Mar 2021 09:42:06 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Christian König <christian.koenig@....com>
Cc:     Thomas Hellström (Intel) 
        <thomas_os@...pmail.org>, 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>
Subject: Re: [RFC PATCH 1/2] mm,drm/ttm: Block fast GUP to TTM huge pages

On Thu, Mar 25, 2021 at 01:09:14PM +0100, Christian König wrote:
> Am 25.03.21 um 13:01 schrieb Jason Gunthorpe:
> > On Thu, Mar 25, 2021 at 12:53:15PM +0100, Thomas Hellström (Intel) wrote:
> > 
> > > Nope. The point here was that in this case, to make sure mmap uses the
> > > correct VA to give us a reasonable chance of alignement, the driver might
> > > need to be aware of and do trickery with the huge page-table-entry sizes
> > > anyway, although I think in most cases a standard helper for this can be
> > > supplied.
> > Of course the driver needs some way to influence the VA mmap uses,
> > gernally it should align to the natural page size of the device
> 
> Well a mmap() needs to be aligned to the page size of the CPU, but not
> necessarily to the one of the device.
> 
> So I'm pretty sure the device driver should not be involved in any way the
> choosing of the VA for the CPU mapping.

No, if the device wants to use huge pages it must influence the mmap
VA or it can't form huge pgaes.

It is the same reason why mmap returns 2M stuff these days to make THP
possible

Jason