Re: [RFC PATCH 1/2] mm,drm/ttm: Block fast GUP to TTM huge pages
From: Christian König
Date: Thu Mar 25 2021 - 09:03:57 EST
Am 25.03.21 um 13:36 schrieb Thomas Hellström (Intel):
On 3/25/21 1:09 PM, 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.
Christian.
We've had this discussion before and at that time I managed to
convince you by pointing to the shmem helper for this,
shmem_get_umapped_area().
No, you didn't convinced me. I was just surprised that this is something
under driver control.
Basically there are two ways to do this. Either use a standard helper
similar to shmem's, and then the driver needs to align physical
(device) huge page boundaries to address space offset huge page
boundaries. If you don't do that you can just as well use a custom
function that adjusts for you not doing that
(drm_get_unmapped_area()). Both require driver knowledge of the size
of huge pages.
And once more, at least for GPU drivers that looks like the totally
wrong approach to me.
Aligning the VMA so that huge page allocations become possible is the
job of the MM subsystem and not that of the drivers.
Without a function to adjust, mmap will use it's default (16 byte?)
alignment and chance of alignment becomes very small.
Well it's 4KiB at least.
Regards,
Christian.
/Thomas
Jason