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

[Thomas Hellström (Intel)]

Hi,

On 3/25/21 2:02 PM, Christian König wrote:
> 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.
Previous discussion here

https://www.spinics.net/lists/linux-mm/msg205291.html

> > 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.
Yes :/ ...
> Regards,
> Christian.
Thanks,

Thomas