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 10:48:33 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Thomas Hellström (Intel) 
        <thomas_os@...pmail.org>
Cc:     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 Wed, Mar 24, 2021 at 02:35:38PM +0100, Thomas Hellström (Intel) wrote:

> > In an ideal world the creation/destruction of page table levels would
> > by dynamic at this point, like THP.
> 
> Hmm, but I'm not sure what problem we're trying to solve by changing the
> interface in this way?

We are trying to make a sensible driver API to deal with huge pages.

> Currently if the core vm requests a huge pud, we give it one, and if we
> can't or don't want to (because of dirty-tracking, for example, which is
> always done on 4K page-level) we just return VM_FAULT_FALLBACK, and the
> fault is retried at a lower level.

Well, my thought would be to move the pte related stuff into
vmf_insert_range instead of recursing back via VM_FAULT_FALLBACK.

I don't know if the locking works out, but it feels cleaner that the
driver tells the vmf how big a page it can stuff in, not the vm
telling the driver to stuff in a certain size page which it might not
want to do.

Some devices want to work on a in-between page size like 64k so they
can't form 2M pages but they can stuff 64k of 4K pages in a batch on
every fault.

That idea doesn't fit naturally if the VM is driving the size.

Jason