Re: [PATCH 0/6] RFC v2: mm: gup/dma tracking

From: Christopher Lameter
Date: Mon Feb 04 2019 - 11:08:06 EST

On Sun, 3 Feb 2019, john.hubbard@xxxxxxxxx wrote:

> Some kernel components (file systems, device drivers) need to access
> memory that is specified via process virtual address. For a long time, the
> API to achieve that was get_user_pages ("GUP") and its variations. However,
> GUP has critical limitations that have been overlooked; in particular, GUP
> does not interact correctly with filesystems in all situations. That means
> that file-backed memory + GUP is a recipe for potential problems, some of
> which have already occurred in the field.

It may be worth noting a couple of times in this text that this was
designed for anonymous memory and that such use is/was ok. We are talking
about a use case here using mmapped access with a regular filesystem that
was not initially intended. The mmapping of from the hugepages filesystem
is special in that it is not a device that is actually writing things

Any use with a filesystem that actually writes data back to a medium
is something that is broken.