On Tuesday 05 March 2013, Marek Szyprowski wrote:
On 3/5/2013 9:50 AM, Arnd Bergmann wrote:
On Tuesday 05 March 2013, Marek Szyprowski wrote:
The problem is that the opposite approach is imho easier.
I can understand that, yes ;-)
get_user_pages() is used in quite a lot of places (I was quite surprised when I've added some debug to it and saw the logs) and it seems to be easier to identify places where references are kept for significant amount of time. Usually such places are in the device drivers. In our case only videobuf2 and some closed-source driver were causing the real migration problems, so I decided to leave the default approach unchanged.
If we use this workaround for every get_user_pages() call we will sooner or later end with most of the anonymous pages migrated to non-movable pageblocks what make the whole CMA approach a bit pointless.
But you said that most users are in device drivers, and I would expect drivers not to touch that many pages.
We already have two interfaces: the generic get_user_pages and the "fast" version "get_user_pages_fast" that has a number of restrictions. We could add another such restriction to get_user_pages_fast(), which is that it must not hold the page reference count for an extended time because it will not migrate pages out.
I would assume that most of the in-kernel users of get_user_pages() that are called a lot either already use get_user_pages_fast, or can be easily converted to it.
Arnd