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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>