On Thu, Nov 29, 2012 at 11:55:02PM -0800, Andrew Morton wrote: > On Fri, 30 Nov 2012 15:01:26 +0800 Lin Feng <linfeng@xxxxxxxxxxxxxx> wrote: > > > > > > > On 11/30/2012 01:57 PM, Andrew Morton wrote: > > > On Fri, 30 Nov 2012 11:42:05 +0800 Lin Feng <linfeng@xxxxxxxxxxxxxx> wrote: > > > > > >> hi Andrew, > > >> > > >> On 11/30/2012 07:39 AM, Andrew Morton wrote: > > >>> Tricky. > > >>> > > >>> I expect the same problem would occur with pages which are under > > >>> O_DIRECT I/O. Obviously O_DIRECT pages won't be pinned for such long > > >>> periods, but the durations could still be lengthy (seconds). > > >> the offline retry timeout duration is 2 minutes, so to O_DIRECT pages > > >> seem maybe not a problem for the moment. > > >>> > > >>> Worse is a futex page, which could easily remain pinned indefinitely. > > >>> > > >>> The best I can think of is to make changes in or around > > >>> get_user_pages(), to steal the pages from userspace and replace them > > >>> with non-movable ones before pinning them. The performance cost of > > >>> something like this would surely be unacceptable for direct-io, but > > >>> maybe OK for the aio ring and futexes. > > >> thanks for your advice. > > >> I want to limit the impact as little as possible, as mentioned above, > > >> direct-io seems not a problem, we needn't touch them. Maybe we can > > >> just change the use of get_user_pages()(in or around) such as aio > > >> ring pages. I will try to find a way to do this. > > > > > > What about futexes? > > hi Andrew, > > > > Yes, better to find an approach to solve them all. > > > > But I'm worried about that if we just confine get_user_pages() to use > > none-movable pages, it will drain the none-movable pages soon. Because > > there are many places using get_user_pages() such as some drivers. > > Obviously we shouldn't change get_user_pages() for all callers. > > > IMHO in most cases get_user_pages() callers should release the pages soon, > > so pages allocated from movable zone should be OK. But I'm not sure if > > we get such rule upon get_user_pages(). > > And in other cases we specify get_user_pages() to allocate pages from > > none-movable zone. > > > > So could we add a zone-alloc flags when we call get_user_pages()? > > Well, that's a fairly low-level implementation detail. A more typical > approach would be to add a new get_user_pages_non_movable() or such. > That would probably have the same signature as get_user_pages(), with > one additional argument. Then get_user_pages() becomes a one-line > wrapper which passes in a particular value of that argument. > That is going in the direction that all pinned pages become MIGRATE_UNMOVABLE allocations. That will impact THP availability by increasing the number of MIGRATE_UNMOVABLE blocks that exist and it would hit every user -- not just those that care about ZONE_MOVABLE. I'm likely to NAK such a patch if it's only about node hot-remove because it's much more of a corner case than wanting to use THP. I would prefer if get_user_pages() checked if the page it was about to pin was in ZONE_MOVABLE and if so, migrate it at that point before it's pinned. It'll be expensive but will guarantee ZONE_MOVABLE availability if that's what they want. The CMA people might also want to take advantage of this if the page happened to be in the MIGRATE_CMA pageblock. -- Mel Gorman SUSE Labs -- 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>