On Wed, 27 Apr 2016, Kirill A. Shutemov wrote: > On Tue, Apr 26, 2016 at 02:53:41PM +0200, Daniel Vetter wrote: > > On Mon, Apr 25, 2016 at 02:42:50AM +0300, Kirill A. Shutemov wrote: > > > On Mon, Apr 04, 2016 at 02:18:10PM +0100, Chris Wilson wrote: > > > > From: Akash Goel <akash.goel@xxxxxxxxx> > > > > > > > > This provides support for the drivers or shmem file owners to register > > > > a set of callbacks, which can be invoked from the address space > > > > operations methods implemented by shmem. This allow the file owners to > > > > hook into the shmem address space operations to do some extra/custom > > > > operations in addition to the default ones. > > > > > > > > The private_data field of address_space struct is used to store the > > > > pointer to driver specific ops. Currently only one ops field is defined, > > > > which is migratepage, but can be extended on an as-needed basis. > > > > > > > > The need for driver specific operations arises since some of the > > > > operations (like migratepage) may not be handled completely within shmem, > > > > so as to be effective, and would need some driver specific handling also. > > > > Specifically, i915.ko would like to participate in migratepage(). > > > > i915.ko uses shmemfs to provide swappable backing storage for its user > > > > objects, but when those objects are in use by the GPU it must pin the > > > > entire object until the GPU is idle. As a result, large chunks of memory > > > > can be arbitrarily withdrawn from page migration, resulting in premature > > > > out-of-memory due to fragmentation. However, if i915.ko can receive the > > > > migratepage() request, it can then flush the object from the GPU, remove > > > > its pin and thus enable the migration. > > > > > > > > Since gfx allocations are one of the major consumer of system memory, its > > > > imperative to have such a mechanism to effectively deal with > > > > fragmentation. And therefore the need for such a provision for initiating > > > > driver specific actions during address space operations. > > > > > > Hm. Sorry, my ignorance, but shouldn't this kind of flushing be done in > > > response to mmu_notifier's ->invalidate_page? > > > > > > I'm not aware about how i915 works and what's its expectation wrt shmem. > > > Do you have some userspace VMA which is mirrored on GPU side? > > > If yes, migration would cause unmapping of these pages and trigger the > > > mmu_notifier's hook. > > > > We do that for userptr pages (i.e. stuff we steal from userspace address > > spaces). But we also have native gfx buffer objects based on shmem files, > > and thus far we need to allocate them as !GFP_MOVEABLE. And we allocate a > > _lot_ of those. And those files aren't mapped into any cpu address space > > (ofc they're mapped on the gpu side, but that's driver private), from the > > core mm they are pure pagecache. And afaiui for that we need to wire up > > the migratepage hooks through shmem to i915_gem.c > > I see. > > I don't particularly like the way patch hooks into migrate, but don't a > good idea how to implement this better. > > This way allows to hook up to any shmem file, which can be abused by > drivers later. > > I wounder if it would be better for i915 to have its own in-kernel mount > with variant of tmpfs which provides different mapping->a_ops? Or is it > overkill? I don't know. > > Hugh? This, and the 2/2, remain perpetually in my "needs more thought" box. And won't get that thought today either, I'm afraid. Tomorrow. Like you, I don't particularly like these; but recognize that the i915 guys are doing all the rest of us a big favour by going to some trouble to allow migration of their pinned pages. Potential for abuse of migratepage by drivers is already there anyway: we can be grateful that they're offering to use rather than abuse it; but yes, it's a worry that such trickiness gets dispersed into drivers. Amusing to see them grabbing shmem's page_private(), isn't it? Hugh -- 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>