On Wed, Apr 30, 2014 at 04:38:25PM +0100, Steve Capper wrote: > On Wed, Apr 30, 2014 at 04:33:17PM +0100, Catalin Marinas wrote: > > On Wed, Apr 30, 2014 at 04:20:47PM +0100, Catalin Marinas wrote: > > > On Fri, Mar 28, 2014 at 03:01:31PM +0000, Steve Capper wrote: > > > > In order to implement fast_get_user_pages we need to ensure that the > > > > page table walker is protected from page table pages being freed from > > > > under it. > > > > > > > > This patch enables HAVE_RCU_TABLE_FREE, any page table pages belonging > > > > to address spaces with multiple users will be call_rcu_sched freed. > > > > Meaning that disabling interrupts will block the free and protect the > > > > fast gup page walker. > > > > > > > > Signed-off-by: Steve Capper <steve.capper@xxxxxxxxxx> > > > > > > While this patch is simple, I'd like to better understand the reason for > > > it. Currently HAVE_RCU_TABLE_FREE is enabled for powerpc and sparc while > > > __get_user_pages_fast() is supported by a few other architectures that > > > don't select HAVE_RCU_TABLE_FREE. So why do we need it for fast gup on > > > arm/arm64 while not all the other archs need it? > > > > OK, replying to myself. I assume the other architectures that don't need > > HAVE_RCU_TABLE_FREE use IPI for TLB shootdown, hence they gup_fast > > synchronisation for free. > > Yes that is roughly the case. > Essentially we want to RCU free the page table backing pages at a > later time when we aren't walking on them. > > Other arches use IPI, some others have their own RCU logic. I opted to > activate some existing logic to reduce code duplication. Both powerpc and sparc use tlb_remove_table() via their __pte_free_tlb() etc. which implies an IPI for synchronisation if mm_users > 1. For gup_fast we may not need it since we use the RCU for protection. Am I missing anything? -- Catalin -- 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>