On 24 October 2014 00:40, Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx> wrote: > On Thu, 2014-10-23 at 18:40 -0400, David Miller wrote: >> Hey guys, was looking over the generic GUP while working on a sparc64 >> issue and I noticed that you guys do speculative page gets, and after >> talking with Johannes Weiner (CC:'d) about this we don't see how it >> could be necessary. >> >> If interrupts are disabled during the page table scan (which they >> are), no IPI tlb flushes can arrive. Therefore any removal from the >> page tables is guarded by interrupts being re-enabled. And as a >> result, page counts of pages we see in the page tables must always >> have a count > 0. >> >> x86 does direct atomic_add() on &page->_count because of this >> invariant and I would rather see the generic version do this too. > > This is of course only true of archs who use IPIs for TLB flushes, so if > we are going down the path of not being speculative, powerpc would have > to go back to doing its own since our broadcast TLB flush means we > aren't protected (we are only protected vs. the page tables themselves > being freed since we do that via sched RCU). > > AFAIK, ARM also broadcasts TLB flushes... Indeed, for most ARM cores we have hardware TLB broadcasts, thus we need the speculative path. > > Another option would be to make the generic code use something defined > by the arch to decide whether to use speculative get or > not. I like the idea of keeping the bulk of that code generic... It would be nice to have the code generalised further. In addition to the speculative/atomic helpers the implementation would need to be renamed from GENERIC_RCU_GUP to GENERIC_GUP. The other noteworthy assumption made in the RCU GUP is that pte's can be read atomically. For x86 this isn't true when running with 64-bit pte's, thus a helper would be needed. Cheers, -- Steve -- 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>