On Fri, 2014-10-24 at 09:22 -0700, James Bottomley wrote: > Parisc does this. As soon as one CPU issues a TLB purge, it's broadcast > to all the CPUs on the inter-CPU bus. The next instruction isn't > executed until they respond. > > But this is only for our CPU TLB. There's no other external > consequence, so removal from the page tables isn't effected by this TLB > flush, therefore the theory on which Dave bases the change to > atomic_add() should work for us (of course, atomic_add is lock add > unlock on our CPU, so it's not going to be of much benefit). I'm not sure I follow you here. Do you or do you now perform an IPI to do TLB flushes ? If you don't (for example because you have HW broadcast), then you need the speculative get_page(). If you do (and can read a PTE atomically), you can get away with atomic_add(). The reason is that if you remember how zap_pte_range works, we perform the flush before we get rid of the page. So if your using IPIs for the flush, the fact that gup_fast has interrupts disabled will delay the IPI response and thus effectively prevent the pages from being actually freed, allowing us to simply do the atomic_add() on x86. But if we don't use IPIs because we have HW broadcast of TLB invalidations, then we don't have that synchronization. atomic_add won't work, we need get_page_speculative() because the page could be concurrently being freed. Cheers, Ben. > James > > > 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... > > > > Cheers, > > Ben. > > > > > -- > > > 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> > > > > > > -- > > 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> > > > > > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ -- 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>