On Thu, Jul 10, 2014 at 03:17:16PM +0200, Alexander Graf wrote:
>
> On 10.07.14 15:07, Mel Gorman wrote:
> >On Thu, Jul 10, 2014 at 01:05:47PM +0200, Alexander Graf wrote:
> >>On 09.07.14 00:59, Stewart Smith wrote:
> >>>Hi!
> >>>
> >>>Thanks for review, much appreciated!
> >>>
> >>>Alexander Graf <ag...@suse.de> writes:
> >>>>On 08.07.14 07:06, Stewart Smith wrote:
> >>>>>@@ -1528,6 +1535,7 @@ static void kvmppc_run_core(struct kvmppc_vcore
> >>>>>*vc)
> >>>>> int i, need_vpa_update;
> >>>>> int srcu_idx;
> >>>>> struct kvm_vcpu *vcpus_to_update[threads_per_core];
> >>>>>+ phys_addr_t phy_addr, tmp;
> >>>>Please put the variable declarations into the if () branch so that the
> >>>>compiler can catch potential leaks :)
> >>>ack. will fix.
> >>>
> >>>>>@@ -1590,9 +1598,48 @@ static void kvmppc_run_core(struct kvmppc_vcore
> >>>>>*vc)
> >>>>> srcu_idx = srcu_read_lock(&vc->kvm->srcu);
> >>>>>+ /* If we have a saved list of L2/L3, restore it */
> >>>>>+ if (cpu_has_feature(CPU_FTR_ARCH_207S) && vc->mpp_buffer) {
> >>>>>+ phy_addr = virt_to_phys((void *)vc->mpp_buffer);
> >>>>>+#if defined(CONFIG_PPC_4K_PAGES)
> >>>>>+ phy_addr = (phy_addr + 8*4096) & ~(8*4096);
> >>>>get_free_pages() is automatically aligned to the order, no?
> >>>That's what Paul reckoned too, and then we've attempted to find anywhere
> >>>that documents that behaviour. Happen to be able to point to docs/source
> >>>that say this is part of API?
> >>Phew - it's probably buried somewhere. I could only find this
> >>document saying that we always get order-aligned allocations:
> >>
> >>http://www.thehackademy.net/madchat/ebooks/Mem_virtuelle/linux-mm/zonealloc.html
> >>
> >>Mel, do you happen to have any pointer to something that explicitly
> >>(or even properly implicitly) says that get_free_pages() returns
> >>order-aligned memory?
> >>
> >I did not read the whole thread so I lack context and will just answer
> >this part.
> >
> >There is no guarantee that pages are returned in PFN order for multiple
> >requests to the page allocator. This is the relevant comment in
> >rmqueue_bulk
> >
> > /*
> > * Split buddy pages returned by expand() are received here
> > * in physical page order. The page is added to the callers
> > and
> > * list and the list head then moves forward. From the
> > callers
> > * perspective, the linked list is ordered by page number in
> > * some conditions. This is useful for IO devices that can
> > * merge IO requests if the physical pages are ordered
> > * properly.
> > */
> >
> >It will probably be true early in the lifetime of the system but the milage
> >will vary on systems with a lot of uptime. If you depend on this behaviour
> >for correctness then you will have a bad day.
> >
> >High-order page requests to the page allocator are guaranteed to be in
> >physical
> >order. However, this does not apply to vmalloc() where allocations are
> >only guaranteed to be virtually contiguous.
>
> Hrm, ok to be very concrete:
>
> Does __get_free_pages(..., 4); on a 4k page size system give me a
> 64k aligned pointer? :)
>
Yes.
--
Mel Gorman
SUSE Labs
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