On Mon 07-08-17 10:44:50, David Hildenbrand wrote: > On 03.08.2017 13:59, Wei Wang wrote: > > This patch is a revert of 'commit bb01b64cfab7 ("mm/balloon_compaction.c: > > enqueue zero page to balloon device")' > > > > Ballooned pages will be marked as MADV_DONTNEED by the hypervisor and > > shouldn't be given to the host ksmd to scan. Therefore, it is not > > necessary to zero ballooned pages, which is very time consuming when > > the page amount is large. The ongoing fast balloon tests show that the > > time to balloon 7G pages is increased from ~491ms to 2.8 seconds with > > __GFP_ZERO added. So, this patch removes the flag. > > > > Signed-off-by: Wei Wang <wei.w.wang@xxxxxxxxx> > > Cc: Michal Hocko <mhocko@xxxxxxxxxx> > > Cc: Michael S. Tsirkin <mst@xxxxxxxxxx> > > --- > > mm/balloon_compaction.c | 2 +- > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > diff --git a/mm/balloon_compaction.c b/mm/balloon_compaction.c > > index 9075aa5..b06d9fe 100644 > > --- a/mm/balloon_compaction.c > > +++ b/mm/balloon_compaction.c > > @@ -24,7 +24,7 @@ struct page *balloon_page_enqueue(struct balloon_dev_info *b_dev_info) > > { > > unsigned long flags; > > struct page *page = alloc_page(balloon_mapping_gfp_mask() | > > - __GFP_NOMEMALLOC | __GFP_NORETRY | __GFP_ZERO); > > + __GFP_NOMEMALLOC | __GFP_NORETRY); > > if (!page) > > return NULL; > > > > > > Your assumption here is, that the hypervisor will always supply a zero > page. Unfortunately, this assumption is wrong (and it stems from the > lack of different page size support in virtio-balloon). > > Think about these examples: > > 1. Guest is backed by huge pages (hugetbfs). Ballooning kicks in. > > MADV_DONTNEED is simply ignored in the hypervisor (hugetlbfs requires > fallocate punshhole). Also, trying to zap 4k on e.g. 1MB pages will > simply be ignored. > > 2. Guest on PPC uses 4k pages. Hypervisor uses 64k pages. trying to > MADV_DONTNEED 4K on 64k pages will simply be ignored. > > So unfortunately, zeroing the page is the right thing to do to cover all > cases. Maybe it is my absolute lack of familiarity with what the host actually does with balloon pages but I fail to see why the above matters at all. ksm will not try to merge sub page units (4k for hugetlb or a large base page). And if you need to hide the guest contents then the host can clear the respective subpage just fine. So could you be more explicit why MADV_DONTNEED matters at all? Also does any host actually share sub pages between different guests? This sounds like a bad idea to me in general. -- Michal Hocko SUSE Labs -- 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>