Re: [PATCH RESEND] mm: don't zero ballooned pages

On Mon 07-08-17 10:44:50, David Hildenbrand wrote:

On 03.08.2017 13:59, Wei Wang wrote:

quoted

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 <redacted>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Michael S. Tsirkin <mst@redhat.com>
---
 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