Re: [PATCH] mm: don't zero ballooned pages
On 07/31/2017 03:51 PM, Michal Hocko wrote: On Mon 31-07-17 15:41:49, Wei Wang wrote: >On 07/31/2017 02:55 PM, Michal Hocko wrote: > >On Mon 31-07-17 12:13:33, Wei Wang wrote: > >>Ballooned pages will be marked as MADV_DONTNEED by the hypervisor and > >>shouldn't be given to the host ksmd to scan. > >Could you point me where this MADV_DONTNEED is done, please? > >Sure. It's done in the hypervisor when the balloon pages are received. > >Please see line 40 at >https://github.com/qemu/qemu/blob/master/hw/virtio/virtio-balloon.c And one more thing. I am not familiar with ksm much. But how is MADV_DONTNEED even helping? This madvise is not sticky - aka it will unmap the range without leaving any note behind. AFAICS the only way to have vma scanned is to have VM_MERGEABLE and that is an opt in: See Documentation/vm/ksm.txt " KSM only operates on those areas of address space which an application has advised to be likely candidates for merging, by using the madvise(2) system call: int madvise(addr, length, MADV_MERGEABLE). " So what exactly is going on here? The original patch looks highly suspicious as well. If somebody wants to make that memory mergable then the user of that memory should zero them out. Kernel starts a kthread named "ksmd". ksmd scans the VM_MERGEABLE memory, and merge the same pages.(same page means memcmp(page1, page2, PAGESIZE) == 0). Guest can not use ballooned pages, and these pages will not be accessed in a long time. Kswapd on host will swap these pages out and get more free memory. Rather than swapping, KSM has better performence. Presently pages in the balloon device have random value, they usually cannot be merged. So enqueue zero pages will resolve this problem. Because MADV_DONTNEED depends on host os capability and hypervisor capability, I prefer to enqueue zero pages to balloon device and made this patch. ___ Virtualization mailing list Virtualization@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/virtualization
Re: [PATCH] mm: don't zero ballooned pages
on qemu upstream, code in qemu/util/osdep.c
int qemu_madvise(void *addr, size_t len, int advice)
{
if (advice == QEMU_MADV_INVALID) {
errno = EINVAL;
return -1;
}
#if defined(CONFIG_MADVISE)
return madvise(addr, len, advice);
#elif defined(CONFIG_POSIX_MADVISE)
return posix_madvise(addr, len, advice);
#else
errno = EINVAL;
return -1;
#endif
}
Host OS maybe not support MADV_DONTNEED.
And madvise syscall uses more time.
On 07/31/2017 02:55 PM, Michal Hocko wrote:
On Mon 31-07-17 12:13:33, Wei Wang wrote:
Ballooned pages will be marked as MADV_DONTNEED by the hypervisor and
shouldn't be given to the host ksmd to scan.
Could you point me where this MADV_DONTNEED is done, please?
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.
Please make it obvious that this is a revert of bb01b64cfab7
("mm/balloon_compaction.c: enqueue zero page to balloon device").
Signed-off-by: Wei Wang
---
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;
--
2.7.4
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