Messages in this thread | | | Date | Thu, 03 Aug 2017 21:18:46 +0800 | From | Wei Wang <> | Subject | Re: [PATCH RESEND] mm: don't zero ballooned pages |
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On 08/03/2017 08:54 PM, Michal Hocko wrote: > On Thu 03-08-17 19:59:17, 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. > I find MADV_DONTNEED reference still quite confusing. What do you think > about the following wording instead: > " > Zeroying ballon pages is rather time consuming, especially when a lot of > pages are in flight. E.g. 7GB worth of ballooned memory takes 2.8s with > __GFP_ZERO while it takes ~491ms without it. The original commit argued > that zeroying will help ksmd to merge these pages on the host but this > argument is assuming that the host actually marks balloon pages for ksm > which is not universally true. So we pay performance penalty for > something that even might not be used in the end which is wrong. The > host can zero out pages on its own when there is a need. > "
I think it looks good. Thanks.
>> 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. > The only reason why unconditional zeroying makes some sense is the > data leak protection (guest doesn't want to leak potentially sensitive > data to a malicious guest). I am not sure such a thread applies here > though.
I think the unwashed contents left in the balloon pages (also free pages) should be treated non-confidential - if the guest application has confidential content in its memory, the application itself should zero that before giving back that memory to the guest kernel.
Best, Wei
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