linux-kernel - Re: [PATCH Part2 v6 14/49] crypto: ccp: Handle the legacy TMR allocation when SNP is enabled

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Message-ID: <7882353e-2b13-d35a-b462-cef35ee56f51@suse.cz>
Date:   Tue, 15 Nov 2022 16:14:42 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     "Kalra, Ashish" <ashish.kalra@....com>,
        Borislav Petkov <bp@...en8.de>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-coco@...ts.linux.dev, linux-mm@...ck.org,
        linux-crypto@...r.kernel.org, tglx@...utronix.de, mingo@...hat.com,
        jroedel@...e.de, thomas.lendacky@....com, hpa@...or.com,
        ardb@...nel.org, pbonzini@...hat.com, seanjc@...gle.com,
        vkuznets@...hat.com, jmattson@...gle.com, luto@...nel.org,
        dave.hansen@...ux.intel.com, slp@...hat.com, pgonda@...gle.com,
        peterz@...radead.org, srinivas.pandruvada@...ux.intel.com,
        rientjes@...gle.com, dovmurik@...ux.ibm.com, tobin@....com,
        michael.roth@....com, kirill@...temov.name, ak@...ux.intel.com,
        tony.luck@...el.com, marcorr@...gle.com,
        sathyanarayanan.kuppuswamy@...ux.intel.com, alpergun@...gle.com,
        dgilbert@...hat.com, jarkko@...nel.org,
        "Kaplan, David" <David.Kaplan@....com>,
        Naoya Horiguchi <naoya.horiguchi@....com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Oscar Salvador <osalvador@...e.de>
Subject: Re: [PATCH Part2 v6 14/49] crypto: ccp: Handle the legacy TMR
 allocation when SNP is enabled

Cc'ing memory failure folks, the beinning of this subthread is here:

https://lore.kernel.org/all/3a51840f6a80c87b39632dc728dbd9b5dd444cd7.1655761627.git.ashish.kalra@amd.com/

On 11/15/22 00:36, Kalra, Ashish wrote:
> Hello Boris,
> 
> On 11/2/2022 6:22 AM, Borislav Petkov wrote:
>> On Mon, Oct 31, 2022 at 04:58:38PM -0500, Kalra, Ashish wrote:
>>>       if (snp_lookup_rmpentry(pfn, &rmp_level)) {
>>>              do_sigbus(regs, error_code, address, VM_FAULT_SIGBUS);
>>>              return RMP_PF_RETRY;
>>
>> Does this issue some halfway understandable error message why the
>> process got killed?
>>
>>> Will look at adding our own recovery function for the same, but that will
>>> again mark the pages as poisoned, right ?
>>
>> Well, not poisoned but PG_offlimits or whatever the mm folks agree upon.
>> Semantically, it'll be handled the same way, ofc.
> 
> Added a new PG_offlimits flag and a simple corresponding handler for it.

One thing is, there's not enough page flags to be adding more (except
aliases for existing) for cases that can avoid it, but as Boris says, if
using alias to PG_hwpoison it depends what will become confused with the
actual hwpoison.

> But there is still added complexity of handling hugepages as part of
> reclamation failures (both HugeTLB and transparent hugepages) and that
> means calling more static functions in mm/memory_failure.c
> 
> There is probably a more appropriate handler in mm/memory-failure.c:
> 
> soft_offline_page() - this will mark the page as HWPoisoned and also has
> handling for hugepages. And we can avoid adding a new page flag too.
> 
> soft_offline_page - Soft offline a page.
> Soft offline a page, by migration or invalidation, without killing anything.
> 
> So, this looks like a good option to call
> soft_offline_page() instead of memory_failure() in case of
> failure to transition the page back to HV/shared state via SNP_RECLAIM_CMD
> and/or RMPUPDATE instruction.

So it's a bit unclear to me what exact situation we are handling here. The
original patch here seems to me to be just leaking back pages that are
unsafe for further use. soft_offline_page() seems to fit that scenario of a
graceful leak before something is irrepairably corrupt and we page fault on it.
But then in the thread you discus PF handling and killing. So what is the
case here? If we detect this need to call snp_leak_pages() does it mean:

a) nobody that could page fault at them (the guest?) is running anymore, we
are tearing it down, we just can't reuse the pages further on the host
- seem like soft_offline_page() could work, but maybe we could just put the
pages on some leaked lists without special page? The only thing that should
matter is not to free the pages to the page allocator so they would be
reused by something else.

b) something can stil page fault at them (what?) - AFAIU can't be resolved
without killing something, memory_failure() might limit the damage

> Thanks,
> Ashish
> 
>>
>>> Still waiting for some/more feedback from mm folks on the same.
>>
>> Just send the patch and they'll give it.
>>
>> Thx.
>>