From: Liran Alon <liran.alon@oracle.com>
To: Barret Rhoden <brho@google.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>,
Dan Williams <dan.j.williams@intel.com>,
David Hildenbrand <david@redhat.com>,
Dave Jiang <dave.jiang@intel.com>,
Alexander Duyck <alexander.h.duyck@linux.intel.com>,
Sean Christopherson <sean.j.christopherson@intel.com>,
linux-nvdimm@lists.01.org, x86@kernel.org, kvm@vger.kernel.org,
linux-kernel@vger.kernel.org, jason.zeng@intel.com
Subject: Re: [PATCH v5 2/2] kvm: Use huge pages for DAX-backed files
Date: Thu, 12 Dec 2019 20:49:52 +0200 [thread overview]
Message-ID: <ED482280-CB47-4AB6-9E7E-EEE7848E0F8B@oracle.com> (raw)
In-Reply-To: <06108004-1720-41EB-BCAB-BFA8FEBF4772@oracle.com>
> On 12 Dec 2019, at 20:47, Liran Alon <liran.alon@oracle.com> wrote:
>
>
>
>> On 12 Dec 2019, at 20:22, Barret Rhoden <brho@google.com> wrote:
>>
>> This change allows KVM to map DAX-backed files made of huge pages with
>> huge mappings in the EPT/TDP.
>
> This change isn’t only relevant for TDP. It also affects when KVM use shadow-paging.
> See how FNAME(page_fault)() calls transparent_hugepage_adjust().
>
>>
>> DAX pages are not PageTransCompound. The existing check is trying to
>> determine if the mapping for the pfn is a huge mapping or not.
>
> I would rephrase “The existing check is trying to determine if the pfn
> is mapped as part of a transparent huge-page”.
>
>> For
>> non-DAX maps, e.g. hugetlbfs, that means checking PageTransCompound.
>
> This is not related to hugetlbfs but rather THP.
>
>> For DAX, we can check the page table itself.
>>
>> Note that KVM already faulted in the page (or huge page) in the host's
>> page table, and we hold the KVM mmu spinlock. We grabbed that lock in
>> kvm_mmu_notifier_invalidate_range_end, before checking the mmu seq.
>>
>> Signed-off-by: Barret Rhoden <brho@google.com>
>
> I don’t think the right place to change for this functionality is transparent_hugepage_adjust()
> which is meant to handle PFNs that are mapped as part of a transparent huge-page.
>
> For example, this would prevent mapping DAX-backed file page as 1GB.
> As transparent_hugepage_adjust() only handles the case (level == PT_PAGE_TABLE_LEVEL).
>
> As you are parsing the page-tables to discover the page-size the PFN is mapped in,
> I think you should instead modify kvm_host_page_size() to parse page-tables instead
> of rely on vma_kernel_pagesize() (Which relies on vma->vm_ops->pagesize()) in case
> of is_zone_device_page().
> The main complication though of doing this is that at this point you don’t yet have the PFN
> that is retrieved by try_async_pf(). So maybe you should consider modifying the order of calls
> in tdp_page_fault() & FNAME(page_fault)().
>
> -Liran
Or alternatively when thinking about it more, maybe just rename transparent_hugepage_adjust()
to not be specific to THP and better handle the case of parsing page-tables changing mapping-level to 1GB.
That is probably easier and more elegant.
-Liran
next prev parent reply other threads:[~2019-12-12 18:51 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-12-12 18:22 [PATCH v5 0/2] kvm: Use huge pages for DAX-backed files Barret Rhoden
2019-12-12 18:22 ` [PATCH v5 1/2] mm: make dev_pagemap_mapping_shift() externally visible Barret Rhoden
2019-12-13 17:47 ` Sean Christopherson
2019-12-13 18:13 ` Dan Williams
2019-12-16 17:59 ` Barret Rhoden
2019-12-18 0:18 ` Sean Christopherson
2020-01-15 18:33 ` Paolo Bonzini
2019-12-12 18:22 ` [PATCH v5 2/2] kvm: Use huge pages for DAX-backed files Barret Rhoden
2019-12-12 18:47 ` Liran Alon
2019-12-12 18:49 ` Liran Alon [this message]
2019-12-12 19:55 ` Barret Rhoden
2019-12-13 1:07 ` Liran Alon
2019-12-13 14:13 ` Barret Rhoden
2019-12-13 17:19 ` Sean Christopherson
2019-12-13 17:31 ` Liran Alon
2019-12-13 17:50 ` Sean Christopherson
2019-12-13 18:08 ` Liran Alon
2019-12-16 16:05 ` Barret Rhoden
2020-01-07 19:05 ` Sean Christopherson
2020-01-07 19:19 ` Barret Rhoden
2020-01-08 1:20 ` Sean Christopherson
2020-01-08 1:39 ` Dan Williams
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