From: Sean Christopherson <sean.j.christopherson@intel.com>
To: Liran Alon <liran.alon@oracle.com>
Cc: Barret Rhoden <brho@google.com>,
Paolo Bonzini <pbonzini@redhat.com>,
David Hildenbrand <david@redhat.com>,
Alexander Duyck <alexander.h.duyck@linux.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: Fri, 13 Dec 2019 09:19:50 -0800 [thread overview]
Message-ID: <20191213171950.GA31552@linux.intel.com> (raw)
In-Reply-To: <65FB6CC1-3AD2-4D6F-9481-500BD7037203@oracle.com>
On Fri, Dec 13, 2019 at 03:07:31AM +0200, Liran Alon wrote:
>
> > On 12 Dec 2019, at 21:55, Barret Rhoden <brho@google.com> wrote:
> >
> >>>> 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).
Teaching thp_adjust() how to handle 1GB wouldn't be a bad thing. It's
unlikely THP itself will support 1GB pages any time soon, but having the
logic there wouldn't hurt anything.
> >>> 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.
Agreed.
> > I can rename it to hugepage_adjust(), since it's not just THP anymore.
Or maybe allowed_hugepage_adjust()? To pair with disallowed_hugepage_adjust(),
which adjusts KVM's page size in the opposite direction to avoid the iTLB
multi-hit issue.
>
> Sounds good.
>
> >
> > I was a little hesitant to change the this to handle 1 GB pages with this
> > patchset at first. I didn't want to break the non-DAX case stuff by doing
> > so.
>
> Why would it affect non-DAX case?
> Your patch should just make hugepage_adjust() to parse page-tables only in case is_zone_device_page(). Otherwise, page tables shouldn’t be parsed.
> i.e. THP merged pages should still be detected by PageTransCompoundMap().
I think what Barret is saying is that teaching thp_adjust() how to do 1gb
mappings would naturally affect the code path for THP pages. But I agree
that it would be superficial.
> > Specifically, can a THP page be 1 GB, and if so, how can you tell? If you
> > can't tell easily, I could walk the page table for all cases, instead of
> > just zone_device().
No, THP doesn't currently support 1gb pages. Expliciting returning
PMD_SIZE on PageTransCompoundMap() would be a good thing from a readability
perspective.
> I prefer to walk page-tables only for is_zone_device_page().
>
> >
> > I'd also have to drop the "level == PT_PAGE_TABLE_LEVEL" check, I think,
> > which would open this up to hugetlbfs pages (based on the comments). Is
> > there any reason why that would be a bad idea?
No, the "level == PT_PAGE_TABLE_LEVEL" check is to filter out the case
where KVM is already planning on using a large page, e.g. when the memory
is backed by hugetlbs.
> KVM already supports mapping 1GB hugetlbfs pages. As level is set to
> PUD-level by
> tdp_page_fault()->mapping_level()->host_mapping_level()->kvm_host_page_size()->vma_kernel_pagesize().
> As VMA which is mmap of hugetlbfs sets vma->vm_ops to hugetlb_vm_ops() where
> hugetlb_vm_op_pagesize() will return appropriate page-size.
>
> Specifically, I don’t think THP ever merges small pages to 1GB pages. I think
> this is why transparent_hugepage_adjust() checks PageTransCompoundMap() only
> in case level == PT_PAGE_TABLE_LEVEL. I think you should keep this check in
> the case of !is_zone_device_page().
I would add 1gb support for DAX as a third patch in this series. To pave
the way in patch 2/2, change it to replace "bool pfn_is_huge_mapped()" with
"int host_pfn_mapping_level()", and maybe also renaming host_mapping_level()
to host_vma_mapping_level() to avoid confusion.
Then allowed_hugepage_adjust() would look something like:
static void allowed_hugepage_adjust(struct kvm_vcpu *vcpu, gfn_t gfn,
kvm_pfn_t *pfnp, int *levelp, int max_level)
{
kvm_pfn_t pfn = *pfnp;
int level = *levelp;
unsigned long mask;
if (is_error_noslot_pfn(pfn) || !kvm_is_reserved_pfn(pfn) ||
level == PT_PAGE_TABLE_LEVEL)
return;
/*
* mmu_notifier_retry() was successful and mmu_lock is held, so
* the pmd/pud can't be split from under us.
*/
level = host_pfn_mapping_level(vcpu->kvm, gfn, pfn);
*levelp = level = min(level, max_level);
mask = KVM_PAGES_PER_HPAGE(level) - 1;
VM_BUG_ON((gfn & mask) != (pfn & mask));
*pfnp = pfn & ~mask;
}
_______________________________________________
Linux-nvdimm mailing list -- linux-nvdimm@lists.01.org
To unsubscribe send an email to linux-nvdimm-leave@lists.01.org
next prev parent reply other threads:[~2019-12-13 20:58 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
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 [this message]
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
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20191213171950.GA31552@linux.intel.com \
--to=sean.j.christopherson@intel.com \
--cc=alexander.h.duyck@linux.intel.com \
--cc=brho@google.com \
--cc=david@redhat.com \
--cc=jason.zeng@intel.com \
--cc=kvm@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-nvdimm@lists.01.org \
--cc=liran.alon@oracle.com \
--cc=pbonzini@redhat.com \
--cc=x86@kernel.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).