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From: Barret Rhoden <brho@google.com>
To: Liran Alon <liran.alon@oracle.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>,
	David Hildenbrand <david@redhat.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: Fri, 13 Dec 2019 09:13:05 -0500	[thread overview]
Message-ID: <90a9af31-304c-e8d5-b17c-0ddb4c98fddb@google.com> (raw)
In-Reply-To: <65FB6CC1-3AD2-4D6F-9481-500BD7037203@oracle.com>

On 12/12/19 8:07 PM, Liran Alon wrote:
>> 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().

That's what I already do.  But if I wanted to make the hugepage_adjust() 
function also handle the change to 1 GB, then that code would apply to 
THP too.  I didn't want to do that without knowing the implications for THP.

>> 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().
> 
> I prefer to walk page-tables only for is_zone_device_page().

Is there another way to tell if a THP page is 1 GB?  Anyway, this is the 
sort of stuff I didn't want to mess around with.

hugepage_adjust() seemed like a reasonable place to get a huge (2MB) 
page table entry out of a DAX mapping.  I didn't want to proliferate 
another special case for upgrading to a larger PTE size (i.e. how 
hugetlbfs and THP have separate mechanisms), so I hopped on to the "can 
we do a 2MB mapping even though host_mapping_level() didn't say so" case 
- which is my interpretation of what huge_adjust() is for.

Barret

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WARNING: multiple messages have this Message-ID (diff)
From: Barret Rhoden <brho@google.com>
To: Liran Alon <liran.alon@oracle.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: Fri, 13 Dec 2019 09:13:05 -0500	[thread overview]
Message-ID: <90a9af31-304c-e8d5-b17c-0ddb4c98fddb@google.com> (raw)
In-Reply-To: <65FB6CC1-3AD2-4D6F-9481-500BD7037203@oracle.com>

On 12/12/19 8:07 PM, Liran Alon wrote:
>> 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().

That's what I already do.  But if I wanted to make the hugepage_adjust() 
function also handle the change to 1 GB, then that code would apply to 
THP too.  I didn't want to do that without knowing the implications for THP.

>> 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().
> 
> I prefer to walk page-tables only for is_zone_device_page().

Is there another way to tell if a THP page is 1 GB?  Anyway, this is the 
sort of stuff I didn't want to mess around with.

hugepage_adjust() seemed like a reasonable place to get a huge (2MB) 
page table entry out of a DAX mapping.  I didn't want to proliferate 
another special case for upgrading to a larger PTE size (i.e. how 
hugetlbfs and THP have separate mechanisms), so I hopped on to the "can 
we do a 2MB mapping even though host_mapping_level() didn't say so" case 
- which is my interpretation of what huge_adjust() is for.

Barret



  reply	other threads:[~2019-12-13 20:48 UTC|newest]

Thread overview: 44+ 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 ` Barret Rhoden
2019-12-12 18:22 ` [PATCH v5 1/2] mm: make dev_pagemap_mapping_shift() externally visible Barret Rhoden
2019-12-12 18:22   ` Barret Rhoden
2019-12-13 17:47   ` Sean Christopherson
2019-12-13 17:47     ` Sean Christopherson
2019-12-13 18:13     ` Dan Williams
2019-12-13 18:13       ` Dan Williams
2019-12-16 17:59     ` Barret Rhoden
2019-12-16 17:59       ` Barret Rhoden
2019-12-18  0:18       ` Sean Christopherson
2019-12-18  0:18         ` Sean Christopherson
2020-01-15 18:33       ` Paolo Bonzini
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 18:22   ` Barret Rhoden
2019-12-12 18:47   ` Liran Alon
2019-12-12 18:47     ` Liran Alon
2019-12-12 18:49     ` Liran Alon
2019-12-12 18:49       ` Liran Alon
2019-12-12 19:55       ` Barret Rhoden
2019-12-12 19:55         ` Barret Rhoden
2019-12-13  1:07         ` Liran Alon
2019-12-13  1:07           ` Liran Alon
2019-12-13 14:13           ` Barret Rhoden [this message]
2019-12-13 14:13             ` Barret Rhoden
2019-12-13 17:19           ` Sean Christopherson
2019-12-13 17:19             ` Sean Christopherson
2019-12-13 17:31             ` Liran Alon
2019-12-13 17:31               ` Liran Alon
2019-12-13 17:50               ` Sean Christopherson
2019-12-13 17:50                 ` Sean Christopherson
2019-12-13 18:08                 ` Liran Alon
2019-12-13 18:08                   ` Liran Alon
2019-12-16 16:05             ` Barret Rhoden
2019-12-16 16:05               ` Barret Rhoden
2020-01-07 19:05               ` Sean Christopherson
2020-01-07 19:05                 ` Sean Christopherson
2020-01-07 19:19                 ` Barret Rhoden
2020-01-07 19:19                   ` Barret Rhoden
2020-01-08  1:20                   ` Sean Christopherson
2020-01-08  1:20                     ` Sean Christopherson
2020-01-08  1:39                     ` Dan Williams
2020-01-08  1:39                       ` Dan Williams

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