linux-kernel - Re: [RFC PATCH v3 06/10] mm/mshare: Add vm flag for shared PTEs

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Message-ID: <04c4314e-7958-47bd-8281-23c3e35fc10e@redhat.com>
Date: Mon, 7 Oct 2024 12:24:56 +0200
From: David Hildenbrand <david@...hat.com>
To: James Houghton <jthoughton@...gle.com>,
 Anthony Yznaga <anthony.yznaga@...cle.com>
Cc: akpm@...ux-foundation.org, willy@...radead.org, markhemm@...glemail.com,
 viro@...iv.linux.org.uk, khalid@...nel.org, andreyknvl@...il.com,
 dave.hansen@...el.com, luto@...nel.org, brauner@...nel.org, arnd@...db.de,
 ebiederm@...ssion.com, catalin.marinas@....com, linux-arch@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-mm@...ck.org, mhiramat@...nel.org,
 rostedt@...dmis.org, vasily.averin@...ux.dev, xhao@...ux.alibaba.com,
 pcc@...gle.com, neilb@...e.de, maz@...nel.org
Subject: Re: [RFC PATCH v3 06/10] mm/mshare: Add vm flag for shared PTEs

On 04.09.24 01:40, James Houghton wrote:
> On Tue, Sep 3, 2024 at 4:23 PM Anthony Yznaga <anthony.yznaga@...cle.com> wrote:
>>
>> From: Khalid Aziz <khalid@...nel.org>
>>
>> Add a bit to vm_flags to indicate a vma shares PTEs with others. Add
>> a function to determine if a vma shares PTEs by checking this flag.
>> This is to be used to find the shared page table entries on page fault
>> for vmas sharing PTEs.
>>
>> Signed-off-by: Khalid Aziz <khalid@...nel.org>
>> Signed-off-by: Matthew Wilcox (Oracle) <willy@...radead.org>
>> Signed-off-by: Anthony Yznaga <anthony.yznaga@...cle.com>
>> ---
>>   include/linux/mm.h             | 7 +++++++
>>   include/trace/events/mmflags.h | 3 +++
>>   mm/internal.h                  | 5 +++++
>>   3 files changed, 15 insertions(+)
>>
>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>> index 6549d0979b28..3aa0b3322284 100644
>> --- a/include/linux/mm.h
>> +++ b/include/linux/mm.h
>> @@ -413,6 +413,13 @@ extern unsigned int kobjsize(const void *objp);
>>   #define VM_DROPPABLE           VM_NONE
>>   #endif
>>
>> +#ifdef CONFIG_64BIT
>> +#define VM_SHARED_PT_BIT       41
>> +#define VM_SHARED_PT           BIT(VM_SHARED_PT_BIT)
>> +#else
>> +#define VM_SHARED_PT           VM_NONE
>> +#endif
>> +
>>   #ifdef CONFIG_64BIT
>>   /* VM is sealed, in vm_flags */
>>   #define VM_SEALED      _BITUL(63)
>> diff --git a/include/trace/events/mmflags.h b/include/trace/events/mmflags.h
>> index b63d211bd141..e1ae1e60d086 100644
>> --- a/include/trace/events/mmflags.h
>> +++ b/include/trace/events/mmflags.h
>> @@ -167,8 +167,10 @@ IF_HAVE_PG_ARCH_X(arch_3)
>>
>>   #ifdef CONFIG_64BIT
>>   # define IF_HAVE_VM_DROPPABLE(flag, name) {flag, name},
>> +# define IF_HAVE_VM_SHARED_PT(flag, name) {flag, name},
>>   #else
>>   # define IF_HAVE_VM_DROPPABLE(flag, name)
>> +# define IF_HAVE_VM_SHARED_PT(flag, name)
>>   #endif
>>
>>   #define __def_vmaflag_names                                            \
>> @@ -204,6 +206,7 @@ IF_HAVE_VM_SOFTDIRTY(VM_SOFTDIRTY,  "softdirty"     )               \
>>          {VM_HUGEPAGE,                   "hugepage"      },              \
>>          {VM_NOHUGEPAGE,                 "nohugepage"    },              \
>>   IF_HAVE_VM_DROPPABLE(VM_DROPPABLE,     "droppable"     )               \
>> +IF_HAVE_VM_SHARED_PT(VM_SHARED_PT,     "sharedpt"      )               \
>>          {VM_MERGEABLE,                  "mergeable"     }               \
>>
>>   #define show_vma_flags(flags)                                          \
>> diff --git a/mm/internal.h b/mm/internal.h
>> index b4d86436565b..8005d5956b6e 100644
>> --- a/mm/internal.h
>> +++ b/mm/internal.h
>> @@ -1578,4 +1578,9 @@ void unlink_file_vma_batch_init(struct unlink_vma_file_batch *);
>>   void unlink_file_vma_batch_add(struct unlink_vma_file_batch *, struct vm_area_struct *);
>>   void unlink_file_vma_batch_final(struct unlink_vma_file_batch *);
>>
> 
> Hi Anthony,
> 
> I'm really excited to see this series on the mailing list again! :) I
> won't have time to review this series in too much detail, but I hope
> something like it gets merged eventually.
> 
>> +static inline bool vma_is_shared(const struct vm_area_struct *vma)
>> +{
>> +       return VM_SHARED_PT && (vma->vm_flags & VM_SHARED_PT);
>> +}
> 
> Tiny comment - I find vma_is_shared() to be a bit of a confusing name,
> especially given how vma_is_shared_maywrite() is defined. (Sorry if
> this has already been discussed before.)
> 
> How about vma_is_shared_pt()?

vma_is_mshare() ? ;)

The whole "shared PT / shared PTE" is a bit misleading IMHO and a bit 
too dominant in the series. Yes, we're sharing PTEs/page tables, but the 
main point is that a single mshare VMA might cover multiple different 
VMAs (in a different process).

I would describe mshare VMAs as being something that shares page tables 
with another MM, BUT, also that the VMA is a container and what exactly 
the *actual* VMAs in there are (including holes), only the owner knows.

E.g., is_vm_hugetlb_page() might be *false* for an mshare VMA, but there 
might be hugetlb folios mapped into the page tables, described by a 
is_vm_hugetlb_page() VMA in the owner MM.

So again, it's not just "sharing page tables".

-- 
Cheers,

David / dhildenb