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

[Anthony Yznaga <anthony.yznaga@oracle.com>]

On 10/7/24 3:24 AM, David Hildenbrand wrote:
> On 04.09.24 01:40, James Houghton wrote:
>> On Tue, Sep 3, 2024 at 4:23 PM Anthony Yznaga 
>> <anthony.yznaga@oracle.com> wrote:
>>>
>>> From: Khalid Aziz <khalid@kernel.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@kernel.org>
>>> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
>>> Signed-off-by: Anthony Yznaga <anthony.yznaga@oracle.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() ? ;)

vma_is_vmas()? :-D


>
> 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".

Understood. I'm okay with something like vma_is_mshare() or some other 
shorthand for a "container" VMA. And I recognize that I need to identify 
which code paths need to be enlightened to container VMAs and which 
should expect to be operating on a real VMA or don't care.


Anthony