Re: [PATCHSET 0/2] Allow single pagefault in write access of a VM_MIXEDMAP mapping

[Dan Williams <dan.j.williams@intel.com>]

On Mon, Jan 11, 2016 at 1:22 AM, Boaz Harrosh <boaz@plexistor.com> wrote:
> On 01/11/2016 03:19 AM, Dan Williams wrote:
>> On Sun, Jan 10, 2016 at 5:59 AM, Boaz Harrosh <boaz@plexistor.com> wrote:
>>> Hi
>>>
>>> Today any VM_MIXEDMAP or VM_PFN mapping when enabling a write access
>>> to their mapping, will have a double pagefault for every write access.
>>>
>>> This is because vma->vm_page_prot defines how a page/pfn is inserted into
>>> the page table (see vma_wants_writenotify in mm/mmap.c).
>>>
>>> Which means that it is always inserted with read-only under the
>>> assumption that we want to be notified when write access occurs.
>>>
>>> But this is not always true and adds an unnecessary page-fault on
>>> every new mmap-write access
>>>
>>> This patchset is trying to give the fault handler more choice by passing
>>> an pgprot_t to vm_insert_mixed() via a new vm_insert_mixed_prot() API.
>>>
>>> If the mm guys feel that the pgprot_t and its helpers and flags are private
>>> to mm/memory.c I can easily do a new: vm_insert_mixed_rw() instead. of the
>>> above vm_insert_mixed_prot() which enables any control not only write.
>>>
>>> Following is a patch to DAX to optimize out the extra page-fault.
>>>
>>> TODO: I only did 4k mapping perhaps 2M mapping can enjoy the same single
>>> fault on write access. If interesting to anyone I can attempt a fix.
>>>
>>> Dan Andrew who needs to pick this up please?
>>
>> This collides with the patches currently pending in -mm for 4.5, lets
>> take a look at this for 4.6.
>>
>
> OK thanks, I will try to work this over current linux-next and sure we
> will wait for 4.5-rc1 to look at this again.
>
> Do you have any comments in general about this?

Looks worthwhile at first glance, the only concern that comes to mind
is integration with Ross' fsync/msync enabling.  How much does this
change matter in practice?  If the mapping is long standing then I
expect this cost gets hidden?

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