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

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

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.

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