Allow single pagefault in write access of a VM_MIXEDMAP mapping

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?

list of patches:
[PATCH 1/2] mm: Allow single pagefault on mmap-write with VM_MIXEDMAP [PATCH 2/2] dax: Only fault once on mmap write access

Thank you
Boaz

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