On Mon, Jan 11, 2016 at 1:22 AM, Boaz Harrosh <boaz@xxxxxxxxxxxxx> wrote: > On 01/11/2016 03:19 AM, Dan Williams wrote: >> On Sun, Jan 10, 2016 at 5:59 AM, Boaz Harrosh <boaz@xxxxxxxxxxxxx> 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? -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>