Messages in this thread | | | Date | Wed, 2 Dec 2015 10:06:36 -0800 | Subject | Re: [PATCH] mm: Fix mmap MAP_POPULATE for DAX pmd mapping | From | Dan Williams <> |
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On Wed, Dec 2, 2015 at 9:01 AM, Dan Williams <dan.j.williams@intel.com> wrote: > On Wed, Dec 2, 2015 at 9:43 AM, Toshi Kani <toshi.kani@hpe.com> wrote: >> Oh, I see. I will setup the memmap array and run the tests again. >> >> But, why does the PMD mapping depend on the memmap array? We have observed >> major performance improvement with PMD. This feature should always be enabled >> with DAX regardless of the option to allocate the memmap array. >> > > Several factors drove this decision, I'm open to considering > alternatives but here's the reasoning: > > 1/ DAX pmd mappings caused crashes in the get_user_pages path leading > to commit e82c9ed41e8 "dax: disable pmd mappings". The reason pte > mappings don't crash and instead trigger -EFAULT is due to the > _PAGE_SPECIAL pte bit. > > 2/ To enable get_user_pages for DAX, in both the page and huge-page > case, we need a new pte bit _PAGE_DEVMAP. > > 3/ Given the pte bits are hard to come I'm assuming we won't get two, > i.e. both _PAGE_DEVMAP and a new _PAGE_SPECIAL for pmds. Even if we > could get a _PAGE_SPECIAL for pmds I'm not in favor of pursuing it.
Actually, Dave says they aren't that hard to come by for pmds, so we could go add _PMD_SPECIAL if we really wanted to support the limited page-less DAX-pmd case.
But I'm still of the opinion that we run away from the page-less case until it can be made a full class citizen with O_DIRECT for pfn support.
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