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Date: Mon, 13 Oct 2014 23:36:15 +0300 From: "Kirill A. Shutemov" <kirill@...temov.name> To: Matthew Wilcox <willy@...ux.intel.com> Cc: Matthew Wilcox <matthew.r.wilcox@...el.com>, linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org, linux-mm@...ck.org Subject: Re: [PATCH v1 2/7] mm: Prepare for DAX huge pages On Thu, Oct 09, 2014 at 04:40:26PM -0400, Matthew Wilcox wrote: > On Wed, Oct 08, 2014 at 10:43:35PM +0300, Kirill A. Shutemov wrote: > > On Wed, Oct 08, 2014 at 11:57:58AM -0400, Matthew Wilcox wrote: > > > On Wed, Oct 08, 2014 at 06:21:24PM +0300, Kirill A. Shutemov wrote: > > > > On Wed, Oct 08, 2014 at 09:25:24AM -0400, Matthew Wilcox wrote: > > > > > From: Matthew Wilcox <willy@...ux.intel.com> > > > > > > > > > > DAX wants to use the 'special' bit to mark PMD entries that are not backed > > > > > by struct page, just as for PTEs. > > > > > > > > Hm. I don't see where you use PMD without special set. > > > > > > Right ... I don't currently insert PMDs that point to huge pages of DRAM, > > > only to huge pages of PMEM. > > > > Looks like you don't need pmd_{mk,}special() then. It seems you have all > > inforamtion you need -- vma -- to find out what's going on. Right? > > That would prevent us from putting huge pages of DRAM into a VM_MIXEDMAP | > VM_HUGEPAGE vma. Is that acceptable to the wider peanut gallery? We didn't have huge pages on VM_MIXEDMAP | VM_HUGEPAGE before and we don't have them there after the patchset. Nothing changed. It probably worth adding VM_BUG_ON() in some code path to be able to catch this situation. > > > > No private THP pages with THP? Why? > > > > It should be trivial: we already have a code path for !page case for zero > > > > page and it shouldn't be too hard to modify do_dax_pmd_fault() to support > > > > COW. > > > > > > > > I remeber I've mentioned that you don't think it's reasonable to allocate > > > > 2M page on COW, but that's what we do for anon memory... > > > > > > I agree that it shouldn't be too hard, but I have no evidence that it'll > > > be a performance win to COW 2MB pages for MAP_PRIVATE. I'd rather be > > > cautious for now and we can explore COWing 2MB chunks in a future patch. > > > > I would rather make it other way around: use the same apporoach as for > > anon memory until data shows it's doesn't make any good. Then consider > > switching COW for *both* anon and file THP to fallback path. > > This way we will get consistent behaviour for both types of mappings. > > I'm not sure that we want consistent behaviour for both types of mappings. > My understanding is that they're used for different purposes, and having > different bahaviour is acceptable. This should be described in commit message along with other design solutions (split wrt. mlock, etc) with their pros and cons. -- Kirill A. Shutemov -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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