Dan Williams <dan.j.willi...@intel.com> writes: > On Wed, Mar 6, 2019 at 1:18 AM Aneesh Kumar K.V > <aneesh.ku...@linux.ibm.com> wrote: >> >> Dan Williams <dan.j.willi...@intel.com> writes: >> >> > On Thu, Feb 28, 2019 at 1:40 AM Oliver <ooh...@gmail.com> wrote: >> >> >> >> On Thu, Feb 28, 2019 at 7:35 PM Aneesh Kumar K.V >> >> <aneesh.ku...@linux.ibm.com> wrote: >> >> > >> >> > Add a flag to indicate the ability to do huge page dax mapping. On >> >> > architecture >> >> > like ppc64, the hypervisor can disable huge page support in the guest. >> >> > In >> >> > such a case, we should not enable huge page dax mapping. This patch adds >> >> > a flag which the architecture code will update to indicate huge page >> >> > dax mapping support. >> >> >> >> *groan* >> >> >> >> > Architectures mostly do transparent_hugepage_flag = 0; if they can't >> >> > do hugepages. That also takes care of disabling dax hugepage mapping >> >> > with this change. >> >> > >> >> > Without this patch we get the below error with kvm on ppc64. >> >> > >> >> > [ 118.849975] lpar: Failed hash pte insert with error -4 >> >> > >> >> > NOTE: The patch also use >> >> > >> >> > echo never > /sys/kernel/mm/transparent_hugepage/enabled >> >> > to disable dax huge page mapping. >> >> > >> >> > Signed-off-by: Aneesh Kumar K.V <aneesh.ku...@linux.ibm.com> >> >> > --- >> >> > TODO: >> >> > * Add Fixes: tag >> >> > >> >> > include/linux/huge_mm.h | 4 +++- >> >> > mm/huge_memory.c | 4 ++++ >> >> > 2 files changed, 7 insertions(+), 1 deletion(-) >> >> > >> >> > diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h >> >> > index 381e872bfde0..01ad5258545e 100644 >> >> > --- a/include/linux/huge_mm.h >> >> > +++ b/include/linux/huge_mm.h >> >> > @@ -53,6 +53,7 @@ vm_fault_t vmf_insert_pfn_pud(struct vm_area_struct >> >> > *vma, unsigned long addr, >> >> > pud_t *pud, pfn_t pfn, bool write); >> >> > enum transparent_hugepage_flag { >> >> > TRANSPARENT_HUGEPAGE_FLAG, >> >> > + TRANSPARENT_HUGEPAGE_DAX_FLAG, >> >> > TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG, >> >> > TRANSPARENT_HUGEPAGE_DEFRAG_DIRECT_FLAG, >> >> > TRANSPARENT_HUGEPAGE_DEFRAG_KSWAPD_FLAG, >> >> > @@ -111,7 +112,8 @@ static inline bool >> >> > __transparent_hugepage_enabled(struct vm_area_struct *vma) >> >> > if (transparent_hugepage_flags & (1 << >> >> > TRANSPARENT_HUGEPAGE_FLAG)) >> >> > return true; >> >> > >> >> > - if (vma_is_dax(vma)) >> >> > + if (vma_is_dax(vma) && >> >> > + (transparent_hugepage_flags & (1 << >> >> > TRANSPARENT_HUGEPAGE_DAX_FLAG))) >> >> > return true; >> >> >> >> Forcing PTE sized faults should be fine for fsdax, but it'll break >> >> devdax. The devdax driver requires the fault size be >= the namespace >> >> alignment since devdax tries to guarantee hugepage mappings will be >> >> used and PMD alignment is the default. We can probably have devdax >> >> fall back to the largest size the hypervisor has made available, but >> >> it does run contrary to the design. Ah well, I suppose it's better off >> >> being degraded rather than unusable. >> > >> > Given this is an explicit setting I think device-dax should explicitly >> > fail to enable in the presence of this flag to preserve the >> > application visible behavior. >> > >> > I.e. if device-dax was enabled after this setting was made then I >> > think future faults should fail as well. >> >> Not sure I understood that. Now we are disabling the ability to map >> pages as huge pages. I am now considering that this should not be >> user configurable. Ie, this is something that platform can use to avoid >> dax forcing huge page mapping, but if the architecture can enable huge >> dax mapping, we should always default to using that. > > No, that's an application visible behavior regression. The side effect > of this setting is that all huge-page configured device-dax instances > must be disabled.
So if the device was created with a nd_pfn->align value of PMD_SIZE, that is an indication that we would map the pages in PMD_SIZE? Ok with that understanding, If the align value is not a supported mapping size, we fail initializing the device. > >> Now w.r.t to failures, can device-dax do an opportunistic huge page >> usage? > > device-dax explicitly disclaims the ability to do opportunistic mappings. > >> I haven't looked at the device-dax details fully yet. Do we make the >> assumption of the mapping page size as a format w.r.t device-dax? Is that >> derived from nd_pfn->align value? > > Correct. > >> >> Here is what I am working on: >> 1) If the platform doesn't support huge page and if the device superblock >> indicated that it was created with huge page support, we fail the device >> init. > > Ok. > >> 2) Now if we are creating a new namespace without huge page support in >> the platform, then we force the align details to PAGE_SIZE. In such a >> configuration when handling dax fault even with THP enabled during >> the build, we should not try to use hugepage. This I think we can >> achieve by using TRANSPARENT_HUGEPAEG_DAX_FLAG. > > How is this dynamic property communicated to the guest? via device tree on powerpc. We have a device tree node indicating supported page sizes. > >> >> Also even if the user decided to not use THP, by >> echo "never" > transparent_hugepage/enabled , we should continue to map >> dax fault using huge page on platforms that can support huge pages. >> >> This still doesn't cover the details of a device-dax created with >> PAGE_SIZE align later booted with a kernel that can do hugepage dax.How >> should we handle that? That makes me think, this should be a VMA flag >> which got derived from device config? May be use VM_HUGEPAGE to indicate >> if device should use a hugepage mapping or not? > > device-dax configured with PAGE_SIZE always gets PAGE_SIZE mappings. Now what will be page size used for mapping vmemmap? Architectures possibly will use PMD_SIZE mapping if supported for vmemmap. Now a device-dax with struct page in the device will have pfn reserve area aligned to PAGE_SIZE with the above example? We can't map that using PMD_SIZE page size? -aneesh