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Subject[PATCH 2/2] mm/dax: Don't enable huge dax mapping by default
Date
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.

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.kumar@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;

if (transparent_hugepage_flags &
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index faf357eaf0ce..43d742fe0341 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -53,6 +53,7 @@ unsigned long transparent_hugepage_flags __read_mostly =
#ifdef CONFIG_TRANSPARENT_HUGEPAGE_MADVISE
(1<<TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG)|
#endif
+ (1 << TRANSPARENT_HUGEPAGE_DAX_FLAG) |
(1<<TRANSPARENT_HUGEPAGE_DEFRAG_REQ_MADV_FLAG)|
(1<<TRANSPARENT_HUGEPAGE_DEFRAG_KHUGEPAGED_FLAG)|
(1<<TRANSPARENT_HUGEPAGE_USE_ZERO_PAGE_FLAG);
@@ -475,6 +476,8 @@ static int __init setup_transparent_hugepage(char *str)
&transparent_hugepage_flags);
clear_bit(TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG,
&transparent_hugepage_flags);
+ clear_bit(TRANSPARENT_HUGEPAGE_DAX_FLAG,
+ &transparent_hugepage_flags);
ret = 1;
}
out:
@@ -753,6 +756,7 @@ static void insert_pfn_pmd(struct vm_area_struct *vma, unsigned long addr,
spinlock_t *ptl;

ptl = pmd_lock(mm, pmd);
+ /* should we check for none here again? */
entry = pmd_mkhuge(pfn_t_pmd(pfn, prot));
if (pfn_t_devmap(pfn))
entry = pmd_mkdevmap(entry);
--
2.20.1
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