* [PATCH V2 0/2] Fixes for contiguous hugetlb
@ 2018-09-21 15:34 Steve Capper
2018-09-21 15:34 ` [PATCH V2 1/2] arm64: hugetlb: Fix handling of young ptes Steve Capper
` (2 more replies)
0 siblings, 3 replies; 4+ messages in thread
From: Steve Capper @ 2018-09-21 15:34 UTC (permalink / raw)
To: linux-arm-kernel
This patch series addresses a couple of issues found with contiguous hugetlb.
1) It is possible for old hugetlb's to be created by remove_migration_pte, the
first patch adjusts the code to capture any young ptes that could have been
created by hardware dirty bit management.
2) huge_ptep_set_access_flags has been changed in the second patch to prevent it
from unnecessarily invalidating contiguous hugetlb mappings (which also stops
it from potentially evoking back-to-back page faults).
These patches have not yet been tested on hardware.
Steve Capper (2):
arm64: hugetlb: Fix handling of young ptes
arm64: hugetlb: Avoid unnecessary clearing in
huge_ptep_set_access_flags
arch/arm64/mm/hugetlbpage.c | 50 ++++++++++++++++++++++++++++++++++++++-------
1 file changed, 43 insertions(+), 7 deletions(-)
--
2.11.0
^ permalink raw reply [flat|nested] 4+ messages in thread
* [PATCH V2 1/2] arm64: hugetlb: Fix handling of young ptes
2018-09-21 15:34 [PATCH V2 0/2] Fixes for contiguous hugetlb Steve Capper
@ 2018-09-21 15:34 ` Steve Capper
2018-09-21 15:34 ` [PATCH V2 2/2] arm64: hugetlb: Avoid unnecessary clearing in huge_ptep_set_access_flags Steve Capper
2018-09-24 16:25 ` [PATCH V2 0/2] Fixes for contiguous hugetlb Will Deacon
2 siblings, 0 replies; 4+ messages in thread
From: Steve Capper @ 2018-09-21 15:34 UTC (permalink / raw)
To: linux-arm-kernel
In the contiguous bit hugetlb break-before-make code we assume that all
hugetlb pages are young.
In fact, remove_migration_pte is able to place an old hugetlb pte so
this assumption is not valid.
This patch fixes the contiguous hugetlb scanning code to preserve young
ptes.
Fixes: d8bdcff28764 ("arm64: hugetlb: Add break-before-make logic for contiguous entries")
Signed-off-by: Steve Capper <steve.capper@arm•com>
---
arch/arm64/mm/hugetlbpage.c | 12 +++++++++---
1 file changed, 9 insertions(+), 3 deletions(-)
diff --git a/arch/arm64/mm/hugetlbpage.c b/arch/arm64/mm/hugetlbpage.c
index 192b3ba07075..f85be2f8b140 100644
--- a/arch/arm64/mm/hugetlbpage.c
+++ b/arch/arm64/mm/hugetlbpage.c
@@ -117,11 +117,14 @@ static pte_t get_clear_flush(struct mm_struct *mm,
/*
* If HW_AFDBM is enabled, then the HW could turn on
- * the dirty bit for any page in the set, so check
- * them all. All hugetlb entries are already young.
+ * the dirty or accessed bit for any page in the set,
+ * so check them all.
*/
if (pte_dirty(pte))
orig_pte = pte_mkdirty(orig_pte);
+
+ if (pte_young(pte))
+ orig_pte = pte_mkyoung(orig_pte);
}
if (valid) {
@@ -340,10 +343,13 @@ int huge_ptep_set_access_flags(struct vm_area_struct *vma,
if (!pte_same(orig_pte, pte))
changed = 1;
- /* Make sure we don't lose the dirty state */
+ /* Make sure we don't lose the dirty or young state */
if (pte_dirty(orig_pte))
pte = pte_mkdirty(pte);
+ if (pte_young(orig_pte))
+ pte = pte_mkyoung(pte);
+
hugeprot = pte_pgprot(pte);
for (i = 0; i < ncontig; i++, ptep++, addr += pgsize, pfn += dpfn)
set_pte_at(vma->vm_mm, addr, ptep, pfn_pte(pfn, hugeprot));
--
2.11.0
^ permalink raw reply related [flat|nested] 4+ messages in thread
* [PATCH V2 2/2] arm64: hugetlb: Avoid unnecessary clearing in huge_ptep_set_access_flags
2018-09-21 15:34 [PATCH V2 0/2] Fixes for contiguous hugetlb Steve Capper
2018-09-21 15:34 ` [PATCH V2 1/2] arm64: hugetlb: Fix handling of young ptes Steve Capper
@ 2018-09-21 15:34 ` Steve Capper
2018-09-24 16:25 ` [PATCH V2 0/2] Fixes for contiguous hugetlb Will Deacon
2 siblings, 0 replies; 4+ messages in thread
From: Steve Capper @ 2018-09-21 15:34 UTC (permalink / raw)
To: linux-arm-kernel
For contiguous hugetlb, huge_ptep_set_access_flags performs a
get_clear_flush (which then flushes the TLBs) even when no change of ptes
is necessary.
Unfortunately, this behaviour can lead to back-to-back page faults being
generated when running with multiple threads that access the same
contiguous huge page.
Thread 1 | Thread 2
-----------------------------+------------------------------
hugetlb_fault |
huge_ptep_set_access_flags |
-> invalidate pte range | hugetlb_fault
continue processing | wait for hugetlb_fault_mutex
release mutex and return | huge_ptep_set_access_flags
| -> invalidate pte range
hugetlb_fault
...
This patch changes huge_ptep_set_access_flags s.t. we first read the
contiguous range of ptes (whilst preserving dirty information); the pte
range is only then invalidated where necessary and this prevents further
spurious page faults.
Fixes: d8bdcff28764 ("arm64: hugetlb: Add break-before-make logic for contiguous entries")
Reported-by: Lei Zhang <zhang.lei@jp•fujitsu.com>
Signed-off-by: Steve Capper <steve.capper@arm•com>
---
Changed in v2: we just check dirty, young for the contiguous range and
writable for the first pte in the range.
---
arch/arm64/mm/hugetlbpage.c | 38 ++++++++++++++++++++++++++++++++++----
1 file changed, 34 insertions(+), 4 deletions(-)
diff --git a/arch/arm64/mm/hugetlbpage.c b/arch/arm64/mm/hugetlbpage.c
index f85be2f8b140..f58ea503ad01 100644
--- a/arch/arm64/mm/hugetlbpage.c
+++ b/arch/arm64/mm/hugetlbpage.c
@@ -323,11 +323,40 @@ pte_t huge_ptep_get_and_clear(struct mm_struct *mm,
return get_clear_flush(mm, addr, ptep, pgsize, ncontig);
}
+/*
+ * huge_ptep_set_access_flags will update access flags (dirty, accesssed)
+ * and write permission.
+ *
+ * For a contiguous huge pte range we need to check whether or not write
+ * permission has to change only on the first pte in the set. Then for
+ * all the contiguous ptes we need to check whether or not there is a
+ * discrepancy between dirty or young.
+ */
+static int __cont_access_flags_changed(pte_t *ptep, pte_t pte, int ncontig)
+{
+ int i;
+
+ if (pte_write(pte) != pte_write(huge_ptep_get(ptep)))
+ return 1;
+
+ for (i = 0; i < ncontig; i++) {
+ pte_t orig_pte = huge_ptep_get(ptep + i);
+
+ if (pte_dirty(pte) != pte_dirty(orig_pte))
+ return 1;
+
+ if (pte_young(pte) != pte_young(orig_pte))
+ return 1;
+ }
+
+ return 0;
+}
+
int huge_ptep_set_access_flags(struct vm_area_struct *vma,
unsigned long addr, pte_t *ptep,
pte_t pte, int dirty)
{
- int ncontig, i, changed = 0;
+ int ncontig, i;
size_t pgsize = 0;
unsigned long pfn = pte_pfn(pte), dpfn;
pgprot_t hugeprot;
@@ -339,9 +368,10 @@ int huge_ptep_set_access_flags(struct vm_area_struct *vma,
ncontig = find_num_contig(vma->vm_mm, addr, ptep, &pgsize);
dpfn = pgsize >> PAGE_SHIFT;
+ if (!__cont_access_flags_changed(ptep, pte, ncontig))
+ return 0;
+
orig_pte = get_clear_flush(vma->vm_mm, addr, ptep, pgsize, ncontig);
- if (!pte_same(orig_pte, pte))
- changed = 1;
/* Make sure we don't lose the dirty or young state */
if (pte_dirty(orig_pte))
@@ -354,7 +384,7 @@ int huge_ptep_set_access_flags(struct vm_area_struct *vma,
for (i = 0; i < ncontig; i++, ptep++, addr += pgsize, pfn += dpfn)
set_pte_at(vma->vm_mm, addr, ptep, pfn_pte(pfn, hugeprot));
- return changed;
+ return 1;
}
void huge_ptep_set_wrprotect(struct mm_struct *mm,
--
2.11.0
^ permalink raw reply related [flat|nested] 4+ messages in thread
* [PATCH V2 0/2] Fixes for contiguous hugetlb
2018-09-21 15:34 [PATCH V2 0/2] Fixes for contiguous hugetlb Steve Capper
2018-09-21 15:34 ` [PATCH V2 1/2] arm64: hugetlb: Fix handling of young ptes Steve Capper
2018-09-21 15:34 ` [PATCH V2 2/2] arm64: hugetlb: Avoid unnecessary clearing in huge_ptep_set_access_flags Steve Capper
@ 2018-09-24 16:25 ` Will Deacon
2 siblings, 0 replies; 4+ messages in thread
From: Will Deacon @ 2018-09-24 16:25 UTC (permalink / raw)
To: linux-arm-kernel
On Fri, Sep 21, 2018 at 04:34:03PM +0100, Steve Capper wrote:
> This patch series addresses a couple of issues found with contiguous hugetlb.
>
> 1) It is possible for old hugetlb's to be created by remove_migration_pte, the
> first patch adjusts the code to capture any young ptes that could have been
> created by hardware dirty bit management.
>
> 2) huge_ptep_set_access_flags has been changed in the second patch to prevent it
> from unnecessarily invalidating contiguous hugetlb mappings (which also stops
> it from potentially evoking back-to-back page faults).
Thanks Steve, these look good to me. I'll pick them up as fixes.
Will
^ permalink raw reply [flat|nested] 4+ messages in thread
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2018-09-21 15:34 [PATCH V2 0/2] Fixes for contiguous hugetlb Steve Capper
2018-09-21 15:34 ` [PATCH V2 1/2] arm64: hugetlb: Fix handling of young ptes Steve Capper
2018-09-21 15:34 ` [PATCH V2 2/2] arm64: hugetlb: Avoid unnecessary clearing in huge_ptep_set_access_flags Steve Capper
2018-09-24 16:25 ` [PATCH V2 0/2] Fixes for contiguous hugetlb Will Deacon
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