[PATCH v7 0/4] MIPS: page fault handling optimization

[PATCH v7 0/4] MIPS: page fault handling optimization

[Bibo Mao]

V7:
- define function flush_tlb_fix_spurious_fault as inline rather
  than macro
V6:
- Add update_mmu_tlb function as empty on all platform except mips
  system, we use this function to update local tlb for page fault
  smp-race handling
V5:
- define update_mmu_cache function specified on MIPS platform, and
  add page fault smp-race stats info
V4:
- add pte_sw_mkyoung function to implement readable privilege, and
  this function is  only in effect on MIPS system.
- add page valid bit judgement in function pte_modify
V3:
- add detailed changelog, modify typo issue in patch V2
v2:
- split flush_tlb_fix_spurious_fault and tlb update into two patches
- comments typo modification
- separate tlb update and add pte readable privilege into two patches

Bibo Mao (4):
  MIPS: Do not flush tlb page when updating PTE entry
  mm/memory.c: Update local TLB if PTE entry exists
  mm/memory.c: Add memory read privilege on page fault handling
  MIPS: mm: add page valid judgement in function pte_modify

 arch/mips/include/asm/pgtable.h | 37 +++++++++++++++++++++++++++++++++++--
 include/asm-generic/pgtable.h   | 33 +++++++++++++++++++++++++++++++++
 mm/memory.c                     | 30 ++++++++++++++++++++++--------
 3 files changed, 90 insertions(+), 10 deletions(-)

-- 
1.8.3.1

[PATCH v7 1/4] MIPS: Do not flush tlb page when updating PTE entry

[Bibo Mao]

It is not necessary to flush tlb page on all CPUs if suitable PTE
entry exists already during page fault handling, just updating
TLB is fine.

Here redefine flush_tlb_fix_spurious_fault as empty on MIPS system.

Signed-off-by: Bibo Mao <maobibo@loongson.cn>
---
 arch/mips/include/asm/pgtable.h | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/arch/mips/include/asm/pgtable.h b/arch/mips/include/asm/pgtable.h
index 9b01d2d..f8f48fc 100644
--- a/arch/mips/include/asm/pgtable.h
+++ b/arch/mips/include/asm/pgtable.h
@@ -478,6 +478,11 @@ static inline pgprot_t pgprot_writecombine(pgprot_t _prot)
 	return __pgprot(prot);
 }
 
+static inline void flush_tlb_fix_spurious_fault(struct vm_area_struct *vma,
+						unsigned long address)
+{
+}
+
 /*
  * Conversion functions: convert a page and protection to a page entry,
  * and a page entry and page directory to the page they refer to.
-- 
1.8.3.1

[PATCH v7 2/4] mm/memory.c: Update local TLB if PTE entry exists

[Bibo Mao]

If two threads concurrently fault at the same page, the thread that
won the race updates the PTE and its local TLB. For now, the other
thread gives up, simply does nothing, and continues.

It could happen that this second thread triggers another fault, whereby
it only updates its local TLB while handling the fault. Instead of
triggering another fault, let's directly update the local TLB of the
second thread. Function update_mmu_tlb is used here to update local
TLB on the second thread, and it is defined as empty on other arches.

Signed-off-by: Bibo Mao <maobibo@loongson.cn>
Acked-by: Andrew Morton <akpm@linux-foundation.org>
---
 arch/mips/include/asm/pgtable.h | 23 +++++++++++++++++++++++
 include/asm-generic/pgtable.h   | 17 +++++++++++++++++
 mm/memory.c                     | 27 +++++++++++++++++++--------
 3 files changed, 59 insertions(+), 8 deletions(-)

diff --git a/arch/mips/include/asm/pgtable.h b/arch/mips/include/asm/pgtable.h
index f8f48fc..6f40612 100644
--- a/arch/mips/include/asm/pgtable.h
+++ b/arch/mips/include/asm/pgtable.h
@@ -483,6 +483,26 @@ static inline void flush_tlb_fix_spurious_fault(struct vm_area_struct *vma,
 {
 }
 
+#define __HAVE_ARCH_PTE_SAME
+static inline int pte_same(pte_t pte_a, pte_t pte_b)
+{
+	return pte_val(pte_a) == pte_val(pte_b);
+}
+
+#define __HAVE_ARCH_PTEP_SET_ACCESS_FLAGS
+static inline int ptep_set_access_flags(struct vm_area_struct *vma,
+					unsigned long address, pte_t *ptep,
+					pte_t entry, int dirty)
+{
+	if (!pte_same(*ptep, entry))
+		set_pte_at(vma->vm_mm, address, ptep, entry);
+	/*
+	 * update_mmu_cache will unconditionally execute, handling both
+	 * the case that the PTE changed and the spurious fault case.
+	 */
+	return true;
+}
+
 /*
  * Conversion functions: convert a page and protection to a page entry,
  * and a page entry and page directory to the page they refer to.
@@ -526,6 +546,9 @@ static inline void update_mmu_cache(struct vm_area_struct *vma,
 	__update_tlb(vma, address, pte);
 }
 
+#define	__HAVE_ARCH_UPDATE_MMU_TLB
+#define update_mmu_tlb	update_mmu_cache
+
 static inline void update_mmu_cache_pmd(struct vm_area_struct *vma,
 	unsigned long address, pmd_t *pmdp)
 {
diff --git a/include/asm-generic/pgtable.h b/include/asm-generic/pgtable.h
index 329b8c8..fa5c73f 100644
--- a/include/asm-generic/pgtable.h
+++ b/include/asm-generic/pgtable.h
@@ -188,6 +188,23 @@ static inline pte_t ptep_get_and_clear_full(struct mm_struct *mm,
 }
 #endif
 
+
+/*
+ * If two threads concurrently fault at the same page, the thread that
+ * won the race updates the PTE and its local TLB/Cache. The other thread
+ * gives up, simply does nothing, and continues; on architectures where
+ * software can update TLB,  local TLB can be updated here to avoid next page
+ * fault. This function updates TLB only, do nothing with cache or others.
+ * It is the difference with function update_mmu_cache.
+ */
+#ifndef __HAVE_ARCH_UPDATE_MMU_TLB
+static inline void update_mmu_tlb(struct vm_area_struct *vma,
+				unsigned long address, pte_t *ptep)
+{
+}
+#define __HAVE_ARCH_UPDATE_MMU_TLB
+#endif
+
 /*
  * Some architectures may be able to avoid expensive synchronization
  * primitives when modifications are made to PTE's which are already
diff --git a/mm/memory.c b/mm/memory.c
index f703fe8..8bb31c4 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2436,10 +2436,9 @@ static inline bool cow_user_page(struct page *dst, struct page *src,
 		if (!likely(pte_same(*vmf->pte, vmf->orig_pte))) {
 			/*
 			 * Other thread has already handled the fault
-			 * and we don't need to do anything. If it's
-			 * not the case, the fault will be triggered
-			 * again on the same address.
+			 * and update local tlb only
 			 */
+			update_mmu_tlb(vma, addr, vmf->pte);
 			ret = false;
 			goto pte_unlock;
 		}
@@ -2463,7 +2462,8 @@ static inline bool cow_user_page(struct page *dst, struct page *src,
 		vmf->pte = pte_offset_map_lock(mm, vmf->pmd, addr, &vmf->ptl);
 		locked = true;
 		if (!likely(pte_same(*vmf->pte, vmf->orig_pte))) {
-			/* The PTE changed under us. Retry page fault. */
+			/* The PTE changed under us, update local tlb */
+			update_mmu_tlb(vma, addr, vmf->pte);
 			ret = false;
 			goto pte_unlock;
 		}
@@ -2752,6 +2752,7 @@ static vm_fault_t wp_page_copy(struct vm_fault *vmf)
 		new_page = old_page;
 		page_copied = 1;
 	} else {
+		update_mmu_tlb(vma, vmf->address, vmf->pte);
 		mem_cgroup_cancel_charge(new_page, memcg, false);
 	}
 
@@ -2812,6 +2813,7 @@ vm_fault_t finish_mkwrite_fault(struct vm_fault *vmf)
 	 * pte_offset_map_lock.
 	 */
 	if (!pte_same(*vmf->pte, vmf->orig_pte)) {
+		update_mmu_tlb(vmf->vma, vmf->address, vmf->pte);
 		pte_unmap_unlock(vmf->pte, vmf->ptl);
 		return VM_FAULT_NOPAGE;
 	}
@@ -2936,6 +2938,7 @@ static vm_fault_t do_wp_page(struct vm_fault *vmf)
 			vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd,
 					vmf->address, &vmf->ptl);
 			if (!pte_same(*vmf->pte, vmf->orig_pte)) {
+				update_mmu_tlb(vma, vmf->address, vmf->pte);
 				unlock_page(vmf->page);
 				pte_unmap_unlock(vmf->pte, vmf->ptl);
 				put_page(vmf->page);
@@ -3341,8 +3344,10 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
 						vma->vm_page_prot));
 		vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd,
 				vmf->address, &vmf->ptl);
-		if (!pte_none(*vmf->pte))
+		if (!pte_none(*vmf->pte)) {
+			update_mmu_tlb(vma, vmf->address, vmf->pte);
 			goto unlock;
+		}
 		ret = check_stable_address_space(vma->vm_mm);
 		if (ret)
 			goto unlock;
@@ -3378,8 +3383,10 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
 
 	vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd, vmf->address,
 			&vmf->ptl);
-	if (!pte_none(*vmf->pte))
+	if (!pte_none(*vmf->pte)) {
+		update_mmu_cache(vma, vmf->address, vmf->pte);
 		goto release;
+	}
 
 	ret = check_stable_address_space(vma->vm_mm);
 	if (ret)
@@ -3646,8 +3653,10 @@ vm_fault_t alloc_set_pte(struct vm_fault *vmf, struct mem_cgroup *memcg,
 	}
 
 	/* Re-check under ptl */
-	if (unlikely(!pte_none(*vmf->pte)))
+	if (unlikely(!pte_none(*vmf->pte))) {
+		update_mmu_tlb(vma, vmf->address, vmf->pte);
 		return VM_FAULT_NOPAGE;
+	}
 
 	flush_icache_page(vma, page);
 	entry = mk_pte(page, vma->vm_page_prot);
@@ -4224,8 +4233,10 @@ static vm_fault_t handle_pte_fault(struct vm_fault *vmf)
 	vmf->ptl = pte_lockptr(vmf->vma->vm_mm, vmf->pmd);
 	spin_lock(vmf->ptl);
 	entry = vmf->orig_pte;
-	if (unlikely(!pte_same(*vmf->pte, entry)))
+	if (unlikely(!pte_same(*vmf->pte, entry))) {
+		update_mmu_tlb(vmf->vma, vmf->address, vmf->pte);
 		goto unlock;
+	}
 	if (vmf->flags & FAULT_FLAG_WRITE) {
 		if (!pte_write(entry))
 			return do_wp_page(vmf);
-- 
1.8.3.1

Re: [PATCH v7 2/4] mm/memory.c: Update local TLB if PTE entry exists

[Andrew Morton]

On Wed, 27 May 2020 10:25:18 +0800 Bibo Mao <maobibo@loongson.cn> wrote:

> If two threads concurrently fault at the same page, the thread that
> won the race updates the PTE and its local TLB. For now, the other
> thread gives up, simply does nothing, and continues.
> 
> It could happen that this second thread triggers another fault, whereby
> it only updates its local TLB while handling the fault. Instead of
> triggering another fault, let's directly update the local TLB of the
> second thread. Function update_mmu_tlb is used here to update local
> TLB on the second thread, and it is defined as empty on other arches.
> 
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -2752,6 +2752,7 @@ static vm_fault_t wp_page_copy(struct vm_fault *vmf)
>  		new_page = old_page;
>  		page_copied = 1;
>  	} else {
> +		update_mmu_tlb(vma, vmf->address, vmf->pte);
>  		mem_cgroup_cancel_charge(new_page, memcg, false);
>  	}
>  

When applying your patches on top of the -mm tree's changes, the above
hunk didn't apply.  The entire `else' block was removed by
https://ozlabs.org/~akpm/mmotm/broken-out/mm-memcontrol-convert-anon-and-file-thp-to-new-mem_cgroup_charge-api.patch

I assumed that dropping this hunk was appropriate.  Please check?

Re: [PATCH v7 2/4] mm/memory.c: Update local TLB if PTE entry exists

[maobibo]

On 05/29/2020 03:23 AM, Andrew Morton wrote:
> On Wed, 27 May 2020 10:25:18 +0800 Bibo Mao <maobibo@loongson.cn> wrote:
> 
>> If two threads concurrently fault at the same page, the thread that
>> won the race updates the PTE and its local TLB. For now, the other
>> thread gives up, simply does nothing, and continues.
>>
>> It could happen that this second thread triggers another fault, whereby
>> it only updates its local TLB while handling the fault. Instead of
>> triggering another fault, let's directly update the local TLB of the
>> second thread. Function update_mmu_tlb is used here to update local
>> TLB on the second thread, and it is defined as empty on other arches.
>>
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -2752,6 +2752,7 @@ static vm_fault_t wp_page_copy(struct vm_fault *vmf)
>>  		new_page = old_page;
>>  		page_copied = 1;
>>  	} else {
>> +		update_mmu_tlb(vma, vmf->address, vmf->pte);
>>  		mem_cgroup_cancel_charge(new_page, memcg, false);
>>  	}
>>  
> 
> When applying your patches on top of the -mm tree's changes, the above
> hunk didn't apply.  The entire `else' block was removed by
> https://ozlabs.org/~akpm/mmotm/broken-out/mm-memcontrol-convert-anon-and-file-thp-to-new-mem_cgroup_charge-api.patch
> 
> I assumed that dropping this hunk was appropriate.  Please check?
yes, that is appropriate. Sorry to bother you, originally I should format the
patch based on mm-tree.

[PATCH v7 3/4] mm/memory.c: Add memory read privilege on page fault handling

[Bibo Mao]

Here add pte_sw_mkyoung function to make page readable on MIPS
platform during page fault handling. This patch improves page
fault latency about 10% on my MIPS machine with lmbench
lat_pagefault case.

It is noop function on other arches, there is no negative
influence on those architectures.

Signed-off-by: Bibo Mao <maobibo@loongson.cn>
Acked-by: Andrew Morton <akpm@linux-foundation.org>
---
 arch/mips/include/asm/pgtable.h |  2 ++
 include/asm-generic/pgtable.h   | 16 ++++++++++++++++
 mm/memory.c                     |  3 +++
 3 files changed, 21 insertions(+)

diff --git a/arch/mips/include/asm/pgtable.h b/arch/mips/include/asm/pgtable.h
index 6f40612..d9772aff 100644
--- a/arch/mips/include/asm/pgtable.h
+++ b/arch/mips/include/asm/pgtable.h
@@ -414,6 +414,8 @@ static inline pte_t pte_mkyoung(pte_t pte)
 	return pte;
 }
 
+#define pte_sw_mkyoung	pte_mkyoung
+
 #ifdef CONFIG_MIPS_HUGE_TLB_SUPPORT
 static inline int pte_huge(pte_t pte)	{ return pte_val(pte) & _PAGE_HUGE; }
 
diff --git a/include/asm-generic/pgtable.h b/include/asm-generic/pgtable.h
index fa5c73f..b5278ec 100644
--- a/include/asm-generic/pgtable.h
+++ b/include/asm-generic/pgtable.h
@@ -244,6 +244,22 @@ static inline void ptep_set_wrprotect(struct mm_struct *mm, unsigned long addres
 }
 #endif
 
+/*
+ * On some architectures hardware does not set page access bit when accessing
+ * memory page, it is responsibilty of software setting this bit. It brings
+ * out extra page fault penalty to track page access bit. For optimization page
+ * access bit can be set during all page fault flow on these arches.
+ * To be differentiate with macro pte_mkyoung, this macro is used on platforms
+ * where software maintains page access bit.
+ */
+#ifndef pte_sw_mkyoung
+static inline pte_t pte_sw_mkyoung(pte_t pte)
+{
+	return pte;
+}
+#define pte_sw_mkyoung	pte_sw_mkyoung
+#endif
+
 #ifndef pte_savedwrite
 #define pte_savedwrite pte_write
 #endif
diff --git a/mm/memory.c b/mm/memory.c
index 8bb31c4..c7c8960 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2704,6 +2704,7 @@ static vm_fault_t wp_page_copy(struct vm_fault *vmf)
 		}
 		flush_cache_page(vma, vmf->address, pte_pfn(vmf->orig_pte));
 		entry = mk_pte(new_page, vma->vm_page_prot);
+		entry = pte_sw_mkyoung(entry);
 		entry = maybe_mkwrite(pte_mkdirty(entry), vma);
 		/*
 		 * Clear the pte entry and flush it first, before updating the
@@ -3378,6 +3379,7 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
 	__SetPageUptodate(page);
 
 	entry = mk_pte(page, vma->vm_page_prot);
+	entry = pte_sw_mkyoung(entry);
 	if (vma->vm_flags & VM_WRITE)
 		entry = pte_mkwrite(pte_mkdirty(entry));
 
@@ -3660,6 +3662,7 @@ vm_fault_t alloc_set_pte(struct vm_fault *vmf, struct mem_cgroup *memcg,
 
 	flush_icache_page(vma, page);
 	entry = mk_pte(page, vma->vm_page_prot);
+	entry = pte_sw_mkyoung(entry);
 	if (write)
 		entry = maybe_mkwrite(pte_mkdirty(entry), vma);
 	/* copy-on-write page */
-- 
1.8.3.1

[PATCH v7 4/4] MIPS: mm: add page valid judgement in function pte_modify

[Bibo Mao]

If original PTE has _PAGE_ACCESSED bit set, and new pte has no
_PAGE_NO_READ bit set, we can add _PAGE_SILENT_READ bit to enable
page valid bit.

Signed-off-by: Bibo Mao <maobibo@loongson.cn>
---
 arch/mips/include/asm/pgtable.h | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/arch/mips/include/asm/pgtable.h b/arch/mips/include/asm/pgtable.h
index d9772aff..85b39c9 100644
--- a/arch/mips/include/asm/pgtable.h
+++ b/arch/mips/include/asm/pgtable.h
@@ -532,8 +532,11 @@ static inline pte_t pte_modify(pte_t pte, pgprot_t newprot)
 #else
 static inline pte_t pte_modify(pte_t pte, pgprot_t newprot)
 {
-	return __pte((pte_val(pte) & _PAGE_CHG_MASK) |
-		     (pgprot_val(newprot) & ~_PAGE_CHG_MASK));
+	pte_val(pte) &= _PAGE_CHG_MASK;
+	pte_val(pte) |= pgprot_val(newprot) & ~_PAGE_CHG_MASK;
+	if ((pte_val(pte) & _PAGE_ACCESSED) && !(pte_val(pte) & _PAGE_NO_READ))
+		pte_val(pte) |= _PAGE_SILENT_READ;
+	return pte;
 }
 #endif
 
-- 
1.8.3.1

Re: [PATCH v7 0/4] MIPS: page fault handling optimization

[Thomas Bogendoerfer]

On Wed, May 27, 2020 at 10:25:16AM +0800, Bibo Mao wrote:
> V7:
> - define function flush_tlb_fix_spurious_fault as inline rather
>   than macro
> V6:
> - Add update_mmu_tlb function as empty on all platform except mips
>   system, we use this function to update local tlb for page fault
>   smp-race handling
> V5:
> - define update_mmu_cache function specified on MIPS platform, and
>   add page fault smp-race stats info
> V4:
> - add pte_sw_mkyoung function to implement readable privilege, and
>   this function is  only in effect on MIPS system.
> - add page valid bit judgement in function pte_modify
> V3:
> - add detailed changelog, modify typo issue in patch V2
> v2:
> - split flush_tlb_fix_spurious_fault and tlb update into two patches
> - comments typo modification
> - separate tlb update and add pte readable privilege into two patches
> 
> Bibo Mao (4):
>   MIPS: Do not flush tlb page when updating PTE entry
>   mm/memory.c: Update local TLB if PTE entry exists
>   mm/memory.c: Add memory read privilege on page fault handling
>   MIPS: mm: add page valid judgement in function pte_modify
> 
>  arch/mips/include/asm/pgtable.h | 37 +++++++++++++++++++++++++++++++++++--
>  include/asm-generic/pgtable.h   | 33 +++++++++++++++++++++++++++++++++
>  mm/memory.c                     | 30 ++++++++++++++++++++++--------
>  3 files changed, 90 insertions(+), 10 deletions(-)

series applied to mips-next.

Thomas.

-- 
Crap can work. Given enough thrust pigs will fly, but it's not necessarily a
good idea.                                                [ RFC1925, 2.3 ]