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Subject[PATCH v10 24/25] x86/mm: add speculative pagefault handling
Date
From: Peter Zijlstra <peterz@infradead.org>

Try a speculative fault before acquiring mmap_sem, if it returns with
VM_FAULT_RETRY continue with the mmap_sem acquisition and do the
traditional fault.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>

[Clearing of FAULT_FLAG_ALLOW_RETRY is now done in
handle_speculative_fault()]
[Retry with usual fault path in the case VM_ERROR is returned by
handle_speculative_fault(). This allows signal to be delivered]
[Don't build SPF call if !CONFIG_SPECULATIVE_PAGE_FAULT]
[Try speculative fault path only for multi threaded processes]
[Try reuse to the VMA fetch during the speculative path in case of retry]
[Call reuse_spf_or_find_vma()]
[Handle memory protection key fault]
Signed-off-by: Laurent Dufour <ldufour@linux.vnet.ibm.com>
---
arch/x86/mm/fault.c | 42 ++++++++++++++++++++++++++++++++++++++----
1 file changed, 38 insertions(+), 4 deletions(-)

diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index 73bd8c95ac71..59f778386df5 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -1220,7 +1220,7 @@ __do_page_fault(struct pt_regs *regs, unsigned long error_code,
struct mm_struct *mm;
int fault, major = 0;
unsigned int flags = FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;
- u32 pkey;
+ u32 pkey, *pt_pkey = &pkey;

tsk = current;
mm = tsk->mm;
@@ -1310,6 +1310,30 @@ __do_page_fault(struct pt_regs *regs, unsigned long error_code,
flags |= FAULT_FLAG_INSTRUCTION;

/*
+ * Do not try speculative page fault for kernel's pages and if
+ * the fault was due to protection keys since it can't be resolved.
+ */
+ if (IS_ENABLED(CONFIG_SPECULATIVE_PAGE_FAULT) &&
+ !(error_code & X86_PF_PK)) {
+ fault = handle_speculative_fault(mm, address, flags, &vma);
+ if (fault != VM_FAULT_RETRY) {
+ perf_sw_event(PERF_COUNT_SW_SPF, 1, regs, address);
+ /*
+ * Do not advertise for the pkey value since we don't
+ * know it.
+ * This is not a matter as we checked for X86_PF_PK
+ * earlier, so we should not handle pkey fault here,
+ * but to be sure that mm_fault_error() callees will
+ * not try to use it, we invalidate the pointer.
+ */
+ pt_pkey = NULL;
+ goto done;
+ }
+ } else {
+ vma = NULL;
+ }
+
+ /*
* When running in the kernel we expect faults to occur only to
* addresses in user space. All other faults represent errors in
* the kernel and should generate an OOPS. Unfortunately, in the
@@ -1342,7 +1366,8 @@ __do_page_fault(struct pt_regs *regs, unsigned long error_code,
might_sleep();
}

- vma = find_vma(mm, address);
+ if (!vma || !can_reuse_spf_vma(vma, address))
+ vma = find_vma(mm, address);
if (unlikely(!vma)) {
bad_area(regs, error_code, address);
return;
@@ -1409,8 +1434,15 @@ __do_page_fault(struct pt_regs *regs, unsigned long error_code,
if (flags & FAULT_FLAG_ALLOW_RETRY) {
flags &= ~FAULT_FLAG_ALLOW_RETRY;
flags |= FAULT_FLAG_TRIED;
- if (!fatal_signal_pending(tsk))
+ if (!fatal_signal_pending(tsk)) {
+ /*
+ * Do not try to reuse this vma and fetch it
+ * again since we will release the mmap_sem.
+ */
+ if (IS_ENABLED(CONFIG_SPECULATIVE_PAGE_FAULT))
+ vma = NULL;
goto retry;
+ }
}

/* User mode? Just return to handle the fatal exception */
@@ -1423,8 +1455,10 @@ __do_page_fault(struct pt_regs *regs, unsigned long error_code,
}

up_read(&mm->mmap_sem);
+
+done:
if (unlikely(fault & VM_FAULT_ERROR)) {
- mm_fault_error(regs, error_code, address, &pkey, fault);
+ mm_fault_error(regs, error_code, address, pt_pkey, fault);
return;
}

--
2.7.4
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