[PATCH v4 0/4] kvm/mm: Allow GUP to respond to non fatal signals

[PATCH v4 0/4] kvm/mm: Allow GUP to respond to non fatal signals

[Peter Xu]

v4:
- Split patch 2+3 into three patches [Sean]

rfc: https://lore.kernel.org/r/20220617014147.7299-1-peterx@redhat.com
v1:  https://lore.kernel.org/r/20220622213656.81546-1-peterx@redhat.com
v2:  https://lore.kernel.org/r/20220721000318.93522-1-peterx@redhat.com
v3:  https://lore.kernel.org/r/20220817003614.58900-1-peterx@redhat.com

One issue was reported that libvirt won't be able to stop the virtual
machine using QMP command "stop" during a paused postcopy migration [1].

It won't work because "stop the VM" operation requires the hypervisor to
kick all the vcpu threads out using SIG_IPI in QEMU (which is translated to
a SIGUSR1).  However since during a paused postcopy, the vcpu threads are
hang death at handle_userfault() so there're simply not responding to the
kicks.  Further, the "stop" command will further hang the QMP channel.

The mm has facility to process generic signal (FAULT_FLAG_INTERRUPTIBLE),
however it's only used in the PF handlers only, not in GUP. Unluckily, KVM
is a heavy GUP user on guest page faults.  It means we won't be able to
interrupt a long page fault for KVM fetching guest pages with what we have
right now.

I think it's reasonable for GUP to only listen to fatal signals, as most of
the GUP users are not really ready to handle such case.  But actually KVM
is not such an user, and KVM actually has rich infrastructure to handle
even generic signals, and properly deliver the signal to the userspace.
Then the page fault can be retried in the next KVM_RUN.

This patchset added FOLL_INTERRUPTIBLE to enable FAULT_FLAG_INTERRUPTIBLE,
and let KVM be the first one to use it.  KVM and mm/gup can always be able
to respond to fatal signals, but not non-fatal ones until this patchset.

One thing to mention is that this is not allowing all KVM paths to be able
to respond to non fatal signals, but only on x86 slow page faults.  In the
future when more code is ready for handling signal interruptions, we can
explore possibility to have more gup callers using FOLL_INTERRUPTIBLE.

Tests
=====

I created a postcopy environment, pause the migration by shutting down the
network to emulate a network failure (so the handle_userfault() will stuck
for a long time), then I tried three things:

  (1) Sending QMP command "stop" to QEMU monitor,
  (2) Hitting Ctrl-C from QEMU cmdline,
  (3) GDB attach to the dest QEMU process.

Before this patchset, all three use case hang.  After the patchset, all
work just like when there's not network failure at all.

Please have a look, thanks.

[1] https://gitlab.com/qemu-project/qemu/-/issues/1052

Peter Xu (4):
  mm/gup: Add FOLL_INTERRUPTIBLE
  kvm: Add KVM_PFN_ERR_SIGPENDING
  kvm: Add interruptible flag to __gfn_to_pfn_memslot()
  kvm: x86: Allow to respond to generic signals during slow PF

 arch/arm64/kvm/mmu.c                   |  2 +-
 arch/powerpc/kvm/book3s_64_mmu_hv.c    |  2 +-
 arch/powerpc/kvm/book3s_64_mmu_radix.c |  2 +-
 arch/x86/kvm/mmu/mmu.c                 | 18 ++++++++++----
 include/linux/kvm_host.h               | 14 +++++++++--
 include/linux/mm.h                     |  1 +
 mm/gup.c                               | 33 ++++++++++++++++++++++----
 mm/hugetlb.c                           |  5 +++-
 virt/kvm/kvm_main.c                    | 30 ++++++++++++++---------
 virt/kvm/kvm_mm.h                      |  4 ++--
 virt/kvm/pfncache.c                    |  2 +-
 11 files changed, 85 insertions(+), 28 deletions(-)

-- 
2.37.3

[PATCH v4 1/4] mm/gup: Add FOLL_INTERRUPTIBLE

[Peter Xu]

We have had FAULT_FLAG_INTERRUPTIBLE but it was never applied to GUPs.  One
issue with it is that not all GUP paths are able to handle signal delivers
besides SIGKILL.

That's not ideal for the GUP users who are actually able to handle these
cases, like KVM.

KVM uses GUP extensively on faulting guest pages, during which we've got
existing infrastructures to retry a page fault at a later time.  Allowing
the GUP to be interrupted by generic signals can make KVM related threads
to be more responsive.  For examples:

  (1) SIGUSR1: which QEMU/KVM uses to deliver an inter-process IPI,
      e.g. when the admin issues a vm_stop QMP command, SIGUSR1 can be
      generated to kick the vcpus out of kernel context immediately,

  (2) SIGINT: which can be used with interactive hypervisor users to stop a
      virtual machine with Ctrl-C without any delays/hangs,

  (3) SIGTRAP: which grants GDB capability even during page faults that are
      stuck for a long time.

Normally hypervisor will be able to receive these signals properly, but not
if we're stuck in a GUP for a long time for whatever reason.  It happens
easily with a stucked postcopy migration when e.g. a network temp failure
happens, then some vcpu threads can hang death waiting for the pages.  With
the new FOLL_INTERRUPTIBLE, we can allow GUP users like KVM to selectively
enable the ability to trap these signals.

Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
---
 include/linux/mm.h |  1 +
 mm/gup.c           | 33 +++++++++++++++++++++++++++++----
 mm/hugetlb.c       |  5 ++++-
 3 files changed, 34 insertions(+), 5 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 21f8b27bd9fd..488a9f4cce07 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2897,6 +2897,7 @@ struct page *follow_page(struct vm_area_struct *vma, unsigned long address,
 #define FOLL_SPLIT_PMD	0x20000	/* split huge pmd before returning */
 #define FOLL_PIN	0x40000	/* pages must be released via unpin_user_page */
 #define FOLL_FAST_ONLY	0x80000	/* gup_fast: prevent fall-back to slow gup */
+#define FOLL_INTERRUPTIBLE  0x100000 /* allow interrupts from generic signals */
 
 /*
  * FOLL_PIN and FOLL_LONGTERM may be used in various combinations with each
diff --git a/mm/gup.c b/mm/gup.c
index 5abdaf487460..d51e7ccaef32 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -970,8 +970,17 @@ static int faultin_page(struct vm_area_struct *vma,
 		fault_flags |= FAULT_FLAG_WRITE;
 	if (*flags & FOLL_REMOTE)
 		fault_flags |= FAULT_FLAG_REMOTE;
-	if (locked)
+	if (locked) {
 		fault_flags |= FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;
+		/*
+		 * FAULT_FLAG_INTERRUPTIBLE is opt-in. GUP callers must set
+		 * FOLL_INTERRUPTIBLE to enable FAULT_FLAG_INTERRUPTIBLE.
+		 * That's because some callers may not be prepared to
+		 * handle early exits caused by non-fatal signals.
+		 */
+		if (*flags & FOLL_INTERRUPTIBLE)
+			fault_flags |= FAULT_FLAG_INTERRUPTIBLE;
+	}
 	if (*flags & FOLL_NOWAIT)
 		fault_flags |= FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_RETRY_NOWAIT;
 	if (*flags & FOLL_TRIED) {
@@ -1380,6 +1389,22 @@ int fixup_user_fault(struct mm_struct *mm,
 }
 EXPORT_SYMBOL_GPL(fixup_user_fault);
 
+/*
+ * GUP always responds to fatal signals.  When FOLL_INTERRUPTIBLE is
+ * specified, it'll also respond to generic signals.  The caller of GUP
+ * that has FOLL_INTERRUPTIBLE should take care of the GUP interruption.
+ */
+static bool gup_signal_pending(unsigned int flags)
+{
+	if (fatal_signal_pending(current))
+		return true;
+
+	if (!(flags & FOLL_INTERRUPTIBLE))
+		return false;
+
+	return signal_pending(current);
+}
+
 /*
  * Please note that this function, unlike __get_user_pages will not
  * return 0 for nr_pages > 0 without FOLL_NOWAIT
@@ -1461,11 +1486,11 @@ static __always_inline long __get_user_pages_locked(struct mm_struct *mm,
 		 * Repeat on the address that fired VM_FAULT_RETRY
 		 * with both FAULT_FLAG_ALLOW_RETRY and
 		 * FAULT_FLAG_TRIED.  Note that GUP can be interrupted
-		 * by fatal signals, so we need to check it before we
+		 * by fatal signals of even common signals, depending on
+		 * the caller's request. So we need to check it before we
 		 * start trying again otherwise it can loop forever.
 		 */
-
-		if (fatal_signal_pending(current)) {
+		if (gup_signal_pending(flags)) {
 			if (!pages_done)
 				pages_done = -EINTR;
 			break;
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index e070b8593b37..202f3ad7f35c 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -6206,9 +6206,12 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
 				fault_flags |= FAULT_FLAG_WRITE;
 			else if (unshare)
 				fault_flags |= FAULT_FLAG_UNSHARE;
-			if (locked)
+			if (locked) {
 				fault_flags |= FAULT_FLAG_ALLOW_RETRY |
 					FAULT_FLAG_KILLABLE;
+				if (flags & FOLL_INTERRUPTIBLE)
+					fault_flags |= FAULT_FLAG_INTERRUPTIBLE;
+			}
 			if (flags & FOLL_NOWAIT)
 				fault_flags |= FAULT_FLAG_ALLOW_RETRY |
 					FAULT_FLAG_RETRY_NOWAIT;
-- 
2.37.3

[PATCH v4 2/4] kvm: Add KVM_PFN_ERR_SIGPENDING

[Peter Xu]

Add a new pfn error to show that we've got a pending signal to handle
during hva_to_pfn_slow() procedure (of -EINTR retval).

Signed-off-by: Peter Xu <peterx@redhat.com>
---
 include/linux/kvm_host.h | 10 ++++++++++
 virt/kvm/kvm_main.c      |  2 ++
 2 files changed, 12 insertions(+)

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 32f259fa5801..92baa930b891 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -96,6 +96,7 @@
 #define KVM_PFN_ERR_FAULT	(KVM_PFN_ERR_MASK)
 #define KVM_PFN_ERR_HWPOISON	(KVM_PFN_ERR_MASK + 1)
 #define KVM_PFN_ERR_RO_FAULT	(KVM_PFN_ERR_MASK + 2)
+#define KVM_PFN_ERR_SIGPENDING	(KVM_PFN_ERR_MASK + 3)
 
 /*
  * error pfns indicate that the gfn is in slot but faild to
@@ -106,6 +107,15 @@ static inline bool is_error_pfn(kvm_pfn_t pfn)
 	return !!(pfn & KVM_PFN_ERR_MASK);
 }
 
+/*
+ * KVM_PFN_ERR_SIGPENDING indicates that fetching the PFN was interrupted
+ * by a pending signal.  Note, the signal may or may not be fatal.
+ */
+static inline bool is_sigpending_pfn(kvm_pfn_t pfn)
+{
+	return pfn == KVM_PFN_ERR_SIGPENDING;
+}
+
 /*
  * error_noslot pfns indicate that the gfn can not be
  * translated to pfn - it is not in slot or failed to
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index e30f1b4ecfa5..e20a59dcda32 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -2667,6 +2667,8 @@ kvm_pfn_t hva_to_pfn(unsigned long addr, bool atomic, bool *async,
 	npages = hva_to_pfn_slow(addr, async, write_fault, writable, &pfn);
 	if (npages == 1)
 		return pfn;
+	if (npages == -EINTR)
+		return KVM_PFN_ERR_SIGPENDING;
 
 	mmap_read_lock(current->mm);
 	if (npages == -EHWPOISON ||
-- 
2.37.3

[PATCH v4 3/4] kvm: Add interruptible flag to __gfn_to_pfn_memslot()

[Peter Xu]

Add a new "interruptible" flag showing that the caller is willing to be
interrupted by signals during the __gfn_to_pfn_memslot() request.  Wire it
up with a FOLL_INTERRUPTIBLE flag that we've just introduced.

This prepares KVM to be able to respond to SIGUSR1 (for QEMU that's the
SIGIPI) even during e.g. handling an userfaultfd page fault.

No functional change intended.

Signed-off-by: Peter Xu <peterx@redhat.com>
---
 arch/arm64/kvm/mmu.c                   |  2 +-
 arch/powerpc/kvm/book3s_64_mmu_hv.c    |  2 +-
 arch/powerpc/kvm/book3s_64_mmu_radix.c |  2 +-
 arch/x86/kvm/mmu/mmu.c                 |  4 ++--
 include/linux/kvm_host.h               |  4 ++--
 virt/kvm/kvm_main.c                    | 28 ++++++++++++++++----------
 virt/kvm/kvm_mm.h                      |  4 ++--
 virt/kvm/pfncache.c                    |  2 +-
 8 files changed, 27 insertions(+), 21 deletions(-)

diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index 34c5feed9dc1..7b990b33b337 100644
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c
@@ -1232,7 +1232,7 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
 	 */
 	smp_rmb();
 
-	pfn = __gfn_to_pfn_memslot(memslot, gfn, false, NULL,
+	pfn = __gfn_to_pfn_memslot(memslot, gfn, false, false, NULL,
 				   write_fault, &writable, NULL);
 	if (pfn == KVM_PFN_ERR_HWPOISON) {
 		kvm_send_hwpoison_signal(hva, vma_shift);
diff --git a/arch/powerpc/kvm/book3s_64_mmu_hv.c b/arch/powerpc/kvm/book3s_64_mmu_hv.c
index e9744b41a226..4939f57b6f6a 100644
--- a/arch/powerpc/kvm/book3s_64_mmu_hv.c
+++ b/arch/powerpc/kvm/book3s_64_mmu_hv.c
@@ -598,7 +598,7 @@ int kvmppc_book3s_hv_page_fault(struct kvm_vcpu *vcpu,
 		write_ok = true;
 	} else {
 		/* Call KVM generic code to do the slow-path check */
-		pfn = __gfn_to_pfn_memslot(memslot, gfn, false, NULL,
+		pfn = __gfn_to_pfn_memslot(memslot, gfn, false, false, NULL,
 					   writing, &write_ok, NULL);
 		if (is_error_noslot_pfn(pfn))
 			return -EFAULT;
diff --git a/arch/powerpc/kvm/book3s_64_mmu_radix.c b/arch/powerpc/kvm/book3s_64_mmu_radix.c
index 5d5e12f3bf86..9d3743ca16d5 100644
--- a/arch/powerpc/kvm/book3s_64_mmu_radix.c
+++ b/arch/powerpc/kvm/book3s_64_mmu_radix.c
@@ -846,7 +846,7 @@ int kvmppc_book3s_instantiate_page(struct kvm_vcpu *vcpu,
 		unsigned long pfn;
 
 		/* Call KVM generic code to do the slow-path check */
-		pfn = __gfn_to_pfn_memslot(memslot, gfn, false, NULL,
+		pfn = __gfn_to_pfn_memslot(memslot, gfn, false, false, NULL,
 					   writing, upgrade_p, NULL);
 		if (is_error_noslot_pfn(pfn))
 			return -EFAULT;
diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 6f81539061d6..cc26f425f41c 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -4169,7 +4169,7 @@ static int kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
 	}
 
 	async = false;
-	fault->pfn = __gfn_to_pfn_memslot(slot, fault->gfn, false, &async,
+	fault->pfn = __gfn_to_pfn_memslot(slot, fault->gfn, false, false, &async,
 					  fault->write, &fault->map_writable,
 					  &fault->hva);
 	if (!async)
@@ -4186,7 +4186,7 @@ static int kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
 		}
 	}
 
-	fault->pfn = __gfn_to_pfn_memslot(slot, fault->gfn, false, NULL,
+	fault->pfn = __gfn_to_pfn_memslot(slot, fault->gfn, false, false, NULL,
 					  fault->write, &fault->map_writable,
 					  &fault->hva);
 	return RET_PF_CONTINUE;
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 92baa930b891..1904162a041d 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -1150,8 +1150,8 @@ kvm_pfn_t gfn_to_pfn_prot(struct kvm *kvm, gfn_t gfn, bool write_fault,
 kvm_pfn_t gfn_to_pfn_memslot(const struct kvm_memory_slot *slot, gfn_t gfn);
 kvm_pfn_t gfn_to_pfn_memslot_atomic(const struct kvm_memory_slot *slot, gfn_t gfn);
 kvm_pfn_t __gfn_to_pfn_memslot(const struct kvm_memory_slot *slot, gfn_t gfn,
-			       bool atomic, bool *async, bool write_fault,
-			       bool *writable, hva_t *hva);
+			       bool atomic, bool interruptible, bool *async,
+			       bool write_fault, bool *writable, hva_t *hva);
 
 void kvm_release_pfn_clean(kvm_pfn_t pfn);
 void kvm_release_pfn_dirty(kvm_pfn_t pfn);
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index e20a59dcda32..903ec86c4d54 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -2514,7 +2514,7 @@ static bool hva_to_pfn_fast(unsigned long addr, bool write_fault,
  * 1 indicates success, -errno is returned if error is detected.
  */
 static int hva_to_pfn_slow(unsigned long addr, bool *async, bool write_fault,
-			   bool *writable, kvm_pfn_t *pfn)
+			   bool interruptible, bool *writable, kvm_pfn_t *pfn)
 {
 	unsigned int flags = FOLL_HWPOISON;
 	struct page *page;
@@ -2529,6 +2529,8 @@ static int hva_to_pfn_slow(unsigned long addr, bool *async, bool write_fault,
 		flags |= FOLL_WRITE;
 	if (async)
 		flags |= FOLL_NOWAIT;
+	if (interruptible)
+		flags |= FOLL_INTERRUPTIBLE;
 
 	npages = get_user_pages_unlocked(addr, 1, &page, flags);
 	if (npages != 1)
@@ -2638,6 +2640,7 @@ static int hva_to_pfn_remapped(struct vm_area_struct *vma,
  * Pin guest page in memory and return its pfn.
  * @addr: host virtual address which maps memory to the guest
  * @atomic: whether this function can sleep
+ * @interruptible: whether the process can be interrupted by non-fatal signals
  * @async: whether this function need to wait IO complete if the
  *         host page is not in the memory
  * @write_fault: whether we should get a writable host page
@@ -2648,8 +2651,8 @@ static int hva_to_pfn_remapped(struct vm_area_struct *vma,
  * 2): @write_fault = false && @writable, @writable will tell the caller
  *     whether the mapping is writable.
  */
-kvm_pfn_t hva_to_pfn(unsigned long addr, bool atomic, bool *async,
-		     bool write_fault, bool *writable)
+kvm_pfn_t hva_to_pfn(unsigned long addr, bool atomic, bool interruptible,
+		     bool *async, bool write_fault, bool *writable)
 {
 	struct vm_area_struct *vma;
 	kvm_pfn_t pfn;
@@ -2664,7 +2667,8 @@ kvm_pfn_t hva_to_pfn(unsigned long addr, bool atomic, bool *async,
 	if (atomic)
 		return KVM_PFN_ERR_FAULT;
 
-	npages = hva_to_pfn_slow(addr, async, write_fault, writable, &pfn);
+	npages = hva_to_pfn_slow(addr, async, write_fault, interruptible,
+				 writable, &pfn);
 	if (npages == 1)
 		return pfn;
 	if (npages == -EINTR)
@@ -2699,8 +2703,8 @@ kvm_pfn_t hva_to_pfn(unsigned long addr, bool atomic, bool *async,
 }
 
 kvm_pfn_t __gfn_to_pfn_memslot(const struct kvm_memory_slot *slot, gfn_t gfn,
-			       bool atomic, bool *async, bool write_fault,
-			       bool *writable, hva_t *hva)
+			       bool atomic, bool interruptible, bool *async,
+			       bool write_fault, bool *writable, hva_t *hva)
 {
 	unsigned long addr = __gfn_to_hva_many(slot, gfn, NULL, write_fault);
 
@@ -2725,7 +2729,7 @@ kvm_pfn_t __gfn_to_pfn_memslot(const struct kvm_memory_slot *slot, gfn_t gfn,
 		writable = NULL;
 	}
 
-	return hva_to_pfn(addr, atomic, async, write_fault,
+	return hva_to_pfn(addr, atomic, interruptible, async, write_fault,
 			  writable);
 }
 EXPORT_SYMBOL_GPL(__gfn_to_pfn_memslot);
@@ -2733,20 +2737,22 @@ EXPORT_SYMBOL_GPL(__gfn_to_pfn_memslot);
 kvm_pfn_t gfn_to_pfn_prot(struct kvm *kvm, gfn_t gfn, bool write_fault,
 		      bool *writable)
 {
-	return __gfn_to_pfn_memslot(gfn_to_memslot(kvm, gfn), gfn, false, NULL,
-				    write_fault, writable, NULL);
+	return __gfn_to_pfn_memslot(gfn_to_memslot(kvm, gfn), gfn, false, false,
+				    NULL, write_fault, writable, NULL);
 }
 EXPORT_SYMBOL_GPL(gfn_to_pfn_prot);
 
 kvm_pfn_t gfn_to_pfn_memslot(const struct kvm_memory_slot *slot, gfn_t gfn)
 {
-	return __gfn_to_pfn_memslot(slot, gfn, false, NULL, true, NULL, NULL);
+	return __gfn_to_pfn_memslot(slot, gfn, false, false, NULL, true,
+				    NULL, NULL);
 }
 EXPORT_SYMBOL_GPL(gfn_to_pfn_memslot);
 
 kvm_pfn_t gfn_to_pfn_memslot_atomic(const struct kvm_memory_slot *slot, gfn_t gfn)
 {
-	return __gfn_to_pfn_memslot(slot, gfn, true, NULL, true, NULL, NULL);
+	return __gfn_to_pfn_memslot(slot, gfn, true, false, NULL, true,
+				    NULL, NULL);
 }
 EXPORT_SYMBOL_GPL(gfn_to_pfn_memslot_atomic);
 
diff --git a/virt/kvm/kvm_mm.h b/virt/kvm/kvm_mm.h
index 41da467d99c9..a1ab15006af3 100644
--- a/virt/kvm/kvm_mm.h
+++ b/virt/kvm/kvm_mm.h
@@ -24,8 +24,8 @@
 #define KVM_MMU_READ_UNLOCK(kvm)	spin_unlock(&(kvm)->mmu_lock)
 #endif /* KVM_HAVE_MMU_RWLOCK */
 
-kvm_pfn_t hva_to_pfn(unsigned long addr, bool atomic, bool *async,
-		     bool write_fault, bool *writable);
+kvm_pfn_t hva_to_pfn(unsigned long addr, bool atomic, bool interruptible,
+		     bool *async, bool write_fault, bool *writable);
 
 #ifdef CONFIG_HAVE_KVM_PFNCACHE
 void gfn_to_pfn_cache_invalidate_start(struct kvm *kvm,
diff --git a/virt/kvm/pfncache.c b/virt/kvm/pfncache.c
index 68ff41d39545..6f66808d7793 100644
--- a/virt/kvm/pfncache.c
+++ b/virt/kvm/pfncache.c
@@ -182,7 +182,7 @@ static kvm_pfn_t hva_to_pfn_retry(struct kvm *kvm, struct gfn_to_pfn_cache *gpc)
 		}
 
 		/* We always request a writeable mapping */
-		new_pfn = hva_to_pfn(gpc->uhva, false, NULL, true, NULL);
+		new_pfn = hva_to_pfn(gpc->uhva, false, false, NULL, true, NULL);
 		if (is_error_noslot_pfn(new_pfn))
 			goto out_error;
 
-- 
2.37.3

[PATCH v4 4/4] kvm: x86: Allow to respond to generic signals during slow PF

[Peter Xu]

Enable x86 slow page faults to be able to respond to non-fatal signals,
returning -EINTR properly when it happens.

Signed-off-by: Peter Xu <peterx@redhat.com>
---
 arch/x86/kvm/mmu/mmu.c | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index cc26f425f41c..83b9c034313d 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -3148,8 +3148,13 @@ static void kvm_send_hwpoison_signal(unsigned long address, struct task_struct *
 	send_sig_mceerr(BUS_MCEERR_AR, (void __user *)address, PAGE_SHIFT, tsk);
 }
 
-static int kvm_handle_bad_page(struct kvm_vcpu *vcpu, gfn_t gfn, kvm_pfn_t pfn)
+static int kvm_handle_error_pfn(struct kvm_vcpu *vcpu, gfn_t gfn, kvm_pfn_t pfn)
 {
+	if (is_sigpending_pfn(pfn)) {
+		kvm_handle_signal_exit(vcpu);
+		return -EINTR;
+	}
+
 	/*
 	 * Do not cache the mmio info caused by writing the readonly gfn
 	 * into the spte otherwise read access on readonly gfn also can
@@ -3171,7 +3176,7 @@ static int handle_abnormal_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fau
 {
 	/* The pfn is invalid, report the error! */
 	if (unlikely(is_error_pfn(fault->pfn)))
-		return kvm_handle_bad_page(vcpu, fault->gfn, fault->pfn);
+		return kvm_handle_error_pfn(vcpu, fault->gfn, fault->pfn);
 
 	if (unlikely(!fault->slot)) {
 		gva_t gva = fault->is_tdp ? 0 : fault->addr;
@@ -4186,7 +4191,12 @@ static int kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
 		}
 	}
 
-	fault->pfn = __gfn_to_pfn_memslot(slot, fault->gfn, false, false, NULL,
+	/*
+	 * Allow gup to bail on pending non-fatal signals when it's also allowed
+	 * to wait for IO.  Note, gup always bails if it is unable to quickly
+	 * get a page and a fatal signal, i.e. SIGKILL, is pending.
+	 */
+	fault->pfn = __gfn_to_pfn_memslot(slot, fault->gfn, false, true, NULL,
 					  fault->write, &fault->map_writable,
 					  &fault->hva);
 	return RET_PF_CONTINUE;
-- 
2.37.3

Re: [PATCH v4 2/4] kvm: Add KVM_PFN_ERR_SIGPENDING

[Sean Christopherson]

Uber nit, s/kvm/KVM for the shortlog,

On Tue, Oct 11, 2022, Peter Xu wrote:
> Add a new pfn error to show that we've got a pending signal to handle

Please avoid pronouns in changelogs and comments, they're unnecessarily ambiguous.
E.g. 

  Add a new pfn error to capture that hva_to_pfn_slow() failed due to a
  pending signal.  The new error will be used in future patches to
  propagate -EINTR back to userspace instead of returning -EFAULT.
 

> during hva_to_pfn_slow() procedure (of -EINTR retval).
> 
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---

Nits aside,

Reviewed-by: Sean Christopherson <seanjc@google.com>

Re: [PATCH v4 3/4] kvm: Add interruptible flag to __gfn_to_pfn_memslot()

[Sean Christopherson]

"KVM:" for the shortlog.

On Tue, Oct 11, 2022, Peter Xu wrote:
> Add a new "interruptible" flag showing that the caller is willing to be
> interrupted by signals during the __gfn_to_pfn_memslot() request.  Wire it
> up with a FOLL_INTERRUPTIBLE flag that we've just introduced.
> 
> This prepares KVM to be able to respond to SIGUSR1 (for QEMU that's the
> SIGIPI) even during e.g. handling an userfaultfd page fault.
> 
> No functional change intended.
> 
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---

Reviewed-by: Sean Christopherson <seanjc@google.com>

Re: [PATCH v4 4/4] kvm: x86: Allow to respond to generic signals during slow PF

[Sean Christopherson]

"KVM: x86/mmu:" for the shortlog.

On Tue, Oct 11, 2022, Peter Xu wrote:
> Enable x86 slow page faults to be able to respond to non-fatal signals,
> returning -EINTR properly when it happens.

Probably worth adding:

  Opportunistically rename kvm_handle_bad_page() to kvm_handle_error_pfn()
  as an error pfn doesn't necessarily mean the page is "bad".

> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---

Reviewed-by: Sean Christopherson <seanjc@google.com>

Re: [PATCH v4 0/4] kvm/mm: Allow GUP to respond to non fatal signals

[Sean Christopherson]

On Tue, Oct 11, 2022, Peter Xu wrote:
> Peter Xu (4):
>   mm/gup: Add FOLL_INTERRUPTIBLE
>   kvm: Add KVM_PFN_ERR_SIGPENDING
>   kvm: Add interruptible flag to __gfn_to_pfn_memslot()
>   kvm: x86: Allow to respond to generic signals during slow PF

Some shortlog/changelog nits, but nothing that warrants a v5.  If they get fixed
up when applying, yay, if not, the world won't end.

Re: [PATCH v4 1/4] mm/gup: Add FOLL_INTERRUPTIBLE

[Paolo Bonzini]

On 10/11/22 21:58, Peter Xu wrote:
> We have had FAULT_FLAG_INTERRUPTIBLE but it was never applied to GUPs.  One
> issue with it is that not all GUP paths are able to handle signal delivers
> besides SIGKILL.
> 
> That's not ideal for the GUP users who are actually able to handle these
> cases, like KVM.
> 
> KVM uses GUP extensively on faulting guest pages, during which we've got
> existing infrastructures to retry a page fault at a later time.  Allowing
> the GUP to be interrupted by generic signals can make KVM related threads
> to be more responsive.  For examples:
> 
>    (1) SIGUSR1: which QEMU/KVM uses to deliver an inter-process IPI,
>        e.g. when the admin issues a vm_stop QMP command, SIGUSR1 can be
>        generated to kick the vcpus out of kernel context immediately,
> 
>    (2) SIGINT: which can be used with interactive hypervisor users to stop a
>        virtual machine with Ctrl-C without any delays/hangs,
> 
>    (3) SIGTRAP: which grants GDB capability even during page faults that are
>        stuck for a long time.
> 
> Normally hypervisor will be able to receive these signals properly, but not
> if we're stuck in a GUP for a long time for whatever reason.  It happens
> easily with a stucked postcopy migration when e.g. a network temp failure
> happens, then some vcpu threads can hang death waiting for the pages.  With
> the new FOLL_INTERRUPTIBLE, we can allow GUP users like KVM to selectively
> enable the ability to trap these signals.
> 
> Reviewed-by: John Hubbard <jhubbard@nvidia.com>
> Reviewed-by: David Hildenbrand <david@redhat.com>
> Signed-off-by: Peter Xu <peterx@redhat.com>

If no one shouts I will queue the series for 6.2, but it would be nice 
to get an ACK from the mm folks.

> ---
>   include/linux/mm.h |  1 +
>   mm/gup.c           | 33 +++++++++++++++++++++++++++++----
>   mm/hugetlb.c       |  5 ++++-
>   3 files changed, 34 insertions(+), 5 deletions(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 21f8b27bd9fd..488a9f4cce07 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2897,6 +2897,7 @@ struct page *follow_page(struct vm_area_struct *vma, unsigned long address,
>   #define FOLL_SPLIT_PMD	0x20000	/* split huge pmd before returning */
>   #define FOLL_PIN	0x40000	/* pages must be released via unpin_user_page */
>   #define FOLL_FAST_ONLY	0x80000	/* gup_fast: prevent fall-back to slow gup */
> +#define FOLL_INTERRUPTIBLE  0x100000 /* allow interrupts from generic signals */
>   
>   /*
>    * FOLL_PIN and FOLL_LONGTERM may be used in various combinations with each
> diff --git a/mm/gup.c b/mm/gup.c
> index 5abdaf487460..d51e7ccaef32 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -970,8 +970,17 @@ static int faultin_page(struct vm_area_struct *vma,
>   		fault_flags |= FAULT_FLAG_WRITE;
>   	if (*flags & FOLL_REMOTE)
>   		fault_flags |= FAULT_FLAG_REMOTE;
> -	if (locked)
> +	if (locked) {
>   		fault_flags |= FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;
> +		/*
> +		 * FAULT_FLAG_INTERRUPTIBLE is opt-in. GUP callers must set
> +		 * FOLL_INTERRUPTIBLE to enable FAULT_FLAG_INTERRUPTIBLE.
> +		 * That's because some callers may not be prepared to
> +		 * handle early exits caused by non-fatal signals.
> +		 */
> +		if (*flags & FOLL_INTERRUPTIBLE)
> +			fault_flags |= FAULT_FLAG_INTERRUPTIBLE;
> +	}
>   	if (*flags & FOLL_NOWAIT)
>   		fault_flags |= FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_RETRY_NOWAIT;
>   	if (*flags & FOLL_TRIED) {
> @@ -1380,6 +1389,22 @@ int fixup_user_fault(struct mm_struct *mm,
>   }
>   EXPORT_SYMBOL_GPL(fixup_user_fault);
>   
> +/*
> + * GUP always responds to fatal signals.  When FOLL_INTERRUPTIBLE is
> + * specified, it'll also respond to generic signals.  The caller of GUP
> + * that has FOLL_INTERRUPTIBLE should take care of the GUP interruption.
> + */
> +static bool gup_signal_pending(unsigned int flags)
> +{
> +	if (fatal_signal_pending(current))
> +		return true;
> +
> +	if (!(flags & FOLL_INTERRUPTIBLE))
> +		return false;
> +
> +	return signal_pending(current);
> +}
> +
>   /*
>    * Please note that this function, unlike __get_user_pages will not
>    * return 0 for nr_pages > 0 without FOLL_NOWAIT
> @@ -1461,11 +1486,11 @@ static __always_inline long __get_user_pages_locked(struct mm_struct *mm,
>   		 * Repeat on the address that fired VM_FAULT_RETRY
>   		 * with both FAULT_FLAG_ALLOW_RETRY and
>   		 * FAULT_FLAG_TRIED.  Note that GUP can be interrupted
> -		 * by fatal signals, so we need to check it before we
> +		 * by fatal signals of even common signals, depending on
> +		 * the caller's request. So we need to check it before we
>   		 * start trying again otherwise it can loop forever.
>   		 */
> -
> -		if (fatal_signal_pending(current)) {
> +		if (gup_signal_pending(flags)) {
>   			if (!pages_done)
>   				pages_done = -EINTR;
>   			break;
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index e070b8593b37..202f3ad7f35c 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -6206,9 +6206,12 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
>   				fault_flags |= FAULT_FLAG_WRITE;
>   			else if (unshare)
>   				fault_flags |= FAULT_FLAG_UNSHARE;
> -			if (locked)
> +			if (locked) {
>   				fault_flags |= FAULT_FLAG_ALLOW_RETRY |
>   					FAULT_FLAG_KILLABLE;
> +				if (flags & FOLL_INTERRUPTIBLE)
> +					fault_flags |= FAULT_FLAG_INTERRUPTIBLE;
> +			}
>   			if (flags & FOLL_NOWAIT)
>   				fault_flags |= FAULT_FLAG_ALLOW_RETRY |
>   					FAULT_FLAG_RETRY_NOWAIT;