[PATCH] kvm: Faults which trigger IO release the mmap_sem

[Andres Lagar-Cavilla <andreslc@google.com>]

When KVM handles a tdp fault it uses FOLL_NOWAIT. If the guest memory has been
swapped out or is behind a filemap, this will trigger async readahead and
return immediately. The rationale is that KVM will kick back the guest with an
"async page fault" and allow for some other guest process to take over.

If async PFs are enabled the fault is retried asap from a workqueue, or
immediately if no async PFs. The retry will not relinquish the mmap semaphore
and will block on the IO. This is a bad thing, as other mmap semaphore users
now stall. The fault could take a long time, depending on swap or filemap
latency.

This patch ensures both the regular and async PF path re-enter the fault
allowing for the mmap semaphore to be relinquished in the case of IO wait.

Signed-off-by: Andres Lagar-Cavilla <andreslc@google.com>
---
 include/linux/kvm_host.h |  9 +++++++++
 include/linux/mm.h       |  1 +
 mm/gup.c                 |  4 ++++
 virt/kvm/async_pf.c      |  4 +---
 virt/kvm/kvm_main.c      | 45 ++++++++++++++++++++++++++++++++++++++++++---
 5 files changed, 57 insertions(+), 6 deletions(-)

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 3addcbc..704908d 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -198,6 +198,15 @@ int kvm_setup_async_pf(struct kvm_vcpu *vcpu, gva_t gva, unsigned long hva,
 int kvm_async_pf_wakeup_all(struct kvm_vcpu *vcpu);
 #endif
 
+/*
+ * Retry a fault after a gup with FOLL_NOWAIT. This properly relinquishes mmap
+ * semaphore if the filemap/swap has to wait on page lock (and retries the gup
+ * to completion after that).
+ */
+int kvm_get_user_page_retry(struct task_struct *tsk, struct mm_struct *mm,
+			    unsigned long addr, bool write_fault,
+			    struct page **pagep);
+
 enum {
 	OUTSIDE_GUEST_MODE,
 	IN_GUEST_MODE,
diff --git a/include/linux/mm.h b/include/linux/mm.h
index ebc5f90..13e585f7 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2011,6 +2011,7 @@ static inline struct page *follow_page(struct vm_area_struct *vma,
 #define FOLL_HWPOISON	0x100	/* check page is hwpoisoned */
 #define FOLL_NUMA	0x200	/* force NUMA hinting page fault */
 #define FOLL_MIGRATION	0x400	/* wait for page to replace migration entry */
+#define FOLL_TRIED	0x800	/* a retry, previous pass started an IO */
 
 typedef int (*pte_fn_t)(pte_t *pte, pgtable_t token, unsigned long addr,
 			void *data);
diff --git a/mm/gup.c b/mm/gup.c
index 91d044b..332d1c3 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -281,6 +281,10 @@ static int faultin_page(struct task_struct *tsk, struct vm_area_struct *vma,
 		fault_flags |= FAULT_FLAG_ALLOW_RETRY;
 	if (*flags & FOLL_NOWAIT)
 		fault_flags |= FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_RETRY_NOWAIT;
+	if (*flags & FOLL_TRIED) {
+		WARN_ON_ONCE(fault_flags & FAULT_FLAG_ALLOW_RETRY);
+		fault_flags |= FAULT_FLAG_TRIED;
+	}
 
 	ret = handle_mm_fault(mm, vma, address, fault_flags);
 	if (ret & VM_FAULT_ERROR) {
diff --git a/virt/kvm/async_pf.c b/virt/kvm/async_pf.c
index d6a3d09..17b78b1 100644
--- a/virt/kvm/async_pf.c
+++ b/virt/kvm/async_pf.c
@@ -80,9 +80,7 @@ static void async_pf_execute(struct work_struct *work)
 
 	might_sleep();
 
-	down_read(&mm->mmap_sem);
-	get_user_pages(NULL, mm, addr, 1, 1, 0, NULL, NULL);
-	up_read(&mm->mmap_sem);
+	kvm_get_user_page_retry(NULL, mm, addr, 1, NULL);
 	kvm_async_page_present_sync(vcpu, apf);
 
 	spin_lock(&vcpu->async_pf.lock);
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 7ef6b48..43a9ab9 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1115,6 +1115,39 @@ static int get_user_page_nowait(struct task_struct *tsk, struct mm_struct *mm,
 	return __get_user_pages(tsk, mm, start, 1, flags, page, NULL, NULL);
 }
 
+int kvm_get_user_page_retry(struct task_struct *tsk, struct mm_struct *mm,
+				unsigned long addr, bool write_fault,
+				struct page **pagep)
+{
+	int npages;
+	int locked = 1;
+	int flags = FOLL_TOUCH | FOLL_HWPOISON |
+		    (pagep ? FOLL_GET : 0) |
+		    (write_fault ? FOLL_WRITE : 0);
+
+	/*
+	 * Retrying fault, we get here *not* having allowed the filemap to wait
+	 * on the page lock. We should now allow waiting on the IO with the
+	 * mmap semaphore released.
+	 */
+	down_read(&mm->mmap_sem);
+	npages = __get_user_pages(tsk, mm, addr, 1, flags, pagep, NULL,
+				  &locked);
+	if (!locked) {
+		BUG_ON(npages != -EBUSY);
+		/*
+		 * The previous call has now waited on the IO. Now we can
+		 * retry and complete. Pass TRIED to ensure we do not re
+		 * schedule async IO (see e.g. filemap_fault).
+		 */
+		down_read(&mm->mmap_sem);
+		npages = __get_user_pages(tsk, mm, addr, 1, flags | FOLL_TRIED,
+					  pagep, NULL, NULL);
+	}
+	up_read(&mm->mmap_sem);
+	return npages;
+}
+
 static inline int check_user_page_hwpoison(unsigned long addr)
 {
 	int rc, flags = FOLL_TOUCH | FOLL_HWPOISON | FOLL_WRITE;
@@ -1177,9 +1210,15 @@ static int hva_to_pfn_slow(unsigned long addr, bool *async, bool write_fault,
 		npages = get_user_page_nowait(current, current->mm,
 					      addr, write_fault, page);
 		up_read(&current->mm->mmap_sem);
-	} else
-		npages = get_user_pages_fast(addr, 1, write_fault,
-					     page);
+	} else {
+		/*
+		 * By now we have tried gup_fast, and possible async_pf, and we
+		 * are certainly not atomic. Time to retry the gup, allowing
+		 * mmap semaphore to be relinquished in the case of IO.
+		 */
+		npages = kvm_get_user_page_retry(current, current->mm, addr,
+						 write_fault, page);
+	}
 	if (npages != 1)
 		return npages;
 
-- 
2.1.0.rc2.206.gedb03e5

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