aboutsummaryrefslogtreecommitdiff
diff options
context:
space:
mode:
authorAndres Lagar-Cavilla <andreslc@google.com>2014-09-17 10:51:48 -0700
committerPaolo Bonzini <pbonzini@redhat.com>2014-09-24 14:07:54 +0200
commit234b239bea395316d7f78018c672f4a88b3cdf0d (patch)
tree3defdf8536d0b73b1130d23fc6c79343a9831d59
parentb4619660635732bd2da376bb8f31f94d0f15fc98 (diff)
downloadlinux-234b239bea395316d7f78018c672f4a88b3cdf0d.tar.gz
kvm: Faults which trigger IO release the mmap_sem
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 an async workqueue. If not, it's retried immediately in the same code path. In either case 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 as a function of 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. Reviewed-by: Radim Krčmář <rkrcmar@redhat.com> Signed-off-by: Andres Lagar-Cavilla <andreslc@google.com> Acked-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
-rw-r--r--include/linux/kvm_host.h11
-rw-r--r--include/linux/mm.h1
-rw-r--r--mm/gup.c4
-rw-r--r--virt/kvm/async_pf.c4
-rw-r--r--virt/kvm/kvm_main.c49
5 files changed, 63 insertions, 6 deletions
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index d44a2d64055..45aaeb3360c 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -198,6 +198,17 @@ 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
+/*
+ * Carry out a gup that requires IO. Allow the mm to relinquish the mmap
+ * semaphore if the filemap/swap has to wait on a page lock. pagep == NULL
+ * controls whether we retry the gup one more time to completion in that case.
+ * Typically this is called after a FAULT_FLAG_RETRY_NOWAIT in the main tdp
+ * handler.
+ */
+int kvm_get_user_page_io(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 8981cc882ed..0f4196a0bc2 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1985,6 +1985,7 @@ 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 91d044b1600..af7ea3e0826 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) {
+ VM_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 d6a3d0993d8..5ff7f7f2689 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_io(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 499db0977f3..1c6e8476b24 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1122,6 +1122,43 @@ 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_io(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);
+
+ /*
+ * If retrying the 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) {
+ VM_BUG_ON(npages != -EBUSY);
+
+ if (!pagep)
+ return 0;
+
+ /*
+ * 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;
@@ -1184,9 +1221,15 @@ 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 possibly 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_io(current, current->mm, addr,
+ write_fault, page);
+ }
if (npages != 1)
return npages;