[PATCH v6 2/7] userfaultfd: add minor fault registration mode
From: Axel Rasmussen
Date: Fri Feb 12 2021 - 16:56:12 EST
This feature allows userspace to intercept "minor" faults. By "minor"
faults, I mean the following situation:
Let there exist two mappings (i.e., VMAs) to the same page(s). One of
the mappings is registered with userfaultfd (in minor mode), and the
other is not. Via the non-UFFD mapping, the underlying pages have
already been allocated & filled with some contents. The UFFD mapping
has not yet been faulted in; when it is touched for the first time,
this results in what I'm calling a "minor" fault. As a concrete
example, when working with hugetlbfs, we have huge_pte_none(), but
find_lock_page() finds an existing page.
This commit adds the new feature flag used to enable this behavior. In
the hugetlb fault path, if we find that we have huge_pte_none(), but
find_lock_page() does indeed find an existing page, then we have a
"minor" fault, and if the VMA is UFFD-registered (with VM_UFFD_MISSING),
*and* this feature is enabled, we call into userfaultfd to handle it.
Why not add a new registration mode instead? After all, this being a
feature flag instead has drawbacks:
- You can't handle *only* minor faults, but *not* missing faults.
- This is a per-FD option, not a per-registration option, so if you want
minor faults for some VMAs but not others, you need to open a separate
FD for those two configurations.
- The userfaultfd_minor() check is more expensive, as we have to examine
the userfaultfd_ctx.
- handle_userfault()'s "reason" argument is no longer 1:1 with VM_*
flags, which has to be dealt with (complexity).
Basically, it comes down to the fact that we can't really add a new VM_*
flag. There are no unused bits left. :) With the current design of UFFD,
we don't write down the requested registration mode anywhere except this
flag either - there isn't any extended context we can check. So, I think
this is the only way.
Signed-off-by: Axel Rasmussen <axelrasmussen@xxxxxxxxxx>
---
fs/userfaultfd.c | 37 ++++++++++++++++++++------------
include/linux/mm.h | 2 +-
include/linux/userfaultfd_k.h | 9 ++++++++
include/uapi/linux/userfaultfd.h | 15 +++++++++----
mm/hugetlb.c | 32 +++++++++++++++++++++++++++
5 files changed, 76 insertions(+), 19 deletions(-)
diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index 8d663eae0266..edfdb8f1c740 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -178,6 +178,18 @@ static void userfaultfd_ctx_put(struct userfaultfd_ctx *ctx)
}
}
+bool userfaultfd_minor(struct vm_area_struct *vma)
+{
+ struct userfaultfd_ctx *ctx = vma->vm_userfaultfd_ctx.ctx;
+ unsigned int features = ctx ? ctx->features : 0;
+ bool minor_hugetlbfs = (features & UFFD_FEATURE_MINOR_HUGETLBFS);
+
+ if (!userfaultfd_missing(vma))
+ return false;
+
+ return is_vm_hugetlb_page(vma) && minor_hugetlbfs;
+}
+
static inline void msg_init(struct uffd_msg *msg)
{
BUILD_BUG_ON(sizeof(struct uffd_msg) != 32);
@@ -197,24 +209,21 @@ static inline struct uffd_msg userfault_msg(unsigned long address,
msg_init(&msg);
msg.event = UFFD_EVENT_PAGEFAULT;
msg.arg.pagefault.address = address;
+ /*
+ * These flags indicate why the userfault occurred:
+ * - UFFD_PAGEFAULT_FLAG_WP indicates a write protect fault.
+ * - UFFD_PAGEFAULT_FLAG_MINOR indicates a minor fault.
+ * - Neither of these flags being set indicates a MISSING fault.
+ *
+ * Separately, UFFD_PAGEFAULT_FLAG_WRITE indicates it was a write
+ * fault. Otherwise, it was a read fault.
+ */
if (flags & FAULT_FLAG_WRITE)
- /*
- * If UFFD_FEATURE_PAGEFAULT_FLAG_WP was set in the
- * uffdio_api.features and UFFD_PAGEFAULT_FLAG_WRITE
- * was not set in a UFFD_EVENT_PAGEFAULT, it means it
- * was a read fault, otherwise if set it means it's
- * a write fault.
- */
msg.arg.pagefault.flags |= UFFD_PAGEFAULT_FLAG_WRITE;
if (reason == UFFD_REASON_WP)
- /*
- * If UFFD_FEATURE_PAGEFAULT_FLAG_WP was set in the
- * uffdio_api.features and UFFD_PAGEFAULT_FLAG_WP was
- * not set in a UFFD_EVENT_PAGEFAULT, it means it was
- * a missing fault, otherwise if set it means it's a
- * write protect fault.
- */
msg.arg.pagefault.flags |= UFFD_PAGEFAULT_FLAG_WP;
+ if (reason == UFFD_REASON_MINOR)
+ msg.arg.pagefault.flags |= UFFD_PAGEFAULT_FLAG_MINOR;
if (features & UFFD_FEATURE_THREAD_ID)
msg.arg.pagefault.feat.ptid = task_pid_vnr(current);
return msg;
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 89fca443e6f1..3ddc465e31b0 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -272,7 +272,7 @@ extern unsigned int kobjsize(const void *objp);
#define VM_MAYSHARE 0x00000080
#define VM_GROWSDOWN 0x00000100 /* general info on the segment */
-#define VM_UFFD_MISSING 0x00000200 /* missing pages tracking */
+#define VM_UFFD_MISSING 0x00000200 /* missing or minor fault tracking */
#define VM_PFNMAP 0x00000400 /* Page-ranges managed without "struct page", just pure PFN */
#define VM_DENYWRITE 0x00000800 /* ETXTBSY on write attempts.. */
#define VM_UFFD_WP 0x00001000 /* wrprotect pages tracking */
diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h
index cc1554e7162f..4e03268c65ec 100644
--- a/include/linux/userfaultfd_k.h
+++ b/include/linux/userfaultfd_k.h
@@ -15,6 +15,8 @@ enum uffd_trigger_reason {
UFFD_REASON_MISSING,
/* A write protect fault occurred. */
UFFD_REASON_WP,
+ /* A minor fault occurred. */
+ UFFD_REASON_MINOR,
};
#ifdef CONFIG_USERFAULTFD
@@ -79,6 +81,8 @@ static inline bool userfaultfd_wp(struct vm_area_struct *vma)
return vma->vm_flags & VM_UFFD_WP;
}
+bool userfaultfd_minor(struct vm_area_struct *vma);
+
static inline bool userfaultfd_pte_wp(struct vm_area_struct *vma,
pte_t pte)
{
@@ -140,6 +144,11 @@ static inline bool userfaultfd_wp(struct vm_area_struct *vma)
return false;
}
+static inline bool userfaultfd_minor(struct vm_area_struct *vma)
+{
+ return false;
+}
+
static inline bool userfaultfd_pte_wp(struct vm_area_struct *vma,
pte_t pte)
{
diff --git a/include/uapi/linux/userfaultfd.h b/include/uapi/linux/userfaultfd.h
index 5f2d88212f7c..6b038d56bca7 100644
--- a/include/uapi/linux/userfaultfd.h
+++ b/include/uapi/linux/userfaultfd.h
@@ -22,12 +22,13 @@
#define UFFD_API_FEATURES (UFFD_FEATURE_PAGEFAULT_FLAG_WP | \
UFFD_FEATURE_EVENT_FORK | \
UFFD_FEATURE_EVENT_REMAP | \
- UFFD_FEATURE_EVENT_REMOVE | \
+ UFFD_FEATURE_EVENT_REMOVE | \
UFFD_FEATURE_EVENT_UNMAP | \
UFFD_FEATURE_MISSING_HUGETLBFS | \
UFFD_FEATURE_MISSING_SHMEM | \
UFFD_FEATURE_SIGBUS | \
- UFFD_FEATURE_THREAD_ID)
+ UFFD_FEATURE_THREAD_ID | \
+ UFFD_FEATURE_MINOR_HUGETLBFS)
#define UFFD_API_IOCTLS \
((__u64)1 << _UFFDIO_REGISTER | \
(__u64)1 << _UFFDIO_UNREGISTER | \
@@ -125,8 +126,9 @@ struct uffd_msg {
#define UFFD_EVENT_UNMAP 0x16
/* flags for UFFD_EVENT_PAGEFAULT */
-#define UFFD_PAGEFAULT_FLAG_WRITE (1<<0) /* If this was a write fault */
-#define UFFD_PAGEFAULT_FLAG_WP (1<<1) /* If reason is VM_UFFD_WP */
+#define UFFD_PAGEFAULT_FLAG_WRITE (1<<0) /* write fault */
+#define UFFD_PAGEFAULT_FLAG_WP (1<<1) /* write-protect fault */
+#define UFFD_PAGEFAULT_FLAG_MINOR (1<<2) /* minor fault */
struct uffdio_api {
/* userland asks for an API number and the features to enable */
@@ -171,6 +173,10 @@ struct uffdio_api {
*
* UFFD_FEATURE_THREAD_ID pid of the page faulted task_struct will
* be returned, if feature is not requested 0 will be returned.
+ *
+ * If requested, UFFD_FEATURE_MINOR_HUGETLBFS indicates that hugetlbfs
+ * memory registered with REGISTER_MODE_MISSING will *also* receive
+ * events for minor faults, not just missing faults.
*/
#define UFFD_FEATURE_PAGEFAULT_FLAG_WP (1<<0)
#define UFFD_FEATURE_EVENT_FORK (1<<1)
@@ -181,6 +187,7 @@ struct uffdio_api {
#define UFFD_FEATURE_EVENT_UNMAP (1<<6)
#define UFFD_FEATURE_SIGBUS (1<<7)
#define UFFD_FEATURE_THREAD_ID (1<<8)
+#define UFFD_FEATURE_MINOR_HUGETLBFS (1<<9)
__u64 features;
__u64 ioctls;
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 2a90e0b4bf47..93307fb058b7 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -4366,6 +4366,38 @@ static vm_fault_t hugetlb_no_page(struct mm_struct *mm,
VM_FAULT_SET_HINDEX(hstate_index(h));
goto backout_unlocked;
}
+
+ /* Check for page in userfault range. */
+ if (userfaultfd_minor(vma)) {
+ u32 hash;
+ struct vm_fault vmf = {
+ .vma = vma,
+ .address = haddr,
+ .flags = flags,
+ /*
+ * Hard to debug if it ends up being used by a
+ * callee that assumes something about the
+ * other uninitialized fields... same as in
+ * memory.c
+ */
+ };
+
+ unlock_page(page);
+
+ /*
+ * hugetlb_fault_mutex and i_mmap_rwsem must be dropped
+ * before handling userfault. Reacquire after handling
+ * fault to make calling code simpler.
+ */
+
+ hash = hugetlb_fault_mutex_hash(mapping, idx);
+ mutex_unlock(&hugetlb_fault_mutex_table[hash]);
+ i_mmap_unlock_read(mapping);
+ ret = handle_userfault(&vmf, UFFD_REASON_MINOR);
+ i_mmap_lock_read(mapping);
+ mutex_lock(&hugetlb_fault_mutex_table[hash]);
+ goto out;
+ }
}
/*
--
2.30.0.478.g8a0d178c01-goog