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Subject[PATCH v3 3/7] mm: Introduce FAULT_FLAG_INTERRUPTIBLE
Date
handle_userfaultfd() is currently the only one place in the kernel
page fault procedures that can respond to non-fatal userspace signals.
It was trying to detect such an allowance by checking against USER &
KILLABLE flags, which was "un-official".

In this patch, we introduced a new flag (FAULT_FLAG_INTERRUPTIBLE) to
show that the fault handler allows the fault procedure to respond even
to non-fatal signals. Meanwhile, add this new flag to the default
fault flags so that all the page fault handlers can benefit from the
new flag. With that, replacing the userfault check to this one.

Since the line is getting even longer, clean up the fault flags a bit
too to ease TTY users.

Although we've got a new flag and applied it, we shouldn't have any
functional change with this patch so far.

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Peter Xu <peterx@redhat.com>
---
fs/userfaultfd.c | 4 +---
include/linux/mm.h | 39 ++++++++++++++++++++++++++++-----------
2 files changed, 29 insertions(+), 14 deletions(-)

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index ccbdbd62f0d8..4a8ad2dc2b6f 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -462,9 +462,7 @@ vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason)
uwq.ctx = ctx;
uwq.waken = false;

- return_to_userland =
- (vmf->flags & (FAULT_FLAG_USER|FAULT_FLAG_KILLABLE)) ==
- (FAULT_FLAG_USER|FAULT_FLAG_KILLABLE);
+ return_to_userland = vmf->flags & FAULT_FLAG_INTERRUPTIBLE;
blocking_state = return_to_userland ? TASK_INTERRUPTIBLE :
TASK_KILLABLE;

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 57fb5c535f8e..53ec7abb8472 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -383,22 +383,38 @@ extern unsigned int kobjsize(const void *objp);
*/
extern pgprot_t protection_map[16];

-#define FAULT_FLAG_WRITE 0x01 /* Fault was a write access */
-#define FAULT_FLAG_MKWRITE 0x02 /* Fault was mkwrite of existing pte */
-#define FAULT_FLAG_ALLOW_RETRY 0x04 /* Retry fault if blocking */
-#define FAULT_FLAG_RETRY_NOWAIT 0x08 /* Don't drop mmap_sem and wait when retrying */
-#define FAULT_FLAG_KILLABLE 0x10 /* The fault task is in SIGKILL killable region */
-#define FAULT_FLAG_TRIED 0x20 /* Second try */
-#define FAULT_FLAG_USER 0x40 /* The fault originated in userspace */
-#define FAULT_FLAG_REMOTE 0x80 /* faulting for non current tsk/mm */
-#define FAULT_FLAG_INSTRUCTION 0x100 /* The fault was during an instruction fetch */
+/**
+ * Fault flag definitions.
+ *
+ * @FAULT_FLAG_WRITE: Fault was a write fault.
+ * @FAULT_FLAG_MKWRITE: Fault was mkwrite of existing PTE.
+ * @FAULT_FLAG_ALLOW_RETRY: Allow to retry the fault if blocked.
+ * @FAULT_FLAG_RETRY_NOWAIT: Don't drop mmap_sem and wait when retrying.
+ * @FAULT_FLAG_KILLABLE: The fault task is in SIGKILL killable region.
+ * @FAULT_FLAG_TRIED: The fault has been tried once.
+ * @FAULT_FLAG_USER: The fault originated in userspace.
+ * @FAULT_FLAG_REMOTE: The fault is not for current task/mm.
+ * @FAULT_FLAG_INSTRUCTION: The fault was during an instruction fetch.
+ * @FAULT_FLAG_INTERRUPTIBLE: The fault can be interrupted by non-fatal signals.
+ */
+#define FAULT_FLAG_WRITE 0x01
+#define FAULT_FLAG_MKWRITE 0x02
+#define FAULT_FLAG_ALLOW_RETRY 0x04
+#define FAULT_FLAG_RETRY_NOWAIT 0x08
+#define FAULT_FLAG_KILLABLE 0x10
+#define FAULT_FLAG_TRIED 0x20
+#define FAULT_FLAG_USER 0x40
+#define FAULT_FLAG_REMOTE 0x80
+#define FAULT_FLAG_INSTRUCTION 0x100
+#define FAULT_FLAG_INTERRUPTIBLE 0x200

/*
* The default fault flags that should be used by most of the
* arch-specific page fault handlers.
*/
#define FAULT_FLAG_DEFAULT (FAULT_FLAG_ALLOW_RETRY | \
- FAULT_FLAG_KILLABLE)
+ FAULT_FLAG_KILLABLE | \
+ FAULT_FLAG_INTERRUPTIBLE)

#define FAULT_FLAG_TRACE \
{ FAULT_FLAG_WRITE, "WRITE" }, \
@@ -409,7 +425,8 @@ extern pgprot_t protection_map[16];
{ FAULT_FLAG_TRIED, "TRIED" }, \
{ FAULT_FLAG_USER, "USER" }, \
{ FAULT_FLAG_REMOTE, "REMOTE" }, \
- { FAULT_FLAG_INSTRUCTION, "INSTRUCTION" }
+ { FAULT_FLAG_INSTRUCTION, "INSTRUCTION" }, \
+ { FAULT_FLAG_INTERRUPTIBLE, "INTERRUPTIBLE" }

/*
* vm_fault is filled by the the pagefault handler and passed to the vma's
--
2.21.0
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