[PATCH v4 07/13] iov_iter: Make copy_from_iter() always handle MCE

[David Howells]

Make copy_from_iter() always catch an MCE and return a short copy and make
the coredump code rely on that.  This requires arch support in the form of
a memcpy_mc() function that returns the length copied.

[?] Is it better to kill the thread in the event of an MCE occurring?

Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
cc: Jens Axboe <axboe@xxxxxxxxx>
cc: Christoph Hellwig <hch@xxxxxx>
cc: Christian Brauner <christian@xxxxxxxxxx>
cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
cc: David Laight <David.Laight@xxxxxxxxxx>
cc: linux-block@xxxxxxxxxxxxxxx
cc: linux-fsdevel@xxxxxxxxxxxxxxx
cc: linux-mm@xxxxxxxxx
---
 arch/x86/include/asm/mce.h | 23 +++++++++++++++++++++++
 fs/coredump.c              |  1 -
 lib/iov_iter.c             | 12 +++++-------
 3 files changed, 28 insertions(+), 8 deletions(-)

diff --git a/arch/x86/include/asm/mce.h b/arch/x86/include/asm/mce.h
index 180b1cbfcc4e..77ce2044536c 100644
--- a/arch/x86/include/asm/mce.h
+++ b/arch/x86/include/asm/mce.h
@@ -353,4 +353,27 @@ static inline void mce_hygon_feature_init(struct cpuinfo_x86 *c)	{ return mce_am
 
 unsigned long copy_mc_fragile_handle_tail(char *to, char *from, unsigned len);
 
+static __always_inline __must_check
+size_t memcpy_mc(void *to, const void *from, size_t len)
+{
+#ifdef CONFIG_ARCH_HAS_COPY_MC
+	/*
+	 * If CPU has FSRM feature, use 'rep movs'.
+	 * Otherwise, use rep_movs_alternative.
+	 */
+	asm volatile(
+		"1:\n\t"
+		ALTERNATIVE("rep movsb",
+			    "call rep_movs_alternative", ALT_NOT(X86_FEATURE_FSRM))
+		"2:\n"
+		_ASM_EXTABLE_TYPE(1b, 2b, EX_TYPE_DEFAULT_MCE_SAFE)
+		:"+c" (len), "+D" (to), "+S" (from), ASM_CALL_CONSTRAINT
+		: : "memory", "rax", "r8", "r9", "r10", "r11");
+#else
+	memcpy(to, from, len);
+	return 0;
+#endif
+	return len;
+}
+
 #endif /* _ASM_X86_MCE_H */
diff --git a/fs/coredump.c b/fs/coredump.c
index 9d235fa14ab9..ad54102a5e14 100644
--- a/fs/coredump.c
+++ b/fs/coredump.c
@@ -884,7 +884,6 @@ static int dump_emit_page(struct coredump_params *cprm, struct page *page)
 	pos = file->f_pos;
 	bvec_set_page(&bvec, page, PAGE_SIZE, 0);
 	iov_iter_bvec(&iter, ITER_SOURCE, &bvec, 1, PAGE_SIZE);
-	iov_iter_set_copy_mc(&iter);
 	n = __kernel_write_iter(cprm->file, &iter, &pos);
 	if (n != PAGE_SIZE)
 		return 0;
diff --git a/lib/iov_iter.c b/lib/iov_iter.c
index 65374ee91ecd..b574601783bc 100644
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -14,6 +14,7 @@
 #include <linux/scatterlist.h>
 #include <linux/instrumented.h>
 #include <linux/iov_iter.h>
+#include <asm/mce.h>
 
 static __always_inline
 size_t copy_to_user_iter(void __user *iter_to, size_t progress,
@@ -253,14 +254,11 @@ size_t _copy_mc_to_iter(const void *addr, size_t bytes, struct iov_iter *i)
 EXPORT_SYMBOL_GPL(_copy_mc_to_iter);
 #endif /* CONFIG_ARCH_HAS_COPY_MC */
 
-static size_t memcpy_from_iter_mc(void *iter_from, size_t progress,
-				  size_t len, void *to, void *priv2)
+static __always_inline
+size_t memcpy_from_iter_mc(void *iter_from, size_t progress,
+			   size_t len, void *to, void *priv2)
 {
-	struct iov_iter *iter = priv2;
-
-	if (iov_iter_is_copy_mc(iter))
-		return copy_mc_to_kernel(to + progress, iter_from, len);
-	return memcpy_from_iter(iter_from, progress, len, to, priv2);
+	return memcpy_mc(to + progress, iter_from, len);
 }
 
 size_t _copy_from_iter(void *addr, size_t bytes, struct iov_iter *i)