Re: [PATCH v3 2/2] iov_iter: Don't deal with iter->copy_mc in memcpy_from_iter_mc()

[David Howells]

Would it make sense to always check for MCE in _copy_from_iter() and always
return a short transfer if we encounter one?  It looks pretty cheap in terms
of code size as the exception table stuff handles it, so we don't need to do
anything in the normal path.

I guess this would change the handling of memory errors and DAX errors.

David
---
iov_iter: Always handle MCE in _copy_to_iter()

(incomplete)

---
 arch/x86/include/asm/mce.h |   22 ++++++++++++++++++++++
 fs/coredump.c              |    1 -
 include/linux/uio.h        |   16 ----------------
 lib/iov_iter.c             |   17 +++++------------
 4 files changed, 27 insertions(+), 29 deletions(-)

diff --git a/arch/x86/include/asm/mce.h b/arch/x86/include/asm/mce.h
index 180b1cbfcc4e..ee3ff090360d 100644
--- a/arch/x86/include/asm/mce.h
+++ b/arch/x86/include/asm/mce.h
@@ -353,4 +353,26 @@ 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
+unsigned long memcpy_mc(void *to, const void *from, unsigned long 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
+	return memcpy(to, from, len);
+#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/include/linux/uio.h b/include/linux/uio.h
index 42bce38a8e87..73078ba297b7 100644
--- a/include/linux/uio.h
+++ b/include/linux/uio.h
@@ -40,7 +40,6 @@ struct iov_iter_state {
 
 struct iov_iter {
 	u8 iter_type;
-	bool copy_mc;
 	bool nofault;
 	bool data_source;
 	bool user_backed;
@@ -252,22 +251,8 @@ size_t _copy_from_iter_flushcache(void *addr, size_t bytes, struct iov_iter *i);
 
 #ifdef CONFIG_ARCH_HAS_COPY_MC
 size_t _copy_mc_to_iter(const void *addr, size_t bytes, struct iov_iter *i);
-static inline void iov_iter_set_copy_mc(struct iov_iter *i)
-{
-	i->copy_mc = true;
-}
-
-static inline bool iov_iter_is_copy_mc(const struct iov_iter *i)
-{
-	return i->copy_mc;
-}
 #else
 #define _copy_mc_to_iter _copy_to_iter
-static inline void iov_iter_set_copy_mc(struct iov_iter *i) { }
-static inline bool iov_iter_is_copy_mc(const struct iov_iter *i)
-{
-	return false;
-}
 #endif
 
 size_t iov_iter_zero(size_t bytes, struct iov_iter *);
@@ -382,7 +367,6 @@ static inline void iov_iter_ubuf(struct iov_iter *i, unsigned int direction,
 	WARN_ON(direction & ~(READ | WRITE));
 	*i = (struct iov_iter) {
 		.iter_type = ITER_UBUF,
-		.copy_mc = false,
 		.user_backed = true,
 		.data_source = direction,
 		.ubuf = buf,
diff --git a/lib/iov_iter.c b/lib/iov_iter.c
index d282fd4d348f..887d9cb9be4e 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,
@@ -168,7 +169,6 @@ void iov_iter_init(struct iov_iter *i, unsigned int direction,
 	WARN_ON(direction & ~(READ | WRITE));
 	*i = (struct iov_iter) {
 		.iter_type = ITER_IOVEC,
-		.copy_mc = false,
 		.nofault = false,
 		.user_backed = true,
 		.data_source = direction,
@@ -254,14 +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)
@@ -632,7 +629,6 @@ void iov_iter_kvec(struct iov_iter *i, unsigned int direction,
 	WARN_ON(direction & ~(READ | WRITE));
 	*i = (struct iov_iter){
 		.iter_type = ITER_KVEC,
-		.copy_mc = false,
 		.data_source = direction,
 		.kvec = kvec,
 		.nr_segs = nr_segs,
@@ -649,7 +645,6 @@ void iov_iter_bvec(struct iov_iter *i, unsigned int direction,
 	WARN_ON(direction & ~(READ | WRITE));
 	*i = (struct iov_iter){
 		.iter_type = ITER_BVEC,
-		.copy_mc = false,
 		.data_source = direction,
 		.bvec = bvec,
 		.nr_segs = nr_segs,
@@ -678,7 +673,6 @@ void iov_iter_xarray(struct iov_iter *i, unsigned int direction,
 	BUG_ON(direction & ~1);
 	*i = (struct iov_iter) {
 		.iter_type = ITER_XARRAY,
-		.copy_mc = false,
 		.data_source = direction,
 		.xarray = xarray,
 		.xarray_start = start,
@@ -702,7 +696,6 @@ void iov_iter_discard(struct iov_iter *i, unsigned int direction, size_t count)
 	BUG_ON(direction != READ);
 	*i = (struct iov_iter){
 		.iter_type = ITER_DISCARD,
-		.copy_mc = false,
 		.data_source = false,
 		.count = count,
 		.iov_offset = 0