[PATCH v2 1/1] s390/crash: allow multi-segment iterators

[Alexander Gordeev <agordeev@linux.ibm.com>]

Rework copy_oldmem_page() to allow multi-segment iterators.
Reuse existing iterate_iovec macro as is and only relevant
bits from __iterate_and_advance macro.

Fixes: 49b11524d648 ("s390/crash: add missing iterator advance in copy_oldmem_page())
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>
---
 arch/s390/kernel/crash_dump.c | 65 +++++++++++++++++++++++++++--------
 1 file changed, 50 insertions(+), 15 deletions(-)

diff --git a/arch/s390/kernel/crash_dump.c b/arch/s390/kernel/crash_dump.c
index 28124d0fa1d5..cc1586d64ce2 100644
--- a/arch/s390/kernel/crash_dump.c
+++ b/arch/s390/kernel/crash_dump.c
@@ -210,6 +210,52 @@ static int copy_oldmem_user(void __user *dst, unsigned long src, size_t count)
 	return 0;
 }
 
+#define iterate_iovec(i, n, base, len, off, __p, STEP) {	\
+	size_t off = 0;						\
+	size_t skip = i->iov_offset;				\
+	do {							\
+		len = min(n, __p->iov_len - skip);		\
+		if (likely(len)) {				\
+			base = __p->iov_base + skip;		\
+			len -= (STEP);				\
+			off += len;				\
+			skip += len;				\
+			n -= len;				\
+			if (skip < __p->iov_len)		\
+				break;				\
+		}						\
+		__p++;						\
+		skip = 0;					\
+	} while (n);						\
+	i->iov_offset = skip;					\
+	n = off;						\
+}
+
+#define __iterate_and_advance(i, n, base, len, off, I, K) {	\
+	if (unlikely(i->count < n))				\
+		n = i->count;					\
+	if (likely(n)) {					\
+		if (likely(iter_is_iovec(i))) {			\
+			const struct iovec *iov = i->iov;	\
+			void __user *base;			\
+			size_t len;				\
+			iterate_iovec(i, n, base, len, off,	\
+						iov, (I))	\
+			i->nr_segs -= iov - i->iov;		\
+			i->iov = iov;				\
+		} else if (iov_iter_is_kvec(i)) {		\
+			const struct kvec *kvec = i->kvec;	\
+			void *base;				\
+			size_t len;				\
+			iterate_iovec(i, n, base, len, off,	\
+						kvec, (K))	\
+			i->nr_segs -= kvec - i->kvec;		\
+			i->kvec = kvec;				\
+		}						\
+		i->count -= n;					\
+	}							\
+}
+
 /*
  * Copy one page from "oldmem"
  */
@@ -217,25 +263,14 @@ ssize_t copy_oldmem_page(struct iov_iter *iter, unsigned long pfn, size_t csize,
 			 unsigned long offset)
 {
 	unsigned long src;
-	int rc;
 
 	if (!(iter_is_iovec(iter) || iov_iter_is_kvec(iter)))
 		return -EINVAL;
-	/* Multi-segment iterators are not supported */
-	if (iter->nr_segs > 1)
-		return -EINVAL;
-	if (!csize)
-		return 0;
 	src = pfn_to_phys(pfn) + offset;
-
-	/* XXX: pass the iov_iter down to a common function */
-	if (iter_is_iovec(iter))
-		rc = copy_oldmem_user(iter->iov->iov_base, src, csize);
-	else
-		rc = copy_oldmem_kernel(iter->kvec->iov_base, src, csize);
-	if (rc < 0)
-		return rc;
-	iov_iter_advance(iter, csize);
+	__iterate_and_advance(iter, csize, base, len, off,
+		({ copy_oldmem_user(base, src + off, len) < 0 ? len : 0; }),
+		({ copy_oldmem_kernel(base, src + off, len) < 0 ? len : 0; })
+	)
 	return csize;
 }
 
-- 
2.34.1