D2933.diff
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	Index: head/usr.bin/ar/write.c
	===================================================================
	--- head/usr.bin/ar/write.c
	+++ head/usr.bin/ar/write.c
	@@ -41,6 +41,7 @@
	#include <stdlib.h>
	#include <string.h>
	#include <sysexits.h>
	+#include <unistd.h>

	#include "ar.h"

	@@ -61,6 +62,7 @@
	static void free_obj(struct bsdar bsdar, struct ar_obj obj);
	static void insert_obj(struct bsdar bsdar, struct ar_obj obj,
	struct ar_obj *pos);
	+static void prefault_buffer(const char *buf, size_t s);
	static void read_objs(struct bsdar bsdar, const char archive,
	int checkargv);
	static void write_archive(struct bsdar *bsdar, char mode);
	@@ -551,11 +553,35 @@
	}

	/*
	+ * Fault in the buffer prior to writing as a workaround for poor performance
	+ * due to interaction with kernel fs deadlock avoidance code. See the comment
	+ * above vn_io_fault_doio() in sys/kern/vfs_vnops.c for details of the issue.
	+ */
	+static void
	+prefault_buffer(const char *buf, size_t s)
	+{
	+ volatile const char *p;
	+ size_t page_size;
	+
	+ if (s == 0)
	+ return;
	+ page_size = sysconf(_SC_PAGESIZE);
	+ for (p = buf; p < buf + s; p += page_size)
	+ *p;
	+ /*
	+ * Ensure we touch the last page as well, in case the buffer is not
	+ * page-aligned.
	+ */
	+ (volatile const char )(buf + s - 1);
	+}
	+
	+/*
	* Wrapper for archive_write_data().
	*/
	static void
	write_data(struct bsdar bsdar, struct archive a, const void *buf, size_t s)
	{
	+ prefault_buffer(buf, s);
	if (archive_write_data(a, buf, s) != (ssize_t)s)
	bsdar_errc(bsdar, EX_SOFTWARE, 0, "%s",
	archive_error_string(a));