linux-kernel - [PATCH] use a nocache copy for bvecs in copy_from_iter

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Message-Id: <20161007155511.21502-1-brian.boylston@hpe.com>
Date:   Fri,  7 Oct 2016 10:55:11 -0500
From:   Brian Boylston <brian.boylston@....com>
To:     linux-nvdimm@...ts.01.org
Cc:     linux-kernel@...r.kernel.org, toshi.kani@....com,
        oliver.moreno@....com, Brian Boylston <brian.boylston@....com>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        Al Viro <viro@...IV.linux.org.uk>,
        Dan Williams <dan.j.williams@...el.com>
Subject: [PATCH] use a nocache copy for bvecs in copy_from_iter_nocache()

copy_from_iter_nocache() is only "nocache" for iovecs.  Enhance it to also
use a nocache copy for bvecs.  This improves performance by 2-3X when
splice()ing to a file in a DAX-mounted, pmem-backed file system.

Cc: Ross Zwisler <ross.zwisler@...ux.intel.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: "H. Peter Anvin" <hpa@...or.com>
Cc: <x86@...nel.org>
Cc: Al Viro <viro@...IV.linux.org.uk>
Cc: Dan Williams <dan.j.williams@...el.com>
Signed-off-by: Brian Boylston <brian.boylston@....com>
Reviewed-by: Toshi Kani <toshi.kani@....com>
Reported-by: Oliver Moreno <oliver.moreno@....com>
---
 arch/x86/include/asm/pmem.h |  6 +++---
 lib/iov_iter.c              | 11 +++++++++--
 2 files changed, 12 insertions(+), 5 deletions(-)

diff --git a/arch/x86/include/asm/pmem.h b/arch/x86/include/asm/pmem.h
index 643eba4..d071f45c 100644
--- a/arch/x86/include/asm/pmem.h
+++ b/arch/x86/include/asm/pmem.h
@@ -73,12 +73,12 @@ static inline void arch_wb_cache_pmem(void *addr, size_t size)
 }
 
 /*
- * copy_from_iter_nocache() on x86 only uses non-temporal stores for iovec
- * iterators, so for other types (bvec & kvec) we must do a cache write-back.
+ * copy_from_iter_nocache() on x86 uses non-temporal stores for iovec and
+ * bvec iterators, but for kvec we must do a cache write-back.
  */
 static inline bool __iter_needs_pmem_wb(struct iov_iter *i)
 {
-	return iter_is_iovec(i) == false;
+	return (i->type & ITER_KVEC) == ITER_KVEC;
 }
 
 /**
diff --git a/lib/iov_iter.c b/lib/iov_iter.c
index 7e3138c..df4cb00 100644
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -342,6 +342,13 @@ static void memcpy_from_page(char *to, struct page *page, size_t offset, size_t
 	kunmap_atomic(from);
 }
 
+static void memcpy_from_page_nocache(char *to, struct page *page, size_t offset, size_t len)
+{
+	char *from = kmap_atomic(page);
+	__copy_from_user_inatomic_nocache(to, from, len);
+	kunmap_atomic(from);
+}
+
 static void memcpy_to_page(struct page *page, size_t offset, const char *from, size_t len)
 {
 	char *to = kmap_atomic(page);
@@ -392,8 +399,8 @@ size_t copy_from_iter_nocache(void *addr, size_t bytes, struct iov_iter *i)
 	iterate_and_advance(i, bytes, v,
 		__copy_from_user_nocache((to += v.iov_len) - v.iov_len,
 					 v.iov_base, v.iov_len),
-		memcpy_from_page((to += v.bv_len) - v.bv_len, v.bv_page,
-				 v.bv_offset, v.bv_len),
+		memcpy_from_page_nocache((to += v.bv_len) - v.bv_len,
+					 v.bv_page, v.bv_offset, v.bv_len),
 		memcpy((to += v.iov_len) - v.iov_len, v.iov_base, v.iov_len)
 	)
 
-- 
1.8.3.1