[PATCH] pipe: enable atomic copying of pipe data to/from user space

The pipe ->map() method uses kmap() to virtually map the pages, which
is both slow and has known scalability issues on SMP. This patch enables
atomic copying of pipe pages, by pre-faulting data and using kmap_atomic()
instead.

lmbench bw_pipe and lat_pipe measurements agree this is a Good Thing. Here
are results from that on a UP machine with highmem (1.5GiB of RAM), running
first a UP kernel, SMP kernel, and SMP kernel patched.

Vanilla-UP:
Pipe bandwidth: 1622.28 MB/sec
Pipe bandwidth: 1610.59 MB/sec
Pipe bandwidth: 1608.30 MB/sec
Pipe latency: 7.3275 microseconds
Pipe latency: 7.2995 microseconds
Pipe latency: 7.3097 microseconds

Vanilla-SMP:
Pipe bandwidth: 1382.19 MB/sec
Pipe bandwidth: 1317.27 MB/sec
Pipe bandwidth: 1355.61 MB/sec
Pipe latency: 9.6402 microseconds
Pipe latency: 9.6696 microseconds
Pipe latency: 9.6153 microseconds

Patched-SMP:
Pipe bandwidth: 1578.70 MB/sec
Pipe bandwidth: 1579.95 MB/sec
Pipe bandwidth: 1578.63 MB/sec
Pipe latency: 9.1654 microseconds
Pipe latency: 9.2266 microseconds
Pipe latency: 9.1527 microseconds

Signed-off-by: Jens Axboe <axboe@suse.de>

1 files changed, 118 insertions, 23 deletions

diff --git a/fs/pipe.c b/fs/pipe.c
index d9644fd9cc0d..3941a7f78b5d 100644
--- a/fs/pipe.c
+++ b/fs/pipe.c
@@ -55,7 +55,8 @@ void pipe_wait(struct pipe_inode_info *pipe)
 }
 
 static int
-pipe_iov_copy_from_user(void *to, struct iovec *iov, unsigned long len)
+pipe_iov_copy_from_user(void *to, struct iovec *iov, unsigned long len,
+                        int atomic)
 {
         unsigned long copy;
 
@@ -64,8 +65,13 @@ pipe_iov_copy_from_user(void *to, struct iovec *iov, unsigned long len)
                         iov++;
                 copy = min_t(unsigned long, len, iov->iov_len);
 
-                if (copy_from_user(to, iov->iov_base, copy))
-                        return -EFAULT;
+                if (atomic) {
+                        if (__copy_from_user_inatomic(to, iov->iov_base, copy))
+                                return -EFAULT;
+                } else {
+                        if (copy_from_user(to, iov->iov_base, copy))
+                                return -EFAULT;
+                }
                 to += copy;
                 len -= copy;
                 iov->iov_base += copy;
@@ -75,7 +81,8 @@ pipe_iov_copy_from_user(void *to, struct iovec *iov, unsigned long len)
 }
 
 static int
-pipe_iov_copy_to_user(struct iovec *iov, const void *from, unsigned long len)
+pipe_iov_copy_to_user(struct iovec *iov, const void *from, unsigned long len,
+                      int atomic)
 {
         unsigned long copy;
 
@@ -84,8 +91,13 @@ pipe_iov_copy_to_user(struct iovec *iov, const void *from, unsigned long len)
                         iov++;
                 copy = min_t(unsigned long, len, iov->iov_len);
 
-                if (copy_to_user(iov->iov_base, from, copy))
-                        return -EFAULT;
+                if (atomic) {
+                        if (__copy_to_user_inatomic(iov->iov_base, from, copy))
+                                return -EFAULT;
+                } else {
+                        if (copy_to_user(iov->iov_base, from, copy))
+                                return -EFAULT;
+                }
                 from += copy;
                 len -= copy;
                 iov->iov_base += copy;
@@ -94,6 +106,47 @@ pipe_iov_copy_to_user(struct iovec *iov, const void *from, unsigned long len)
         return 0;
 }
 
+/*
+ * Attempt to pre-fault in the user memory, so we can use atomic copies.
+ * Returns the number of bytes not faulted in.
+ */
+static int iov_fault_in_pages_write(struct iovec *iov, unsigned long len)
+{
+        while (!iov->iov_len)
+                iov++;
+
+        while (len > 0) {
+                unsigned long this_len;
+
+                this_len = min_t(unsigned long, len, iov->iov_len);
+                if (fault_in_pages_writeable(iov->iov_base, this_len))
+                        break;
+
+                len -= this_len;
+                iov++;
+        }
+
+        return len;
+}
+
+/*
+ * Pre-fault in the user memory, so we can use atomic copies.
+ */
+static void iov_fault_in_pages_read(struct iovec *iov, unsigned long len)
+{
+        while (!iov->iov_len)
+                iov++;
+
+        while (len > 0) {
+                unsigned long this_len;
+
+                this_len = min_t(unsigned long, len, iov->iov_len);
+                fault_in_pages_readable(iov->iov_base, this_len);
+                len -= this_len;
+                iov++;
+        }
+}
+
 static void anon_pipe_buf_release(struct pipe_inode_info *pipe,
                                   struct pipe_buffer *buf)
 {
@@ -111,15 +164,24 @@ static void anon_pipe_buf_release(struct pipe_inode_info *pipe,
 }
 
 void *generic_pipe_buf_map(struct pipe_inode_info *pipe,
-                           struct pipe_buffer *buf)
+                           struct pipe_buffer *buf, int atomic)
 {
+        if (atomic) {
+                buf->flags |= PIPE_BUF_FLAG_ATOMIC;
+                return kmap_atomic(buf->page, KM_USER0);
+        }
+
         return kmap(buf->page);
 }
 
 void generic_pipe_buf_unmap(struct pipe_inode_info *pipe,
-                            struct pipe_buffer *buf)
+                            struct pipe_buffer *buf, void *map_data)
 {
-        kunmap(buf->page);
+        if (buf->flags & PIPE_BUF_FLAG_ATOMIC) {
+                buf->flags &= ~PIPE_BUF_FLAG_ATOMIC;
+                kunmap_atomic(map_data, KM_USER0);
+        } else
+                kunmap(buf->page);
 }
 
 static int anon_pipe_buf_steal(struct pipe_inode_info *pipe,
@@ -183,7 +245,7 @@ pipe_readv(struct file *filp, const struct iovec *_iov,
                         struct pipe_buf_operations *ops = buf->ops;
                         void *addr;
                         size_t chars = buf->len;
-                        int error;
+                        int error, atomic;
 
                         if (chars > total_len)
                                 chars = total_len;
@@ -195,12 +257,21 @@ pipe_readv(struct file *filp, const struct iovec *_iov,
                                 break;
                         }
 
-                        addr = ops->map(pipe, buf);
-                        error = pipe_iov_copy_to_user(iov, addr + buf->offset, chars);
-                        ops->unmap(pipe, buf);
+                        atomic = !iov_fault_in_pages_write(iov, chars);
+redo:
+                        addr = ops->map(pipe, buf, atomic);
+                        error = pipe_iov_copy_to_user(iov, addr + buf->offset, chars, atomic);
+                        ops->unmap(pipe, buf, addr);
                         if (unlikely(error)) {
+                                /*
+                                 * Just retry with the slow path if we failed.
+                                 */
+                                if (atomic) {
+                                        atomic = 0;
+                                        goto redo;
+                                }
                                 if (!ret)
-                                        ret = -EFAULT;
+                                        ret = error;
                                 break;
                         }
                         ret += chars;
@@ -304,21 +375,28 @@ pipe_writev(struct file *filp, const struct iovec *_iov,
                 int offset = buf->offset + buf->len;
 
                 if (ops->can_merge && offset + chars <= PAGE_SIZE) {
+                        int error, atomic = 1;
                         void *addr;
-                        int error;
 
                         error = ops->pin(pipe, buf);
                         if (error)
                                 goto out;
 
-                        addr = ops->map(pipe, buf);
+                        iov_fault_in_pages_read(iov, chars);
+redo1:
+                        addr = ops->map(pipe, buf, atomic);
                         error = pipe_iov_copy_from_user(offset + addr, iov,
-                                                        chars);
-                        ops->unmap(pipe, buf);
+                                                        chars, atomic);
+                        ops->unmap(pipe, buf, addr);
                         ret = error;
                         do_wakeup = 1;
-                        if (error)
+                        if (error) {
+                                if (atomic) {
+                                        atomic = 0;
+                                        goto redo1;
+                                }
                                 goto out;
+                        }
                         buf->len += chars;
                         total_len -= chars;
                         ret = chars;
@@ -341,7 +419,8 @@ pipe_writev(struct file *filp, const struct iovec *_iov,
                         int newbuf = (pipe->curbuf + bufs) & (PIPE_BUFFERS-1);
                         struct pipe_buffer *buf = pipe->bufs + newbuf;
                         struct page *page = pipe->tmp_page;
-                        int error;
+                        char *src;
+                        int error, atomic = 1;
 
                         if (!page) {
                                 page = alloc_page(GFP_HIGHUSER);
@@ -361,11 +440,27 @@ pipe_writev(struct file *filp, const struct iovec *_iov,
                         if (chars > total_len)
                                 chars = total_len;
 
-                        error = pipe_iov_copy_from_user(kmap(page), iov, chars);
-                        kunmap(page);
+                        iov_fault_in_pages_read(iov, chars);
+redo2:
+                        if (atomic)
+                                src = kmap_atomic(page, KM_USER0);
+                        else
+                                src = kmap(page);
+
+                        error = pipe_iov_copy_from_user(src, iov, chars,
+                                                        atomic);
+                        if (atomic)
+                                kunmap_atomic(src, KM_USER0);
+                        else
+                                kunmap(page);
+
                         if (unlikely(error)) {
+                                if (atomic) {
+                                        atomic = 0;
+                                        goto redo2;
+                                }
                                 if (!ret)
-                                        ret = -EFAULT;
+                                        ret = error;
                                 break;
                         }
                         ret += chars;