Re: [patch] cache pipe buf page address for non-highmem arch

Ken Chen
2007-03-24T00:48:43Z

On 3/22/07, Andrew Morton <akpm@linux-foundation.org> wrote:
> If we're going to do this then we should also implement pipe_kunmap_atomic().
> Relying upon kunmap_atomic() internals like this is weird-looking, and is fragile
> against future changes to kunmap_atomic().

OK, I added the matching pipe_kunmap variants. Now that we have the
matching pair, I made further optimization to make both pipe_kmap and
pipe_kmap_atomic the same because pipe_kmap does not hold any critical
resource that can't sleep. There is no reason to make them different.

Full patch and changelog:

It is really sad that we always call kmap and friends for every pipe
buffer page on 64-bit arch that doesn't use HIGHMEM, or on
configuration that doesn't turn on HIGHMEM.

The effect of calling kmap* is visible in the execution profile when
pipe code is being stressed. It is especially true on amd's x86-64
platform where kmap() has to traverse through numa node index
calculation in order to convert struct page * to kernel virtual
address. It is fairly pointless to perform that calculation repeatly
on system with no highmem (i.e., 64-bit arch like x86-64). This patch
caches kernel pipe buffer page's kernel vaddr to speed up pipe buffer
mapping functions.

There is another suboptimal block in pipe_read() where wake_up is
called twice. I think it was an oversight since in pipe_write(), it
looks like it is doing the right thing.

Signed-off-by: Ken Chen <kenchen@google.com>

diff --git a/fs/pipe.c b/fs/pipe.c
index ebafde7..4cd1d68 100644
--- a/fs/pipe.c
+++ b/fs/pipe.c
@@ -21,6 +21,21 @@ #include <linux/audit.h>
#include <asm/uaccess.h>
#include <asm/ioctls.h>

+#ifdef CONFIG_HIGHMEM
+#define pipe_kmap kmap
+#define pipe_kmap_atomic kmap_atomic
+#define pipe_kunmap kunmap
+#define pipe_kunmap_atomic kunmap_atomic
+#else /* CONFIG_HIGHMEM */
+static inline void *pipe_kmap(struct page *page)
+{
+ return (void *) page->private;
+}
+#define pipe_kmap_atomic(page, type) pipe_kmap(page)
+#define pipe_kunmap(page) do { } while (0)
+#define pipe_kunmap_atomic(page, type) do { } while (0)
+#endif
+
/*
* We use a start+len construction, which provides full use of the
* allocated memory.
@@ -169,10 +184,10 @@ void *generic_pipe_buf_map(struct pipe_i
{
if (atomic) {
buf->flags |= PIPE_BUF_FLAG_ATOMIC;
- return kmap_atomic(buf->page, KM_USER0);
+ return pipe_kmap_atomic(buf->page, KM_USER0);
}

- return kmap(buf->page);
+ return pipe_kmap(buf->page);
}

void generic_pipe_buf_unmap(struct pipe_inode_info *pipe,
@@ -180,9 +195,9 @@ void generic_pipe_buf_unmap(struct pipe_
{
if (buf->flags & PIPE_BUF_FLAG_ATOMIC) {
buf->flags &= ~PIPE_BUF_FLAG_ATOMIC;
- kunmap_atomic(map_data, KM_USER0);
+ pipe_kunmap_atomic(map_data, KM_USER0);
} else
- kunmap(buf->page);
+ pipe_kunmap(buf->page);
}

int generic_pipe_buf_steal(struct pipe_inode_info *pipe,
@@ -316,6 +331,7 @@ redo:
if (do_wakeup) {
wake_up_interruptible_sync(&pipe->wait);
kill_fasync(&pipe->fasync_writers, SIGIO, POLL_OUT);
+ do_wakeup = 0;
}
pipe_wait(pipe);
}
@@ -423,6 +439,8 @@ redo1:
ret = ret ? : -ENOMEM;
break;
}
+ page->private = (unsigned long)
+ page_address(page);
pipe->tmp_page = page;
}
/* Always wake up, even if the copy fails. Otherwise
@@ -438,16 +456,16 @@ redo1:
iov_fault_in_pages_read(iov, chars);
redo2:
if (atomic)
- src = kmap_atomic(page, KM_USER0);
+ src = pipe_kmap_atomic(page, KM_USER0);
else
- src = kmap(page);
+ src = pipe_kmap(page);

error = pipe_iov_copy_from_user(src, iov, chars,
atomic);
if (atomic)
- kunmap_atomic(src, KM_USER0);
+ pipe_kunmap_atomic(src, KM_USER0);
else
- kunmap(page);
+ pipe_kunmap(page);

if (unlikely(error)) {
if (atomic) {