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Message-ID: <20070604004647.GA8076@mail.ustc.edu.cn>
Date: Mon, 4 Jun 2007 08:46:47 +0800
From: Fengguang Wu <fengguang.wu@...il.com>
To: Jens Axboe <jens.axboe@...cle.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>,
"H. Peter Anvin" <hpa@...or.com>,
Eric Dumazet <dada1@...mosbay.com>,
linux-kernel@...r.kernel.org, cotte@...ibm.com, hugh@...itas.com,
neilb@...e.de, zanussi@...ibm.com, hch@...radead.org
Subject: Re: [PATCH] sendfile removal
Hi Jens,
This is another try, still not in a comfortable state though.
//Busy waiting is possible for interleaved reads.
Fengguang
---
In non-block splicing read, return EAGAIN once for possible I/O waits.
It avoids busy waiting by checking (ra.prev_index != index) in
__generic_file_splice_read(). If there are other readers on the same
fd, busy waiting is still possible.
If keep calling generic_file_splice_read() in non-blocking mode, it
will return partial data before the first wait point; then return
EAGAIN at second call; at last it will block on the I/O page.
Signed-off-by: Fengguang Wu <wfg@...l.ustc.edu.cn>
---
fs/splice.c | 41 +++++++++++++++++++++---------------
include/linux/pipe_fs_i.h | 2 +
2 files changed, 27 insertions(+), 16 deletions(-)
--- linux-2.6.22-rc3-mm1.orig/fs/splice.c
+++ linux-2.6.22-rc3-mm1/fs/splice.c
@@ -264,7 +264,7 @@ static ssize_t splice_to_pipe(struct pip
static int
__generic_file_splice_read(struct file *in, loff_t *ppos,
struct pipe_inode_info *pipe, size_t len,
- unsigned int flags)
+ unsigned int *flags)
{
struct address_space *mapping = in->f_mapping;
unsigned int loff, nr_pages;
@@ -278,7 +278,7 @@ __generic_file_splice_read(struct file *
struct splice_pipe_desc spd = {
.pages = pages,
.partial = partial,
- .flags = flags,
+ .flags = *flags,
.ops = &page_cache_pipe_buf_ops,
};
@@ -299,12 +299,16 @@ __generic_file_splice_read(struct file *
* Lookup the (hopefully) full range of pages we need.
*/
spd.nr_pages = find_get_pages_contig(mapping, index, nr_pages, pages);
+ index += spd.nr_pages;
/*
* If find_get_pages_contig() returned fewer pages than we needed,
- * allocate the rest.
+ * readahead/allocate the rest.
*/
- index += spd.nr_pages;
+ if (spd.nr_pages < nr_pages)
+ page_cache_readahead_ondemand(mapping, &in->f_ra, in,
+ NULL, index, nr_pages - spd.nr_pages);
+
while (spd.nr_pages < nr_pages) {
/*
* Page could be there, find_get_pages_contig() breaks on
@@ -312,9 +316,6 @@ __generic_file_splice_read(struct file *
*/
page = find_get_page(mapping, index);
if (!page) {
- page_cache_readahead_ondemand(mapping, &in->f_ra, in,
- NULL, index, nr_pages - spd.nr_pages);
-
/*
* page didn't exist, allocate one.
*/
@@ -326,8 +327,6 @@ __generic_file_splice_read(struct file *
GFP_KERNEL);
if (unlikely(error)) {
page_cache_release(page);
- if (error == -EEXIST)
- continue;
break;
}
/*
@@ -369,12 +368,19 @@ __generic_file_splice_read(struct file *
*/
if (!PageUptodate(page)) {
/*
- * If in nonblock mode then dont block on waiting
- * for an in-flight io page
+ * On retrying sequential reads in non-blocking mode:
+ * 1. (prev_index <= index - 2) => return partial data
+ * 2. (prev_index == index - 1) => return EAGAIN
+ * 3. (prev_index == index) => wait on I/O
+ * This works nicely for page aligned reads.
*/
- if (flags & SPLICE_F_NONBLOCK) {
- if (TestSetPageLocked(page))
- break;
+ if ((*flags & SPLICE_F_NONBLOCK) &&
+ TestSetPageLocked(page) &&
+ in->f_ra.prev_index != index) {
+ if (in->f_ra.prev_index != index - 1)
+ index--;
+ *flags |= SPLICE_INTERNAL_WILLBLOCK;
+ break;
} else
lock_page(page);
@@ -452,7 +458,6 @@ fill_it:
*/
while (page_nr < nr_pages)
page_cache_release(pages[page_nr++]);
- in->f_ra.prev_index = index;
if (spd.nr_pages)
return splice_to_pipe(pipe, &spd);
@@ -480,7 +485,7 @@ ssize_t generic_file_splice_read(struct
spliced = 0;
while (len) {
- ret = __generic_file_splice_read(in, ppos, pipe, len, flags);
+ ret = __generic_file_splice_read(in, ppos, pipe, len, &flags);
if (ret < 0)
break;
@@ -496,8 +501,12 @@ ssize_t generic_file_splice_read(struct
*ppos += ret;
len -= ret;
spliced += ret;
+
+ if (flags & SPLICE_INTERNAL_WILLBLOCK)
+ break;
}
+ in->f_ra.prev_index = (*ppos - 1) >> PAGE_CACHE_SHIFT;
if (spliced)
return spliced;
--- linux-2.6.22-rc3-mm1.orig/include/linux/pipe_fs_i.h
+++ linux-2.6.22-rc3-mm1/include/linux/pipe_fs_i.h
@@ -82,6 +82,8 @@ int generic_pipe_buf_steal(struct pipe_i
#define SPLICE_F_MORE (0x04) /* expect more data */
#define SPLICE_F_GIFT (0x08) /* pages passed in are a gift */
+#define SPLICE_INTERNAL_WILLBLOCK (0x100) /* read on next page will block */
+
/*
* Passed to the actors
*/
-
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