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Date: Tue, 08 Apr 2008 14:58:43 +0200
From: Eric Dumazet <dada1@...mosbay.com>
To: Evgeniy Polyakov <johnpol@....mipt.ru>
Cc: David Miller <davem@...emloft.net>, Jens Axboe <axboe@...nel.dk>,
netdev@...r.kernel.org, Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [take 2] Fix for the fundamental network/block layer race in
sendfile().
Evgeniy Polyakov a écrit :
> Hi.
>
> Summary of the previous series with this pompous header:
> when sendfile() returns, pages which it sent can still be queued in tcp
> stack or hardware, so subsequent write into them will endup in
> corrupting data which will be eventually sent. This concerns all
> ->sendpage() users namely sendfile() and splice().
>
> With this patch it is now guaranteed that data was transferred over the
> wire after splice/sendfile returns.
>
>
1) So a nonblocking operation (O_NDELAY) can become a blocking one now ?
> This approach (besides the fact that previous one was not 100% correct
> dealing with cloned skbs) uses new destructor field in shared info
> structure (introduced by Rusty, afacs they do not collide), which is
> invoked each time data is about to be released (but before actual
> release of data happens).
>
> Patch works by assigning each page from the sendfile path a couple of
>
>
> pointers: to page itself to differentiate sendfile pages from all
> others and pointer to pipe structure, which wakes up splice code.
> It is safe to assign the same callback for sendfile and non-sendfile
> pages because of above page pointer - pages which do not have it (slab,
> non-sendfile bio layer and others) will not have pipe structure
> dereferenced from private pointers.
>
> P.S. Previous patch was not an April 1 joke as long as this one :)
>
>
Hum, I see you forgot me in CC list, dont try to escape :)
> Signed-off-by: Evgeniy Polyakov <johnpol@....mipt.ru>
>
> diff --git a/fs/read_write.c b/fs/read_write.c
> index 49a9871..1961a46 100644
> --- a/fs/read_write.c
> +++ b/fs/read_write.c
> @@ -16,6 +16,7 @@
> #include <linux/syscalls.h>
> #include <linux/pagemap.h>
> #include <linux/splice.h>
> +#include <net/sock.h>
> #include "read_write.h"
>
> #include <asm/uaccess.h>
> @@ -695,6 +696,24 @@ sys_writev(unsigned long fd, const struct iovec __user *vec, unsigned long vlen)
> return ret;
> }
>
> +static void skb_splice_destructor(struct skb_shared_info *shi)
> +{
> + if (shi->nr_frags) {
> + int i;
> + struct pipe_inode_info *pipe;
> +
> + for (i = 0; i < shi->nr_frags; i++) {
> + struct page *page = shi->frags[i].page;
> +
> + if (page->lru.prev == (struct list_head *)page) {
> + pipe = (struct pipe_inode_info *)page->lru.next;
>
Unless I mistaken, you store in page->lru.next some info to find
your pipe pointer, assuming it is unique for this page.
What happens if two threads are doing a splice()/sendfile() using the
same underlying (source) file (and same pages in this file)
> + if (atomic_dec_return(&pipe->god_blessed_us) == 0)
> + wake_up(&pipe->wait);
> + }
> + }
> + }
> +}
> +
> static ssize_t do_sendfile(int out_fd, int in_fd, loff_t *ppos,
> size_t count, loff_t max)
> {
> @@ -703,6 +722,8 @@ static ssize_t do_sendfile(int out_fd, int in_fd, loff_t *ppos,
> loff_t pos;
> ssize_t retval;
> int fput_needed_in, fput_needed_out, fl;
> + struct sock *sk;
> + struct socket *sock;
>
> /*
> * Get input file, and verify that it is ok..
> @@ -762,6 +783,12 @@ static ssize_t do_sendfile(int out_fd, int in_fd, loff_t *ppos,
> count = max - pos;
> }
>
> + sock = out_file->private_data;
> + sk = sock->sk;
> +
> + sk->sk_user_data = &skb_splice_destructor;
> + set_bit(SOCK_PRIVATE_CALLBACK, &sock->flags);
> +
> fl = 0;
> #if 0
> /*
> @@ -775,6 +802,8 @@ static ssize_t do_sendfile(int out_fd, int in_fd, loff_t *ppos,
> #endif
> retval = do_splice_direct(in_file, ppos, out_file, count, fl);
>
> + clear_bit(SOCK_PRIVATE_CALLBACK, &sock->flags);
> +
> if (retval > 0) {
> add_rchar(current, retval);
> add_wchar(current, retval);
> diff --git a/fs/splice.c b/fs/splice.c
> index 0670c91..dcda32e 100644
> --- a/fs/splice.c
> +++ b/fs/splice.c
> @@ -29,6 +29,7 @@
> #include <linux/syscalls.h>
> #include <linux/uio.h>
> #include <linux/security.h>
> +#include <net/sock.h>
>
> /*
> * Attempt to steal a page from a pipe buffer. This should perhaps go into
> @@ -535,6 +536,8 @@ static int pipe_to_sendpage(struct pipe_inode_info *pipe,
> if (!ret) {
> more = (sd->flags & SPLICE_F_MORE) || sd->len < sd->total_len;
>
> + buf->page->lru.next = (void *)pipe;
>
is it really allowed here, are you the only user ot this page ?
> + buf->page->lru.prev = (void *)buf->page;
> ret = file->f_op->sendpage(file, buf->page, buf->offset,
> sd->len, &pos, more);
> }
> @@ -629,6 +632,9 @@ ssize_t __splice_from_pipe(struct pipe_inode_info *pipe, struct splice_desc *sd,
> ret = 0;
> do_wakeup = 0;
>
--
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