linux-kernel - Re: [PATCH v2] splice: sendfile() at once fails for big files

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Message-ID: <20150427070131.GA21190@gondor.apana.org.au>
Date:	Mon, 27 Apr 2015 15:01:31 +0800
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	Christophe Leroy <christophe.leroy@....fr>
Cc:	viro@...iv.linux.org.uk, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, linux-crypto@...r.kernel.org,
	Jens Axboe <axboe@...com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v2] splice: sendfile() at once fails for big files

Christophe Leroy <christophe.leroy@....fr> wrote:
> Using sendfile with below small program to get MD5 sums of some files,
> it appear that big files (over 64kbytes with 4k pages system) get a
> wrong MD5 sum while small files get the correct sum.
> This program uses sendfile() to send a file to an AF_ALG socket
> for hashing.

Jens, any comments on this patch?

> /* md5sum2.c */
> #include <stdio.h>
> #include <stdlib.h>
> #include <unistd.h>
> #include <string.h>
> #include <fcntl.h>
> #include <sys/socket.h>
> #include <sys/stat.h>
> #include <sys/types.h>
> #include <linux/if_alg.h>
> 
> int main(int argc, char **argv)
> {
>        int sk = socket(AF_ALG, SOCK_SEQPACKET, 0);
>        struct stat st;
>        struct sockaddr_alg sa = {
>                .salg_family = AF_ALG,
>                .salg_type = "hash",
>                .salg_name = "md5",
>        };
>        int n;
> 
>        bind(sk, (struct sockaddr*)&sa, sizeof(sa));
> 
>        for (n = 1; n < argc; n++) {
>                int size;
>                int offset = 0;
>                char buf[4096];
>                int fd;
>                int sko;
>                int i;
> 
>                fd = open(argv[n], O_RDONLY);
>                sko = accept(sk, NULL, 0);
>                fstat(fd, &st);
>                size = st.st_size;
>                sendfile(sko, fd, &offset, size);
>                size = read(sko, buf, sizeof(buf));
>                for (i = 0; i < size; i++)
>                        printf("%2.2x", buf[i]);
>                printf("  %s\n", argv[n]);
>                close(fd);
>                close(sko);
>        }
>        exit(0);
> }
> 
> Test below is done using official linux patch files. First result is
> with a software based md5sum. Second result is with the program above.
> 
> root@...ip:~# ls -l patch-3.6.*
> -rw-r--r--    1 root     root         64011 Aug 24 12:01 patch-3.6.2.gz
> -rw-r--r--    1 root     root         94131 Aug 24 12:01 patch-3.6.3.gz
> 
> root@...ip:~# md5sum patch-3.6.*
> b3ffb9848196846f31b2ff133d2d6443  patch-3.6.2.gz
> c5e8f687878457db77cb7158c38a7e43  patch-3.6.3.gz
> 
> root@...ip:~# ./md5sum2 patch-3.6.*
> b3ffb9848196846f31b2ff133d2d6443  patch-3.6.2.gz
> 5fd77b24e68bb24dcc72d6e57c64790e  patch-3.6.3.gz
> 
> 
> After investivation, it appears that sendfile() sends the files by blocks
> of 64kbytes (16 times PAGE_SIZE). The problem is that at the end of each
> block, the SPLICE_F_MORE flag is missing, therefore the hashing operation
> is reset as if it was the end of the file.
> 
> This patch adds SPLICE_F_MORE to the flags when more data is pending.
> 
> With the patch applied, we get the correct sums:
> 
> root@...ip:~# md5sum patch-3.6.*
> b3ffb9848196846f31b2ff133d2d6443  patch-3.6.2.gz
> c5e8f687878457db77cb7158c38a7e43  patch-3.6.3.gz
> 
> root@...ip:~# ./md5sum2 patch-3.6.*
> b3ffb9848196846f31b2ff133d2d6443  patch-3.6.2.gz
> c5e8f687878457db77cb7158c38a7e43  patch-3.6.3.gz
> 
> Signed-off-by: Christophe Leroy <christophe.leroy@....fr>
> ---
> v2: no change, only new commit text
> 
> fs/splice.c | 7 ++++++-
> 1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/splice.c b/fs/splice.c
> index 476024b..fe61723 100644
> --- a/fs/splice.c
> +++ b/fs/splice.c
> @@ -1161,7 +1161,7 @@ ssize_t splice_direct_to_actor(struct file *in, struct splice_desc *sd,
>        long ret, bytes;
>        umode_t i_mode;
>        size_t len;
> -       int i, flags;
> +       int i, flags, more;
> 
>        /*
>         * We require the input being a regular file, as we don't want to
> @@ -1204,6 +1204,7 @@ ssize_t splice_direct_to_actor(struct file *in, struct splice_desc *sd,
>         * Don't block on output, we have to drain the direct pipe.
>         */
>        sd->flags &= ~SPLICE_F_NONBLOCK;
> +       more = sd->flags & SPLICE_F_MORE;
> 
>        while (len) {
>                size_t read_len;
> @@ -1216,6 +1217,10 @@ ssize_t splice_direct_to_actor(struct file *in, struct splice_desc *sd,
>                read_len = ret;
>                sd->total_len = read_len;
> 
> +               if (read_len < len)
> +                       sd->flags |= SPLICE_F_MORE;
> +               else if (!more)
> +                       sd->flags &= ~SPLICE_F_MORE;
>                /*
>                 * NOTE: nonblocking mode only applies to the input. We
>                 * must not do the output in nonblocking mode as then we

Thanks,
-- 
Email: Herbert Xu <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
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