Re: [RFC][PATCH] vfs: don't fallback to buffered read if the offset of dio read is beyond eof

[Jan Kara <jack@suse.cz>]

On Thu 21-11-13 16:34:14, Zheng Liu wrote:
> From: Zheng Liu <wenqing.lz@taobao.com>
> 
> Currently when we issue a dio read at a given offset that is beyond the
> end of file we will fallback to buffered read.  Then we check i_size in
> buffered read path after we know the page is updated.  But it could
> return some zero-filled pages to the userspace when we do some append
> dio writes.  We could use the following code snippet to reproduce this
> problem in a ext2/3/4 filesystem.
> 
> code snippet:
>   #define _GNU_SOURCE
> 
>   #include <stdio.h>
>   #include <stdlib.h>
>   #include <string.h>
>   #include <memory.h>
> 
>   #include <unistd.h>
>   #include <fcntl.h>
>   #include <sys/types.h>
>   #include <sys/stat.h>
>   #include <errno.h>
> 
>   #include <pthread.h>
> 
>   #define BUF_ALIGN	1024
> 
>   struct writer_data {
>   	int fd;
>   	size_t blksize;
>   	char *buf;
>   };
> 
>   static void *writer(void *arg)
>   {
>   	struct writer_data *data = (struct writer_data *)arg;
>   	int ret;
> 
>   	ret = write(data->fd, data->buf, data->blksize);
>   	if (ret < 0)
>   		fprintf(stderr, "write file failed: %s\n", strerror(errno));
> 
>   	return NULL;
>   }
> 
>   int main(int argc, char *argv[])
>   {
>   	pthread_t tid;
>   	struct writer_data wdata;
>   	size_t max_blocks = 10 * 1024;
>   	size_t blksize = 1 * 1024 * 1024;
>   	char *rbuf, *wbuf;
>   	int readfd, writefd;
>   	int i, j;
> 
>   	if (argc < 2) {
>   		fprintf(stderr, "usage: %s [filename]\n", argv[0]);
>   		exit(1);
>   	}
> 
>   	writefd = open(argv[1], O_CREAT|O_DIRECT|O_WRONLY|O_APPEND|O_TRUNC, S_IRWXU);
>   	if (writefd < 0) {
>   		fprintf(stderr, "failed to open wfile: %s\n", strerror(errno));
>   		exit(1);
>   	}
>   	readfd = open(argv[1], O_DIRECT|O_RDONLY, S_IRWXU);
>   	if (readfd < 0) {
>   		fprintf(stderr, "failed to open rfile: %s\n", strerror(errno));
>   		exit(1);
>   	}
> 
>   	if (posix_memalign((void **)&wbuf, BUF_ALIGN, blksize)) {
>   		fprintf(stderr, "failed to alloc memory: %s\n", strerror(errno));
>   		exit(1);
>   	}
> 
>   	if (posix_memalign((void **)&rbuf, 4096, blksize)) {
>   		fprintf(stderr, "failed to alloc memory: %s\n", strerror(errno));
>   		exit(1);
>   	}
> 
>   	memset(wbuf, 'a', blksize);
> 
>   	wdata.fd = writefd;
>   	wdata.blksize = blksize;
>   	wdata.buf = wbuf;
> 
>   	for (i = 0; i < max_blocks; i++) {
>   		void *retval;
>   		int ret;
> 
>   		ret = pthread_create(&tid, NULL, writer, &wdata);
>   		if (ret) {
>   			fprintf(stderr, "create thread failed: %s\n", strerror(errno));
>   			exit(1);
>   		}
> 
>   		memset(rbuf, 'b', blksize);
>   		do {
>   			ret = pread(readfd, rbuf, blksize, i * blksize);
>   		} while (ret <= 0);
> 
>   		if (ret < 0) {
>   			fprintf(stderr, "read file failed: %s\n", strerror(errno));
>   			exit(1);
>   		}
> 
>   		if (pthread_join(tid, &retval)) {
>   			fprintf(stderr, "pthread join failed: %s\n", strerror(errno));
>   			exit(1);
>   		}
> 
>   		if (ret >= 0) {
>   			for (j = 0; j < ret; j++) {
>   				if (rbuf[j] != 'a') {
>   					fprintf(stderr, "encounter an error: offset %ld\n",
>   						i);
>   					goto err;
>   				}
>   			}
>   		}
>   	}
> 
>   err:
>   	free(wbuf);
>   	free(rbuf);
> 
>   	return 0;
>   }
> 
> build & run:
>   $ gcc code.c -o code -lpthread # build
>   $ ./code ${filename} # run
> 
> As we expected, we should read nothing or data with 'a'.  But now we
> read data with '0'.  I take a closer look at the code and it seems that
> there is a bug in vfs.  Let me describe my found here.
> 
>   reader					writer
>                                                 generic_file_aio_write()
>                                                 ->__generic_file_aio_write()
>                                                   ->generic_file_direct_write()
>   generic_file_aio_read()
>   ->do_generic_file_read()
>     [fallback to buffered read]
> 
>     ->find_get_page()
>     ->page_cache_sync_readahead()
>     ->find_get_page()
>     [in find_page label, we couldn't find a
>      page before and after calling
>      page_cache_sync_readahead().  So go to
>      no_cached_page label]
> 
>     ->page_cache_alloc_cold()
>     ->add_to_page_cache_lru()
>     [in no_cached_page label, we alloc a page
>      and goto readpage label.]
> 
>     ->aops->readpage()
>     [in readpage label, readpage() callback
>      is called and mpage_readpage() return a
>      zero-filled page (e.g. ext3/4), and go
>      to page_ok label]
> 
>                                                   ->a_ops->direct_IO()
>                                                   ->i_size_write()
>                                                   [we enlarge the i_size]
> 
>     Here we check i_size
>     [in page_ok label, we check i_size but
>      it has been enlarged.  Thus, we pass
>      the check and return a zero-filled page]
> 
> This commit let dio read return directly if the current offset of the
> dio read is beyond the end of file in order to avoid this problem.
  This would be OK as well but it doesn't solve the case when the reader
uses a normal buffered read, does it? So I think trimming read count as I
described as a reply to your previous patch might be still better.

								Honza

> 
> Cc: Alexander Viro <viro@zeniv.linux.org.uk>
> Signed-off-by: Zheng Liu <wenqing.lz@taobao.com>
> ---
>  mm/filemap.c |    2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 1e6aec4..4179249 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -1452,6 +1452,8 @@ generic_file_aio_read(struct kiocb *iocb, const struct iovec *iov,
>  				file_accessed(filp);
>  				goto out;
>  			}
> +		} else {
> +			goto out;
>  		}
>  	}
>  
> -- 
> 1.7.9.7
> 
> --
> To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR