(cc linux-mm)
It would be good if we could get some more eyes onto this please.
On Thu, 21 Nov 2013 16:34:14 +0800 Zheng Liu <gnehzuil.liu@xxxxxxxxx> wrote:
> From: Zheng Liu <wenqing.lz@xxxxxxxxxx>
>
> 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]
It's odd that do_generic_file_read() is permitting a "read" outside
i_size. Perhaps we should be checking for this in the `no_cached_page'
block.
> ->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]
OK, so it's a race.
> 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.
>
> ...
>
> --- 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;
> }
> }
OK, so we don't fall back to buffered reading at all if we're outside
i_size.
I'm not sure this 100% fixes the problem. In generic_file_aio_read():
: if (pos < size) {
write() extends i_size now.
: retval = filemap_write_and_wait_range(mapping, pos,
: pos + iov_length(iov, nr_segs) - 1);
: if (!retval) {
: retval = mapping->a_ops->direct_IO(READ, iocb,
: iov, pos, nr_segs);
: }
: if (retval > 0) {
: *ppos = pos + retval;
: count -= retval;
: }
:
: /*
: * Btrfs can have a short DIO read if we encounter
: * compressed extents, so if there was an error, or if
: * we've already read everything we wanted to, or if
: * there was a short read because we hit EOF, go ahead
: * and return. Otherwise fallthrough to buffered io for
: * the rest of the read.
: */
: if (retval < 0 || !count || *ppos >= size) {
: file_accessed(filp);
: goto out;
: }
we can still fall through to buffered read.
: } else {
: goto out;
: }
: }
--
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