On Wed, Mar 01, 2017 at 01:36:26AM +0800, Eryu Guan wrote:
> From: Eryu Guan <eguan@xxxxxxxxxx>
>
> After XFS switching to iomap based DIO (commit acdda3aae146 ("xfs:
> use iomap_dio_rw")), I started to notice dio29/dio30 tests failures
> from LTP run on ppc64 hosts, and they can be reproduced on x86_64
> hosts with 512B/1k block size XFS too.
>
> dio29 diotest3 -b 65536 -n 100 -i 1000 -o 1024000
> dio30 diotest6 -b 65536 -n 100 -i 1000 -o 1024000
>
> The failure message is like:
> bufcmp: offset 0: Expected: 0x62, got 0x0
> diotest03 1 TPASS : Read with Direct IO, Write without
> diotest03 2 TFAIL : diotest3.c:142: comparsion failed; child=98 offset=1425408
> diotest03 3 TFAIL : diotest3.c:194: Write Direct-child 98 failed
>
> Direct write wrote 0x62 but buffer read got zero. This is because,
> when doing direct write to a hole or preallocated file, we
> invalidate the page caches before converting the extent from
> unwritten state to normal state, which is done by
> iomap_dio_complete(), thus leave a window for other buffer reader to
> cache the unwritten state extent.
>
> Consider this case, with sub-page blocksize XFS, two processes are
> direct writing to different blocksize-aligned regions (say 512B) of
> the same preallocated file, and reading the region back via buffered
> I/O to compare contents.
>
> process A, region [0,512] process B, region [512,1024]
> xfs_file_write_iter
> xfs_file_aio_dio_write
> iomap_dio_rw
> iomap_apply
> invalidate_inode_pages2_range
> xfs_file_write_iter
> xfs_file_aio_dio_write
> iomap_dio_rw
> iomap_apply
> invalidate_inode_pages2_range
> iomap_dio_complete
> xfs_file_read_iter
> xfs_file_buffered_aio_read
> generic_file_read_iter
> do_generic_file_read
> <readahead fills pagecache with 0>
> iomap_dio_complete
> xfs_file_read_iter
> <read gets 0 from pagecache>
>
> Process A first invalidates page caches, at this point the
> underlying extent is still in unwritten state (iomap_dio_complete
> not called yet), and process B finishs direct write and populates
> page caches via readahead, which caches zeros in page for region A,
> then process A reads zeros from page cache, instead of the actual
> data.
>
> Fix it by invalidating page caches after converting unwritten extent
> to make sure we read content from disk after extent state changed,
> as what we did before switching to iomap based dio.
/me thinks this looks ok, though I'd rather have Christoph's r-v-b since
it's his code... :)
--D
>
> Also introduce a new 'start' variable to save the original write
> offset (iomap_dio_complete() updates iocb->ki_pos), and a 'res'
> variable for invalidating caches result, cause we can't reuse 'ret'
> anymore.
>
> Signed-off-by: Eryu Guan <eguan@xxxxxxxxxx>
> ---
> fs/iomap.c | 23 +++++++++++++----------
> 1 file changed, 13 insertions(+), 10 deletions(-)
>
> diff --git a/fs/iomap.c b/fs/iomap.c
> index 0f85f24..d5e4ea0 100644
> --- a/fs/iomap.c
> +++ b/fs/iomap.c
> @@ -844,10 +844,12 @@ iomap_dio_rw(struct kiocb *iocb, struct iov_iter *iter,
> struct address_space *mapping = iocb->ki_filp->f_mapping;
> struct inode *inode = file_inode(iocb->ki_filp);
> size_t count = iov_iter_count(iter);
> - loff_t pos = iocb->ki_pos, end = iocb->ki_pos + count - 1, ret = 0;
> + loff_t pos = iocb->ki_pos, start = pos;
> + loff_t end = iocb->ki_pos + count - 1, ret = 0;
> unsigned int flags = IOMAP_DIRECT;
> struct blk_plug plug;
> struct iomap_dio *dio;
> + int res;
>
> lockdep_assert_held(&inode->i_rwsem);
>
> @@ -885,14 +887,13 @@ iomap_dio_rw(struct kiocb *iocb, struct iov_iter *iter,
> }
>
> if (mapping->nrpages) {
> - ret = filemap_write_and_wait_range(mapping, iocb->ki_pos, end);
> + ret = filemap_write_and_wait_range(mapping, start, end);
> if (ret)
> goto out_free_dio;
>
> - ret = invalidate_inode_pages2_range(mapping,
> - iocb->ki_pos >> PAGE_SHIFT, end >> PAGE_SHIFT);
> - WARN_ON_ONCE(ret);
> - ret = 0;
> + res = invalidate_inode_pages2_range(mapping,
> + start >> PAGE_SHIFT, end >> PAGE_SHIFT);
> + WARN_ON_ONCE(res);
> }
>
> inode_dio_begin(inode);
> @@ -939,6 +940,8 @@ iomap_dio_rw(struct kiocb *iocb, struct iov_iter *iter,
> __set_current_state(TASK_RUNNING);
> }
>
> + ret = iomap_dio_complete(dio);
> +
> /*
> * Try again to invalidate clean pages which might have been cached by
> * non-direct readahead, or faulted in by get_user_pages() if the source
> @@ -947,12 +950,12 @@ iomap_dio_rw(struct kiocb *iocb, struct iov_iter *iter,
> * this invalidation fails, tough, the write still worked...
> */
> if (iov_iter_rw(iter) == WRITE && mapping->nrpages) {
> - ret = invalidate_inode_pages2_range(mapping,
> - iocb->ki_pos >> PAGE_SHIFT, end >> PAGE_SHIFT);
> - WARN_ON_ONCE(ret);
> + res = invalidate_inode_pages2_range(mapping,
> + start >> PAGE_SHIFT, end >> PAGE_SHIFT);
> + WARN_ON_ONCE(res);
> }
>
> - return iomap_dio_complete(dio);
> + return ret;
>
> out_free_dio:
> kfree(dio);
> --
> 2.9.3
>
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