Re: [PATCH 3/3] iomap: fall back to buffered writes for invalidation failures

Ritesh Harjani
2020-07-31T07:01:33Z

On 7/22/20 12:01 AM, Christoph Hellwig wrote:
> Failing to invalid the page cache means data in incoherent, which is
> a very bad state for the system. Always fall back to buffered I/O
> through the page cache if we can't invalidate mappings.
>
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> Acked-by: Dave Chinner <dchinner@redhat.com>
> Reviewed-by: Goldwyn Rodrigues <rgoldwyn@suse.com>

Reviewed-by: Ritesh Harjani <riteshh@linux.ibm.com>
mainly for ext4 part.

> ---
> fs/ext4/file.c | 2 ++
> fs/gfs2/file.c | 3 ++-
> fs/iomap/direct-io.c | 16 +++++++++++-----
> fs/iomap/trace.h | 1 +
> fs/xfs/xfs_file.c | 4 ++--
> fs/zonefs/super.c | 7 +++++--
> 6 files changed, 23 insertions(+), 10 deletions(-)
>
> diff --git a/fs/ext4/file.c b/fs/ext4/file.c
> index 2a01e31a032c4c..129cc1dd6b7952 100644
> --- a/fs/ext4/file.c
> +++ b/fs/ext4/file.c
> @@ -544,6 +544,8 @@ static ssize_t ext4_dio_write_iter(struct kiocb *iocb, struct iov_iter *from)
> iomap_ops = &ext4_iomap_overwrite_ops;
> ret = iomap_dio_rw(iocb, from, iomap_ops, &ext4_dio_write_ops,
> is_sync_kiocb(iocb) || unaligned_io || extend);
> + if (ret == -ENOTBLK)
> + ret = 0;
>
> if (extend)
> ret = ext4_handle_inode_extension(inode, offset, ret, count);
> diff --git a/fs/gfs2/file.c b/fs/gfs2/file.c
> index bebde537ac8cf2..b085a3bea4f0fd 100644
> --- a/fs/gfs2/file.c
> +++ b/fs/gfs2/file.c
> @@ -835,7 +835,8 @@ static ssize_t gfs2_file_direct_write(struct kiocb *iocb, struct iov_iter *from)
>
> ret = iomap_dio_rw(iocb, from, &gfs2_iomap_ops, NULL,
> is_sync_kiocb(iocb));
> -
> + if (ret == -ENOTBLK)
> + ret = 0;
> out:
> gfs2_glock_dq(&gh);
> out_uninit:
> diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
> index 190967e87b69e4..c1aafb2ab99072 100644
> --- a/fs/iomap/direct-io.c
> +++ b/fs/iomap/direct-io.c
> @@ -10,6 +10,7 @@
> #include <linux/backing-dev.h>
> #include <linux/uio.h>
> #include <linux/task_io_accounting_ops.h>
> +#include "trace.h"
>
> #include "../internal.h"
>
> @@ -401,6 +402,9 @@ iomap_dio_actor(struct inode *inode, loff_t pos, loff_t length,
> * can be mapped into multiple disjoint IOs and only a subset of the IOs issued
> * may be pure data writes. In that case, we still need to do a full data sync
> * completion.
> + *
> + * Returns -ENOTBLK In case of a page invalidation invalidation failure for
> + * writes. The callers needs to fall back to buffered I/O in this case.
> */
> ssize_t
> iomap_dio_rw(struct kiocb *iocb, struct iov_iter *iter,
> @@ -478,13 +482,15 @@ iomap_dio_rw(struct kiocb *iocb, struct iov_iter *iter,
> if (iov_iter_rw(iter) == WRITE) {
> /*
> * Try to invalidate cache pages for the range we are writing.
> - * If this invalidation fails, tough, the write will still work,
> - * but racing two incompatible write paths is a pretty crazy
> - * thing to do, so we don't support it 100%.
> + * If this invalidation fails, let the caller fall back to
> + * buffered I/O.
> */
> if (invalidate_inode_pages2_range(mapping, pos >> PAGE_SHIFT,
> - end >> PAGE_SHIFT))
> - dio_warn_stale_pagecache(iocb->ki_filp);
> + end >> PAGE_SHIFT)) {
> + trace_iomap_dio_invalidate_fail(inode, pos, count);
> + ret = -ENOTBLK;
> + goto out_free_dio;
> + }
>
> if (!wait_for_completion && !inode->i_sb->s_dio_done_wq) {
> ret = sb_init_dio_done_wq(inode->i_sb);

Just as a note. So if the driver returns -ENOTBLK (from ->iomap_end)then
iomap considers it as a magic value to fall back to buffered-io and it
changes ret=0 And now with this patch, iomap could also return
-ENOTBLK if it gets an error while doing above operation and so the
driver is free to consider this as a fallback mechanism to buffered-io.