Re: [PATCH 2/2] iomap: fall back to buffered writes for invalidation failures

Damien Le Moal
2020-07-14T01:41:39Z

On 2020/07/13 16:51, Christoph Hellwig wrote:
> Failing to invalid the page cache means data in incoherent, which is

s/in incoherent/is incoherent

> 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>
> ---
> fs/ext4/file.c | 2 ++
> fs/gfs2/file.c | 3 ++-
> fs/iomap/direct-io.c | 13 ++++++++-----
> fs/iomap/trace.h | 1 +
> fs/xfs/xfs_file.c | 4 ++--
> fs/zonefs/super.c | 7 +++++--
> 6 files changed, 20 insertions(+), 10 deletions(-)
>
> diff --git a/fs/ext4/file.c b/fs/ext4/file.c
> index 2a01e31a032c4c..0da6c2a2c32c1e 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 == -EREMCHG)
> + 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 fe305e4bfd3734..c7907d40c61d17 100644
> --- a/fs/gfs2/file.c
> +++ b/fs/gfs2/file.c
> @@ -814,7 +814,8 @@ 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 == -EREMCHG)
> + 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..62626235cdbe8d 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"
>
> @@ -478,13 +479,15 @@ 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 = -EREMCHG;

I am wondering if it is OK to unconditionally always return -EREMCHG here.
Shouldn't this depend on the return code of invalidate_inode_pages2_range() ?
ret may be the value returned by mapping->a_ops->launder_page(page) instead of
-EBUSY that invalidate_inode_pages2_range() otherwise returns for a failed
invalidation. Isn't their any error condition that would be better served by not
forcing the fallback to buffered write ? E.g. -ENOMEM ?

> + goto out_free_dio;
> + }
>
> if (!wait_for_completion && !inode->i_sb->s_dio_done_wq) {
> ret = sb_init_dio_done_wq(inode->i_sb);
> diff --git a/fs/iomap/trace.h b/fs/iomap/trace.h
> index 5693a39d52fb63..fdc7ae388476f5 100644
> --- a/fs/iomap/trace.h
> +++ b/fs/iomap/trace.h
> @@ -74,6 +74,7 @@ DEFINE_EVENT(iomap_range_class, name, \
> DEFINE_RANGE_EVENT(iomap_writepage);
> DEFINE_RANGE_EVENT(iomap_releasepage);
> DEFINE_RANGE_EVENT(iomap_invalidatepage);
> +DEFINE_RANGE_EVENT(iomap_dio_invalidate_fail);
>
> #define IOMAP_TYPE_STRINGS \
> { IOMAP_HOLE, "HOLE" }, \
> diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
> index 00db81eac80d6c..551cca39fa3ba6 100644
> --- a/fs/xfs/xfs_file.c
> +++ b/fs/xfs/xfs_file.c
> @@ -553,8 +553,8 @@ xfs_file_dio_aio_write(
> xfs_iunlock(ip, iolock);
>
> /*
> - * No fallback to buffered IO on errors for XFS, direct IO will either
> - * complete fully or fail.
> + * No partial fallback to buffered IO on errors for XFS, direct IO will
> + * either complete fully or fail.
> */
> ASSERT(ret < 0 || ret == count);
> return ret;
> diff --git a/fs/zonefs/super.c b/fs/zonefs/super.c
> index 07bc42d62673ce..793850454b752f 100644
> --- a/fs/zonefs/super.c
> +++ b/fs/zonefs/super.c
> @@ -786,8 +786,11 @@ static ssize_t zonefs_file_write_iter(struct kiocb *iocb, struct iov_iter *from)
> if (iocb->ki_pos >= ZONEFS_I(inode)->i_max_size)
> return -EFBIG;
>
> - if (iocb->ki_flags & IOCB_DIRECT)
> - return zonefs_file_dio_write(iocb, from);
> + if (iocb->ki_flags & IOCB_DIRECT) {
> + ret = zonefs_file_dio_write(iocb, from);
> + if (ret != -EREMCHG)
> + return ret;
> + }

This looks fine. This would happen only for conventional zone writes since
sequential zone writes cannot ever issue direct IOs on top of cached data as
that would be a forbidden "overwrite" operation.

>
> return zonefs_file_buffered_write(iocb, from);
> }
>

--
Damien Le Moal
Western Digital Research