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Subject: Re: [PATCH 3/3] iomap: fall back to buffered writes for invalidation
 failures
To:     Christoph Hellwig <hch@lst.de>, Dave Chinner <david@fromorbit.com>,
        Goldwyn Rodrigues <rgoldwyn@suse.de>
Cc:     Damien Le Moal <damien.lemoal@wdc.com>,
        Naohiro Aota <naohiro.aota@wdc.com>,
        Johannes Thumshirn <jth@kernel.org>,
        Matthew Wilcox <willy@infradead.org>,
        linux-btrfs@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        cluster-devel@redhat.com, linux-ext4@vger.kernel.org,
        linux-xfs@vger.kernel.org, Dave Chinner <dchinner@redhat.com>,
        Goldwyn Rodrigues <rgoldwyn@suse.com>
References: <20200721183157.202276-1-hch@lst.de>
 <20200721183157.202276-4-hch@lst.de>
From:   Ritesh Harjani <riteshh@linux.ibm.com>
Date:   Fri, 31 Jul 2020 12:31:08 +0530
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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.