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From: Matthew Wilcox <[email protected]>
To: Stefan Roesch <[email protected]>
Cc: [email protected], [email protected], [email protected],
	[email protected], [email protected],
	[email protected], [email protected], [email protected],
	[email protected]
Subject: Re: [PATCH v8 06/14] iomap: Return -EAGAIN from iomap_write_iter()
Date: Wed, 8 Jun 2022 20:02:39 +0100	[thread overview]
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>

On Wed, Jun 08, 2022 at 10:17:33AM -0700, Stefan Roesch wrote:
> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> index b06a5c24a4db..f701dcb7c26a 100644
> --- a/fs/iomap/buffered-io.c
> +++ b/fs/iomap/buffered-io.c
> @@ -829,7 +829,13 @@ static loff_t iomap_write_iter(struct iomap_iter *iter, struct iov_iter *i)
>  		length -= status;
>  	} while (iov_iter_count(i) && length);
>  
> -	return written ? written : status;
> +	if (status == -EAGAIN) {
> +		iov_iter_revert(i, written);
> +		return -EAGAIN;
> +	}
> +	if (written)
> +		return written;
> +	return status;
>  }

I still don't understand how this can possibly work.  Walk me through it.

Let's imagine we have a file laid out such that extent 1 is bytes
0-4095 of the file and extent 2 is extent 4096-16385 of the file.
We do a write of 5000 bytes starting at offset 4000 of the file.

iomap_iter() tells us about the first extent and we write the first
96 bytes of our data to the first extent, returning 96.  iomap_iter()
tells us about the second extent, and we write the next 4000 bytes to
the second extent.  Then we get a page fault and get to the -EAGAIN case.
We rewind the iter 4000 bytes.

How do we not end up writing garbage when the kworker does the retry?
I'd understand if we rewound the iter all the way to the start.  Or if
we didn't rewind the iter at all and were able to pick up partway through
the write.  But rewinding to the start of the extent feels like it can't
possibly work.

  reply	other threads:[~2022-06-08 19:06 UTC|newest]

Thread overview: 23+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-06-08 17:17 [PATCH v8 00/14] io-uring/xfs: support async buffered writes Stefan Roesch
2022-06-08 17:17 ` [PATCH v8 01/14] mm: Move starting of background writeback into the main balancing loop Stefan Roesch
2022-06-08 17:17 ` [PATCH v8 02/14] mm: Move updates of dirty_exceeded into one place Stefan Roesch
2022-06-08 17:17 ` [PATCH v8 03/14] mm: Add balance_dirty_pages_ratelimited_flags() function Stefan Roesch
2022-06-08 18:44   ` Matthew Wilcox
2022-06-08 17:17 ` [PATCH v8 04/14] iomap: Add flags parameter to iomap_page_create() Stefan Roesch
2022-06-08 18:51   ` Matthew Wilcox
2022-06-08 17:17 ` [PATCH v8 05/14] iomap: Add async buffered write support Stefan Roesch
2022-06-08 17:17 ` [PATCH v8 06/14] iomap: Return -EAGAIN from iomap_write_iter() Stefan Roesch
2022-06-08 19:02   ` Matthew Wilcox [this message]
2022-06-09 18:49     ` Stefan Roesch
2022-06-08 17:17 ` [PATCH v8 07/14] fs: Add check for async buffered writes to generic_write_checks Stefan Roesch
2022-06-10 11:50   ` Christian Brauner
2022-06-08 17:17 ` [PATCH v8 08/14] fs: add __remove_file_privs() with flags parameter Stefan Roesch
2022-06-10 11:53   ` Christian Brauner
2022-06-08 17:17 ` [PATCH v8 09/14] fs: Split off inode_needs_update_time and __file_update_time Stefan Roesch
2022-06-10 11:55   ` Christian Brauner
2022-06-08 17:17 ` [PATCH v8 10/14] fs: Add async write file modification handling Stefan Roesch
2022-06-10 12:38   ` Christian Brauner
2022-06-08 17:17 ` [PATCH v8 11/14] io_uring: Add support for async buffered writes Stefan Roesch
2022-06-08 17:17 ` [PATCH v8 12/14] io_uring: Add tracepoint for short writes Stefan Roesch
2022-06-08 17:17 ` [PATCH v8 13/14] xfs: Specify lockmode when calling xfs_ilock_for_iomap() Stefan Roesch
2022-06-08 17:17 ` [PATCH v8 14/14] xfs: Add async buffered write support Stefan Roesch

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