linux-kernel - Re: [PATCH 2/3] afs: Fix afs_write

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Message-ID: <YMdZbsvBNYBtZDC2@casper.infradead.org>
Date:   Mon, 14 Jun 2021 14:28:14 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     David Howells <dhowells@...hat.com>
Cc:     jlayton@...nel.org, linux-afs@...ts.infradead.org,
        ceph-devel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] afs: Fix afs_write_end() to handle short writes

On Mon, Jun 14, 2021 at 02:20:25PM +0100, David Howells wrote:
> Fix afs_write_end() to correctly handle a short copy into the intended
> write region of the page.  Two things are necessary:
> 
>  (1) If the page is not up to date, then we should just return 0
>      (ie. indicating a zero-length copy).  The loop in
>      generic_perform_write() will go around again, possibly breaking up the
>      iterator into discrete chunks.

Does this actually work?  What about the situation where you're reading
the last page of a file and thus (almost) always reading fewer bytes
than a PAGE_SIZE?

>      This is analogous to commit b9de313cf05fe08fa59efaf19756ec5283af672a
>      for ceph.
> 
>  (2) The page should not have been set uptodate if it wasn't completely set
>      up by netfs_write_begin() (this will be fixed in the next page), so we
>      need to set PG_uptodate here in such a case.

s/page/patch/

and you have a really bad habit of breaking the layering and referring
to PG_foo.  Please just refer to PageFoo.  Filesystems shouldn't
care what the implementation of PageFoo is.

> +	len = min_t(size_t, len, thp_size(page) - from);
> +	if (!PageUptodate(page)) {
> +		if (copied < len) {
> +			copied = 0;
> +			goto out;
> +		}
> +
> +		SetPageUptodate(page);
> +	}