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Date:   Fri, 7 Jan 2022 15:53:03 +0000
From:   Matthew Wilcox <willy@infradead.org>
To:     Hugh Dickins <hughd@google.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Christoph Hellwig <hch@infradead.org>, Jan Kara <jack@suse.cz>,
        William Kucharski <william.kucharski@oracle.com>,
        linux-fsdevel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH next 2/3] shmem: Fix data loss when folio truncated
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On Sun, Jan 02, 2022 at 05:34:05PM -0800, Hugh Dickins wrote:
> xfstests generic 098 214 263 286 412 used to pass on huge tmpfs (well,
> three of those _require_odirect, enabled by a shmem_direct_IO() stub),
> but still fail even with the partial_end fix.
> 
> generic/098 output mismatch shows actual data loss:
>     --- tests/generic/098.out
>     +++ /home/hughd/xfstests/results//generic/098.out.bad
>     @@ -4,9 +4,7 @@
>      wrote 32768/32768 bytes at offset 262144
>      XXX Bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
>      File content after remount:
>     -0000000 aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa
>     -*
>     -0400000 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>     +0000000 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>     ...

generic/098 is passing for me ;-(  I'm using 'always' for THPs.
I'll have to try harder.  Regardless, I think your fix is good ...

> +static struct folio *shmem_get_partial_folio(struct inode *inode, pgoff_t index)

Love the better calling convention.

> +	folio = __filemap_get_folio(inode->i_mapping, index,
> +					FGP_ENTRY | FGP_LOCK, 0);
> +	if (!folio || !xa_is_value(folio))
> +		return folio;

That first '!folio' is redundant.  xa_is_value(NULL) is false.