Re: [PATCH] btrfs: fix unnecessary flush on close when truncating zero-sized files

[Dave Chen <davechen@synology.com>]

On Mon, Mar 23, 2026 at 03:07:48PM +0100, David Sterba wrote:
> Does this have a measurable impact? Truncating 0 -> 0 without any
> intermediate writes could happen but I kind of doubt it's worth
> optimizing. I'm not against adding the patch but would like to know if
> it's fixing some problem for you. Thanks.

Yes, this comes from a real workload. We have a backup service that
creates temporary files via mkstemp(), closes them, and later reopens
them with O_TRUNC for writing. The O_TRUNC is defensive -- the file is
always empty at that point, but the temp file creation and usage are in
separate components, so removing it from userspace isn't straightforward.

This pattern repeats for a large number of files per backup job, and
each close() triggers an unnecessary filemap_flush().

When oldsize is already 0, the flag provides no protection -- there is
no data being truncated away, so the filemap_flush() on close serves
no purpose. The fix simply makes the condition match what the comment
already describes.

Thanks.

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