Re: [PATCH 2/2] iomap: make zero range flush conditional on unwritten mappings

[Christoph Hellwig <hch@infradead.org>]

On Thu, Aug 22, 2024 at 10:59:10AM -0400, Brian Foster wrote:
> Note that we also flush for hole mappings because iomap_zero_range()
> is used for partial folio zeroing in some cases. For example, if a
> folio straddles EOF on a sub-page FSB size fs, the post-eof portion
> is hole-backed and dirtied/written via mapped write, and then i_size
> increases before writeback can occur (which otherwise zeroes the
> post-eof portion of the EOF folio), then the folio becomes
> inconsistent with disk until reclaimed.

Eww.  I'm not sure iomap_zero_range is the right way to handle this
even if that's what we have now and it kinda work.

> +	/*
> +	 * We can skip pre-zeroed mappings so long as either the mapping was
> +	 * clean before we started or we've flushed at least once since.
> +	 * Otherwise we don't know whether the current mapping had dirty
> +	 * pagecache, so flush it now, stale the current mapping, and proceed
> +	 * from there.
> +	 */
> +	if (srcmap->type == IOMAP_HOLE || srcmap->type == IOMAP_UNWRITTEN) {

.. at very least the above needs to be documented here as a big fat
reminder, though.

Otherwise the series looks sensible to me.