Re: [PATCH v1 4/8] iomap: add writepages support for IOMAP_IN_MEM iomaps

[Christoph Hellwig <hch@infradead.org>]

On Mon, Jun 09, 2025 at 09:57:41AM -0700, Darrick J. Wong wrote:
> > It might also be worth stating what you don't use.  One big thing
> > that springs to mind is ioends.  Which are really useful if you
> > need more than one request to handle a folio, something that is
> > pretty common in network file systems.  I guess you don't need
> > that for fuse?
> 
> My initial thought was "I wonder if Joanne would be better off with a
> totally separate iomap_writepage_map_blocks implementation"

I think that's basically what the patches do, right?


> since I
> *think* fuse just needs a callback from iomap to initiate FUSE_WRITE
> calls on the dirty range(s) of a folio, and then fuse can call
> mapping_set_error and iomap_finish_folio_write when those FUSE_WRITE
> calls complete.  There are no bios, so I don't see much point in using
> the ioend machinery.

Note that the mapping_set_error in iomap_writepage_map is only for
synchronous errors for mapping setup anyway, all the actual I/O error
are handled asynchronously anyway.  Similar, clearing the writeback
bit only happens for synchronous erorr, or the rare case of (almost)
synchronous I/O.