Re: [PATCH 4/8] iomap: split bios to zone append limits in the submission handlers

[Christoph Hellwig <hch@lst.de>]

On Thu, Dec 12, 2024 at 11:51:49AM -0800, Darrick J. Wong wrote:
> > +struct iomap_ioend *iomap_split_ioend(struct iomap_ioend *ioend, bool is_append,
> 
> Can you determine is_append from (ioend->io_flags & ZONE_APPEND)?

That would require us to add that flag first :)  As we don't really
need that as persistent per-iomap that it's probably not worth it.

> Also it's not clear to me what the initial and output state of
> *alloc_len is supposed to be?  I guess you set it to the number of bytes
> the @ioend covers?

It gets set to the number of blocks that the allocator could find,
and iomap_split_ioend decrements the amount of that it used for the
ioend returned, which is min(*alloc_len, max_zone_append_sectors) for
sequential zones, or *alloc_len for conventional zones.

> > +++ b/include/linux/iomap.h
> > @@ -354,6 +354,9 @@ struct iomap_ioend {
> >  	struct list_head	io_list;	/* next ioend in chain */
> >  	u16			io_flags;	/* IOMAP_IOEND_* */
> >  	struct inode		*io_inode;	/* file being written to */
> > +	atomic_t		io_remaining;	/* completetion defer count */
> > +	int			io_error;	/* stashed away status */
> > +	struct iomap_ioend	*io_parent;	/* parent for completions */
> 
> I guess this means ioends can chain together, sort of like how bios can
> when you split them?

Exactly.