Re: [PATCH] iomap: turn the byte variable in iomap_zero_iter into a ssize_t

[Matthew Wilcox <willy@infradead.org>]

On Wed, Dec 08, 2021 at 10:12:03AM +0100, Christoph Hellwig wrote:
> bytes also hold the return value from iomap_write_end, which can contain
> a negative error value.  As bytes is always less than the page size even
> the signed type can hold the entire possible range.

iomap_write_end() can't return an errno.  I went through and checked as
part of the folio conversion.  It actually has two return values -- 0
on error and 'len' on success.  And it can't have an error because
that only occurs if 'copied' is less than 'length'.

So I think this should actually be:

-               bytes = iomap_write_end(iter, pos, bytes, bytes, folio);
-               if (bytes < 0)
-                       return bytes;
+               status = iomap_write_end(iter, pos, bytes, bytes, folio);
+               if (WARN_ON_ONCE(status == 0))
+                       return -EIO;

just like its counterpart loop in iomap_unshare_iter()

(ok this won't apply to Dan's tree, but YKWIM)