Re: [PATCH v2 7/7] iomap: Copy larger chunks from userspace

[Matthew Wilcox <willy@infradead.org>]

On Sun, Jun 04, 2023 at 11:29:52AM -0700, Darrick J. Wong wrote:
> On Fri, Jun 02, 2023 at 11:24:44PM +0100, Matthew Wilcox (Oracle) wrote:
> > -		copied = copy_page_from_iter_atomic(page, offset, bytes, i);
> > +		copied = copy_page_from_iter_atomic(&folio->page, offset, bytes, i);
> 
> I think I've gotten lost in the weeds.  Does copy_page_from_iter_atomic
> actually know how to deal with a multipage folio?  AFAICT it takes a
> page, kmaps it, and copies @bytes starting at @offset in the page.  If
> a caller feeds it a multipage folio, does that all work correctly?  Or
> will the pagecache split multipage folios as needed to make it work
> right?

It's a smidgen inefficient, but it does work.  First, it calls
page_copy_sane() to check that offset & n fit within the compound page
(ie this all predates folios).

... Oh.  copy_page_from_iter() handles this correctly.
copy_page_from_iter_atomic() doesn't.  I'll have to fix this
first.  Looks like Al fixed copy_page_from_iter() in c03f05f183cd
and didn't fix copy_page_from_iter_atomic().

> If we create a 64k folio at pos 0 and then want to write a byte at pos
> 40k, does __filemap_get_folio break up the 64k folio so that the folio
> returned by iomap_get_folio starts at 40k?  Or can the iter code handle
> jumping ten pages into a 16-page folio and I just can't see it?

Well ... it handles it fine unless it's highmem.  p is kaddr + offset,
so if offset is 40k, it works correctly on !highmem.