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.
