On Wed, Nov 17, 2021 at 02:31:26PM +0000, Matthew Wilcox wrote:

On Tue, Nov 16, 2021 at 08:31:27PM -0800, Darrick J. Wong wrote:

quoted

quoted

@@ -764,16 +761,17 @@ static loff_t iomap_write_iter(struct iomap_iter *iter, struct iov_iter *i)
 			break;
 		}
 
-		status = iomap_write_begin(iter, pos, bytes, &page);
+		status = iomap_write_begin(iter, pos, bytes, &folio);
 		if (unlikely(status))
 			break;
 
+		page = folio_file_page(folio, pos >> PAGE_SHIFT);
 		if (mapping_writably_mapped(iter->inode->i_mapping))
 			flush_dcache_page(page);
 
 		copied = copy_page_from_iter_atomic(page, offset, bytes, i);

Hrmm.  In principle (or I guess even a subsequent patch), if we had
multi-page folios, could we simply loop the pages in the folio instead
of doing a single page and then calling back into iomap_write_begin to
get (probably) the same folio?

This looks like a fairly straightforward conversion, but I was wondering
about that one little point...

Theoretically, yes, we should be able to do that.  But all of this code
is pretty subtle ("What if we hit a page fault?  What if we're writing
to part of this folio from an mmap of a different part of this folio?
What if it's !Uptodate?  What if we hit this weird ARM super-mprotect
memory tag thing?  What if ...") and, frankly, I got scared.  So I've
left that as future work; someone else can try to wrap their brain around
all of this.

<nod> That's roughly the same conclusion I came to -- conceptually we
could keep walking pages until we hit /any/ problem or other difference
with the first page that we don't feel like dealing with, and pass that
count to iomap_end... but no need to try that right this second.

Just checking that I grokked what's going on in this series. :)

--D