Re: [PATCH v2 20/28] iomap: Convert iomap_write_begin() and iomap_write_end() to folios

[Matthew Wilcox]

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