Re: [RFC PATCH 3/3] fb_defio: do not use deprecated page->mapping, index fields

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Wed, Jan 08, 2025 at 05:32:54PM +0000, Matthew Wilcox wrote:
> On Wed, Jan 08, 2025 at 04:18:42PM +0000, Lorenzo Stoakes wrote:
> > @@ -280,7 +269,10 @@ static void fb_deferred_io_work(struct work_struct *work)
> >  		struct folio *folio = page_folio(pageref->page);
> >
> >  		folio_lock(folio);
> > -		folio_mkclean(folio);
> > +		rmap_wrprotect_file_page(fbdefio->mapping,
> > +					 pageref->offset >> PAGE_SHIFT,
> > +					 compound_nr(pageref->page),
> > +					 page_to_pfn(pageref->page));
> >  		folio_unlock(folio);
>
> Why do we need to lock the folio?  (since this isn't necessarily a
> folio)  Also, do we need compound_nr() here?  I _think_ for defio,
> the number of pages allocated per object are fixed, so this should be
> an fbdefio->nr_pages field?

I'm trying to keep the code as similar as possible to the way it was before,
even if there are questionable parts.

There is a comment about some timing issue around the locks and so there appears
to be an assumption about that.

As to compound_nr(), we're not write protecting everything, just each invidiual
page in the list that needs it, so we only want to do one at a time. I strongly
suspect it's a single base page each time, but for belts + braces I'm doing
compound_nr().

See below, this is wrong, it should just be '1'.

So this is iterating through a list of pagerefs that can be in any random order.

>
> (something that's always troubled me about compound_nr() is that it
> returns 1 for tail pages and the number you actually expect for head
> pages)
>

OK I changed this from '1' to compound_nr() out of an (apparently) abundance of
caution, but I was wrong:

npagerefs = DIV_ROUND_UP(info->fix.smem_len, PAGE_SIZE);

There are page refs for each PAGE_SIZE (i.e. base page size), so there is no way
anything is compound.

Will switch this to 1.