Re: [PATCH v14 124/138] fs: Convert vfs_dedupe_file_range_compare to folios

Matthew Wilcox
2021-07-15T22:20:50Z

On Thu, Jul 15, 2021 at 03:08:40PM -0700, Darrick J. Wong wrote:
> On Thu, Jul 15, 2021 at 04:36:50AM +0100, Matthew Wilcox (Oracle) wrote:
> > We still only operate on a single page of data at a time due to using
> > kmap(). A more complex implementation would work on each page in a folio,
> > but it's not clear that such a complex implementation would be worthwhile.
>
> Does this break up a compound folio into smaller pages?

No. We just operate on each page in turn. Splitting a folio is an
expensive and unrealiable thing to do, so we avoid it unless necessary.

> > +/* Unlock two folios, being careful not to unlock the same folio twice. */
> > +static void vfs_unlock_two_folios(struct folio *folio1, struct folio *folio2)
> > {
> > - unlock_page(page1);
> > - if (page1 != page2)
> > - unlock_page(page2);
> > + folio_unlock(folio1);
> > + if (folio1 != folio2)
> > + folio_unlock(folio2);
>
> This could result in a lot of folio lock cycling. Do you think it's
> worth the effort to minimize this by keeping the folio locked if the
> next page is going to be from the same one?

I think that might well be a worthwhile optimisation. I'd like to do
that as a separate patch, though (and maybe somebody other than me could
do it ;-)