On Tue, Mar 23, 2021 at 04:30:27PM +0000, Matthew Wilcox wrote:
> On Tue, Mar 23, 2021 at 03:41:25PM +0000, Christoph Hellwig wrote:
> > On Mon, Mar 22, 2021 at 01:19:07AM +0000, Matthew Wilcox wrote:
> > > I'd like to get it down to zero. After all, the !mapping case in
> > > set_page_dirty() is exactly what we want. So is there a problem
> > > with doing this?
> > >
> > > +++ b/mm/page-writeback.c
> > > @@ -2562 +2562 @@ int set_page_dirty(struct page *page)
> > > - if (likely(mapping)) {
> > > + if (likely(mapping && mapping_can_writeback(mapping))) {
> > >
> > > But then I noticed that we have both mapping_can_writeback()
> > > and mapping_use_writeback_tags(), and I'm no longer sure
> > > which one to use. Also, why don't we mirror the results of
> > > inode_to_bdi(mapping->host)->capabilities & BDI_CAP_WRITEBACK into
> > > a mapping->flags & AS_something bit?
> >
> > Probably because no one has bothered to submit a patch yet.
>
> I was hoping for a little more guidance. Are mapping_can_writeback()
> and mapping_use_writeback_tags() really the same thing? I mean,
> obviously the swap spaces actually _can_ writeback, but it doesn't
> use the tags to do it.
Have you looked at the commit adding mapping_use_writeback_tags? It
pretty clearly documents that as of that commit the swap cache does not
use writeback tags and why.
