Re: [PATCH 2/12] mm: let swap use exceptional entries

[Hugh Dickins]

On Sat, 18 Jun 2011, Andrew Morton wrote:
> On Tue, 14 Jun 2011 03:43:47 -0700 (PDT) Hugh Dickins <hughd@xxxxxxxxxx> wrote:
> 
> > --- linux.orig/mm/filemap.c	2011-06-13 13:26:44.430284135 -0700
> > +++ linux/mm/filemap.c	2011-06-13 13:27:34.526532556 -0700
> > @@ -717,9 +717,12 @@ repeat:
> >  		page = radix_tree_deref_slot(pagep);
> >  		if (unlikely(!page))
> >  			goto out;
> > -		if (radix_tree_deref_retry(page))
> > +		if (radix_tree_exception(page)) {
> > +			if (radix_tree_exceptional_entry(page))
> > +				goto out;
> > +			/* radix_tree_deref_retry(page) */
> >  			goto repeat;
> > -
> > +		}
> >  		if (!page_cache_get_speculative(page))
> >  			goto repeat;
> 
> All the crap^Wnice changes made to filemap.c really need some comments,
> please.  Particularly when they're keyed off the bland-sounding
> "radix_tree_exception()".  Apparently they have something to do with
> swap, but how is the poor reader to know this?

The naming was intentionally bland, because other filesystems might
in future have other uses for such exceptional entries.

(I think the field size would generally defeat it, but you can,
for example, imagine a small filesystem wanting to save sector number
there when a page is evicted.)

But let's go bland when it's more familiar, and such uses materialize -
particularly since I only placed those checks in places where they're
needed now for shmem/tmpfs/swap.

I'll keep the bland naming, if that's okay, but send a patch adding
a line of comment in such places.  Mentioning shmem, tmpfs, swap.

> 
> Also, commenting out a function call might be meaningful information for
> Hugh-right-now, but for other people later on, they're just a big WTF.

Ah yes, I hadn't realized at all that those look like commented-out
function calls.  No, they're comments on what the else case is that
we have arrived at there.  I'll make those clearer too.

Hugh
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