Re: [PATCH v2 4/5] mm: Introduce Hinted pages

[Alexander Duyck]

On Thu, Jul 25, 2019 at 1:53 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> On 24.07.19 19:03, Alexander Duyck wrote:
> > From: Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx>

<snip>

> >  /*
> > + * PageHinted() is an alias for Offline, however it is not meant to be an
> > + * exclusive value. It should be combined with PageBuddy() when seen as it
> > + * is meant to indicate that the page has been scrubbed while waiting in
> > + * the buddy system.
> > + */
> > +PAGE_TYPE_OPS(Hinted, offline)
>
>
> CCing Matthew
>
> I am still not sure if I like the idea of having two page types at a time.
>
> 1. Once we run out of page type bits (which can happen easily looking at
> it getting more and more user - e.g., maybe for vmmap pages soon), we
> might want to convert again back to a value-based, not bit-based type
> detection. This will certainly make this switch harder.

Shouldn't we wait to cross that bridge until we get there? It wouldn't
take much to look at either defining the buddy as 2 types for such a
case, or if needed we could then look at the option of moving over to
another bit.

> 2. It will complicate the kexec/kdump handling. I assume it can be fixed
> some way - e.g., making the elf interface aware of the exact notion of
> page type bits compared to mapcount values we have right now (e.g.,
> PAGE_BUDDY_MAPCOUNT_VALUE). Not addressed in this series yet.

It does, but not by much. We were already exposing both the buddy and
offline values. The cahnge could probably be in the executable that
are accessing the interface to allow the combination of buddy and
offline. That is one of the advantages of using the "offline" value to
also mean hinted since then "hinted" is just a combination of the two
known values.

> Can't we reuse one of the traditional page flags for that, not used
> along with buddy pages? E.g., PG_dirty: Pages that were not hinted yet
> are dirty.

Reusing something like the dirty bit would just be confusing in my
opinion. In addition it looks like Xen has also re-purposed PG_dirty
already for another purpose.

If anything I could probably look at seeing if the PG_private flags
are available when a page is in the buddy allocator which I suspect
they probably are since the only users I currently see appear to be
SLOB and compound pages. Either that or maybe something like PG_head
might make sense since once we start allocating them we are popping
the head off of the boundary list.