Re: [PATCH v6 00/33] Split ptdesc from struct page

[David Hildenbrand <david@xxxxxxxxxx>]

On 27.06.23 22:13, Hugh Dickins wrote:
> On Tue, 27 Jun 2023, David Hildenbrand wrote:
> > On 27.06.23 06:44, Hugh Dickins wrote:
> > > On Mon, 26 Jun 2023, Vishal Moola (Oracle) wrote:
> > >
> > > > The MM subsystem is trying to shrink struct page. This patchset
> > > > introduces a memory descriptor for page table tracking - struct ptdesc.
> > > ...
> > > >    39 files changed, 686 insertions(+), 455 deletions(-)
> > > I don't see the point of this patchset: to me it is just obfuscation of
> > > the present-day tight relationship between page table and struct page.
> > >
> > > Matthew already explained:
> > >
> > > > The intent is to get ptdescs to be dynamically allocated at some point
> > > > in the ~2-3 years out future when we have finished the folio project ...
> > > So in a kindly mood, I'd say that this patchset is ahead of its time.
> > > But I can certainly adapt to it, if everyone else sees some point to it.
> > I share your thoughts, that code churn which will help eventually in the far,
> > far future (not wanting to sound too pessimistic, but it's not going to be
> > there tomorrow ;) ).
> >
> > However, if it's just the same as the other conversions we already did (e.g.,
> > struct slab), then I guess there is no reason to stop now -- the obfuscation
> > already happened.
> >
> > ... or is there a difference regarding this conversion and the previous ones?
> I was aware of the struct slab thing, didn't see much point there myself
> either; but it was welcomed by Vlastimil, and barely affected outside of
> slab allocators, so I had no reason to object.
>
> You think that if a little unnecessary churn (a *lot* of churn if you
> include folios, which did save some repeated calls to compound_head())
> has already occurred, that's a good precedent for allowing more and more?
Well, if you phrase it like that ... no, I'm not in favor of unnecessary 
churn at all. Yes, folios were/are a lot of churn (IMHO not unnecessary, 
though).
I'm not a friend of these "overlays"; it all only really makes sense to 
me once we actually allocate the descriptors dynamically. Maybe some of 
the existing/ongoing conversions were different (that's why I was asking 
for the difference, as you said the "struct slab" thing was well received).
If they are primarily only unnecessary churn for now (and unclear 
when/how it will become useful), I share your opinion.
--
Cheers,

David / dhildenb