Re: [PATCH 07/13] huge_memory: Allow mappings of PUD sized pages

[David Hildenbrand]

On 02.07.24 12:19, Alistair Popple wrote:
> David Hildenbrand <david@xxxxxxxxxx> writes:
>
> > On 27.06.24 02:54, Alistair Popple wrote:
> > > Currently DAX folio/page reference counts are managed differently to
> > > normal pages. To allow these to be managed the same as normal pages
> > > introduce dax_insert_pfn_pud. This will map the entire PUD-sized folio
> > > and take references as it would for a normally mapped page.
> > > This is distinct from the current mechanism, vmf_insert_pfn_pud,
> > > which
> > > simply inserts a special devmap PUD entry into the page table without
> > > holding a reference to the page for the mapping.
> >
> > Do we really have to involve mapcounts/rmap for daxfs pages at this
> > point? Or is this only "to make it look more like other pages" ?
>
> The aim of the series is make FS DAX and other ZONE_DEVICE pages look
> like other pages, at least with regards to the way they are refcounted.
>
> At the moment they are not refcounted - instead their refcounts are
> basically statically initialised to one and there are all these special
> cases and functions requiring magic PTE bits (pXX_devmap) to do the
> special DAX reference counting. This then adds some cruft to manage
> pgmap references and to catch the 2->1 page refcount transition. All
> this just goes away if we manage the page references the same as other
> pages (and indeed we already manage DEVICE_PRIVATE and COHERENT pages
> the same as normal pages).
>
> So I think to make this work we at least need the mapcounts.

We only really need the mapcounts if we intend to do something like 
folio_mapcount() == folio_ref_count() to detect unexpected folio 
references, and if we have to have things like folio_mapped() working. 
For now that was not required, that's why I am asking.

Background also being that in a distant future folios will be decoupled 
more from other compound pages, and only folios (or "struct anon_folio" 
/ "struct file_folio") would even have mapcounts.

For example, most stuff we map (and refcount!) via vm_insert_page() 
really must stop involving mapcounts. These won't be "ordinary" 
mapcount-tracked folios in the future, they are simply some refcounted 
pages some ordinary driver allocated.

For FS-DAX, if we'll be using the same "struct file_folio" approach as 
for ordinary pageache memory, then this is the right thing to do here.


> > I'm asking this because:
> >
> > (A) We don't support mixing PUD+PMD mappings yet. I have plans to change
> >      that in the future, but for now you can only map using a single PUD
> >      or by PTEs. I suspect that's good enoug for now for dax fs?
>
> Yep, that's all we support.
>
> > (B) As long as we have subpage mapcounts, this prevents vmemmap
> >      optimizations [1]. Is that only used for device-dax for now and are
> >      there no plans to make use of that for fs-dax?
>
> I don't have any plans to. This is purely focussed on refcounting pages
> "like normal" so we can get rid of all the DAX special casing.
>
> > (C) We managed without so far :)
>
> Indeed, although Christoph has asked repeatedly ([1], [2] and likely
> others) that this gets fixed and I finally got sick of it coming up
> everytime I need to touch something with ZONE_DEVICE pages :)
>
> Also it removes the need for people to understand the special DAX page
> recounting scheme and ends up removing a bunch of cruft as a bonus:
>
>   59 files changed, 485 insertions(+), 869 deletions(-)

I'm not challenging the refcounting scheme. I'm purely asking about 
mapcount handling, which is something related but different.

> And that's before I clean up all the pgmap reference handling. It also
> removes the pXX_trans_huge and pXX_leaf distinction. So we managed, but
> things could be better IMHO.

Again, all nice things.

> > Having that said, with folio->_large_mapcount things like
> > folio_mapcount() are no longer terribly slow once we weould PTE-map a
> > PUD-sized folio.
> >
> > Also, all ZONE_DEVICE pages should currently be marked PG_reserved,
> > translating to "don't touch the memmap". I think we might want to
> > tackle that first.

Missed to add a pointer to [2].

> Ok. I'm keen to get this series finished and I don't quite get the
> connection here, what needs to change there?

include/linux/page-flags.h

"PG_reserved is set for special pages. The "struct page" of such a page 
should in general not be touched (e.g. set dirty) except by its owner. 
Pages marked as PG_reserved include:

...

- Device memory (e.g. PMEM, DAX, HMM)
"

I think we already entered that domain with other ZONE_DEVICE pages 
being returned from vm_normal_folio(), unfortunately. But that really 
must be cleaned up for these pages to not look special anymore.

Agreed that it likely is something that is not blocking this series.

--
Cheers,

David / dhildenb