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
