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

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



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





[Index of Archives]     [Kernel Newbies]     [Security]     [Netfilter]     [Bugtraq]     [Linux FS]     [Yosemite Forum]     [MIPS Linux]     [ARM Linux]     [Linux Security]     [Linux RAID]     [Samba]     [Video 4 Linux]     [Device Mapper]     [Linux Resources]

  Powered by Linux