Date: Tue, 02 Jul 2024 21:30:19 +1000
From: Alistair Popple <apopple@...dia.com>
To: David Hildenbrand <david@...hat.com>
Cc: dan.j.williams@...el.com, vishal.l.verma@...el.com,
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Subject: Re: [PATCH 07/13] huge_memory: Allow mappings of PUD sized pages


David Hildenbrand <david@...hat.com> writes:

> On 02.07.24 12:19, Alistair Popple wrote:
>> David Hildenbrand <david@...hat.com> 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.

Oh I see, thanks for pointing that out. In that case I agree, we don't
need the mapcounts. As you say we don't currently need to detect
unexpect references for FS DAX and this series doesn't seek to introduce
any new behviour/features.

> 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.

Ok, so for FS DAX we should take a reference on the folio for the
mapping but not a mapcount?

> 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.

Got it, thanks. I hadn't quite picked up on the mapcount vs. refcount
distinction you were asking about.

>> 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.

Great. I'd like to see that cleaned up a little too or at least made
more understandable. The first time I looked at this it took a
suprising amount of time to figure out what constituted a "normal"
page so I'd be happy to help. This series does, in a small way, clean
that up by removing the pte_devmap special case.

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