Re: [PATCH 21/46] hugetlb: use struct hugetlb_pte for walk_hugetlb_range

[David Hildenbrand <david@redhat.com>]

On 01.02.23 16:45, James Houghton wrote:
> On Tue, Jan 31, 2023 at 5:24 PM Peter Xu <peterx@redhat.com> wrote:
>>
>> On Tue, Jan 31, 2023 at 04:24:15PM -0800, James Houghton wrote:
>>> On Mon, Jan 30, 2023 at 1:14 PM Peter Xu <peterx@redhat.com> wrote:
>>>>
>>>> On Mon, Jan 30, 2023 at 10:38:41AM -0800, James Houghton wrote:
>>>>> On Mon, Jan 30, 2023 at 9:29 AM Peter Xu <peterx@redhat.com> wrote:
>>>>>>
>>>>>> On Fri, Jan 27, 2023 at 01:02:02PM -0800, James Houghton wrote:
> [snip]
>>>>>> Another way to not use thp mapcount, nor break smaps and similar calls to
>>>>>> page_mapcount() on small page, is to only increase the hpage mapcount only
>>>>>> when hstate pXd (in case of 1G it's PUD) entry being populated (no matter
>>>>>> as leaf or a non-leaf), and the mapcount can be decreased when the pXd
>>>>>> entry is removed (for leaf, it's the same as for now; for HGM, it's when
>>>>>> freeing pgtable of the PUD entry).
>>>>>
>>>>> Right, and this is doable. Also it seems like this is pretty close to
>>>>> the direction Matthew Wilcox wants to go with THPs.
>>>>
>>>> I may not be familiar with it, do you mean this one?
>>>>
>>>> https://lore.kernel.org/all/Y9Afwds%2FJl39UjEp@casper.infradead.org/
>>>
>>> Yep that's it.
>>>
>>>>
>>>> For hugetlb I think it should be easier to maintain rather than any-sized
>>>> folios, because there's the pgtable non-leaf entry to track rmap
>>>> information and the folio size being static to hpage size.
>>>>
>>>> It'll be different to folios where it can be random sized pages chunk, so
>>>> it needs to be managed by batching the ptes when install/zap.
>>>
>>> Agreed. It's probably easier for HugeTLB because they're always
>>> "naturally aligned" and yeah they can't change sizes.
>>>
>>>>
>>>>>
>>>>> Something I noticed though, from the implementation of
>>>>> folio_referenced()/folio_referenced_one(), is that folio_mapcount()
>>>>> ought to report the total number of PTEs that are pointing on the page
>>>>> (or the number of times page_vma_mapped_walk returns true). FWIW,
>>>>> folio_referenced() is never called for hugetlb folios.
>>>>
>>>> FWIU folio_mapcount is the thing it needs for now to do the rmap walks -
>>>> it'll walk every leaf page being mapped, big or small, so IIUC that number
>>>> should match with what it expects to see later, more or less.
>>>
>>> I don't fully understand what you mean here.
>>
>> I meant the rmap_walk pairing with folio_referenced_one() will walk all the
>> leaves for the folio, big or small.  I think that will match the number
>> with what got returned from folio_mapcount().
> 
> See below.
> 
>>
>>>
>>>>
>>>> But I agree the mapcount/referenced value itself is debatable to me, just
>>>> like what you raised in the other thread on page migration.  Meanwhile, I
>>>> am not certain whether the mapcount is accurate either because AFAICT the
>>>> mapcount can be modified if e.g. new page mapping established as long as
>>>> before taking the page lock later in folio_referenced().
>>>>
>>>> It's just that I don't see any severe issue either due to any of above, as
>>>> long as that information is only used as a hint for next steps, e.g., to
>>>> swap which page out.
>>>
>>> I also don't see a big problem with folio_referenced() (and you're
>>> right that folio_mapcount() can be stale by the time it takes the
>>> folio lock). It still seems like folio_mapcount() should return the
>>> total number of PTEs that map the page though. Are you saying that
>>> breaking this would be ok?
>>
>> I didn't quite follow - isn't that already doing so?
>>
>> folio_mapcount() is total_compound_mapcount() here, IIUC it is an
>> accumulated value of all possible PTEs or PMDs being mapped as long as it's
>> all or part of the folio being mapped.
> 
> We've talked about 3 ways of handling mapcount:
> 
> 1. The RFC v2 way, which is head-only, and we increment the compound
> mapcount for each PT mapping we have. So a PTE-mapped 2M page,
> compound_mapcount=512, subpage->_mapcount=0 (ignoring the -1 bias).
> 2. The THP-like way. If we are fully mapping the hugetlb page with the
> hstate-level PTE, we increment the compound mapcount, otherwise we
> increment subpage->_mapcount.
> 3. The RFC v1 way (the way you have suggested above), which is
> head-only, and we increment the compound mapcount if the hstate-level
> PTE is made present.
> 
> With #1 and #2, there is no concern with folio_mapcount(). But with
> #3, folio_mapcount() for a PTE-mapped 2M page mapped in a single VMA
> would yield 1 instead of 512 (right?). That's what I mean.

My 2 cents:

The mapcount is primarily used (in hugetlb context) to

(a) Detect if a page might be shared. mapcount > 1 implies that two 
independent page table hierarchies are mapping the page. We care about 
mapcount == 1 vs. mapcount != 1.

(b) Detect if unmapping was sucessfull. We care about mapcount == 0 vs. 
mapcount != 0.

For hugetlb, I don't see why we should care about the subpage mapcount 
at all.

For (a) it's even good to count "somehow mapped into a single page table 
structure" as "mapcount == 1" For (b), we don't care as long as "still 
mapped" implies "mapcount != 0".

-- 
Thanks,

David / dhildenb