Messages in this thread | | | Date | Wed, 1 Feb 2023 16:56:39 +0100 | Subject | Re: [PATCH 21/46] hugetlb: use struct hugetlb_pte for walk_hugetlb_range | From | David Hildenbrand <> |
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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
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