Messages in this thread | | | Date | Fri, 26 Apr 2024 22:15:55 +0200 | Subject | Re: [PATCH v5] mm/rmap: do not add fully unmapped large folio to deferred split list | From | David Hildenbrand <> |
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On 26.04.24 21:20, Zi Yan wrote: > On 26 Apr 2024, at 15:08, David Hildenbrand wrote: > >> On 26.04.24 21:02, Zi Yan wrote: >>> From: Zi Yan <ziy@nvidia.com> >>> >>> In __folio_remove_rmap(), a large folio is added to deferred split list >>> if any page in a folio loses its final mapping. But it is possible that >>> the folio is fully unmapped and adding it to deferred split list is >>> unnecessary. >>> >>> For PMD-mapped THPs, that was not really an issue, because removing the >>> last PMD mapping in the absence of PTE mappings would not have added the >>> folio to the deferred split queue. >>> >>> However, for PTE-mapped THPs, which are now more prominent due to mTHP, >>> they are always added to the deferred split queue. One side effect >>> is that the THP_DEFERRED_SPLIT_PAGE stat for a PTE-mapped folio can be >>> unintentionally increased, making it look like there are many partially >>> mapped folios -- although the whole folio is fully unmapped stepwise. >>> >>> Core-mm now tries batch-unmapping consecutive PTEs of PTE-mapped THPs >>> where possible starting from commit b06dc281aa99 ("mm/rmap: introduce >>> folio_remove_rmap_[pte|ptes|pmd]()"). When it happens, a whole PTE-mapped >>> folio is unmapped in one go and can avoid being added to deferred split >>> list, reducing the THP_DEFERRED_SPLIT_PAGE noise. But there will still be >>> noise when we cannot batch-unmap a complete PTE-mapped folio in one go >>> -- or where this type of batching is not implemented yet, e.g., migration. >>> >>> To avoid the unnecessary addition, folio->_nr_pages_mapped is checked >>> to tell if the whole folio is unmapped. If the folio is already on >>> deferred split list, it will be skipped, too. >>> >>> Note: commit 98046944a159 ("mm: huge_memory: add the missing >>> folio_test_pmd_mappable() for THP split statistics") tried to exclude >>> mTHP deferred split stats from THP_DEFERRED_SPLIT_PAGE, but it does not >>> fix the above issue. A fully unmapped PTE-mapped order-9 THP was still >>> added to deferred split list and counted as THP_DEFERRED_SPLIT_PAGE, >>> since nr is 512 (non zero), level is RMAP_LEVEL_PTE, and inside >>> deferred_split_folio() the order-9 folio is folio_test_pmd_mappable(). >>> >>> Suggested-by: David Hildenbrand <david@redhat.com> >>> Signed-off-by: Zi Yan <ziy@nvidia.com> >>> --- >>> mm/rmap.c | 12 +++++++++--- >>> 1 file changed, 9 insertions(+), 3 deletions(-) >>> >>> diff --git a/mm/rmap.c b/mm/rmap.c >>> index 2608c40dffad..a9bd64ebdd9a 100644 >>> --- a/mm/rmap.c >>> +++ b/mm/rmap.c >>> @@ -1495,6 +1495,7 @@ static __always_inline void __folio_remove_rmap(struct folio *folio, >>> { >>> atomic_t *mapped = &folio->_nr_pages_mapped; >>> int last, nr = 0, nr_pmdmapped = 0; >>> + bool partially_mapped = false; >>> enum node_stat_item idx; >>> __folio_rmap_sanity_checks(folio, page, nr_pages, level); >>> @@ -1515,6 +1516,8 @@ static __always_inline void __folio_remove_rmap(struct folio *folio, >>> nr++; >>> } >>> } while (page++, --nr_pages > 0); >>> + >>> + partially_mapped = !!nr && !!atomic_read(mapped); >> >> Nit: The && should remove the need for both !!. > > My impression was that !! is needed to convert from int to bool and I do > find "!!int && !!int" use in the kernel.
I might be wrong about this, but if you wouldn't write
if (!!nr && !!atomic_read(mapped))
then
bool partially_mapped = nr && atomic_read(mapped);
is sufficient.
&& would make sure that the result is either 0 or 1, which you can store safely in a bool, no matter which underlying type is used to store that value.
But I *think* nowdays, the compiler will always handle that correctly, even without the "&&" (ever since C99 added _Bool).
Likely, also
bool partially_mapped = nr & atomic_read(mapped);
Would nowadays work, but looks stupid.
Related: https://lkml.org/lkml/2013/8/31/138
--- #include <stdio.h> #include <stdbool.h> #include <stdint.h> #include <inttypes.h>
volatile uint64_t a = 0x8000000000000000ull;
void main (void) { printf("uint64_t a = a: 0x%" PRIx64 "\n", a);
int i1 = a; printf("int i1 = a: %d\n", i1);
int i2 = !!a; printf("int i2 = !!a: %d\n", i2);
bool b1 = a; printf("bool b1 = a: %d\n", b1);
bool b2 = !!a; printf("bool b2 = !!a: %d\n", b2); } --- $ ./test uint64_t a = a: 0x8000000000000000 int i1 = a: 0 int i2 = !!a: 1 bool b1 = a: 1 bool b2 = !!a: 1 ---
Note that if bool would be defined as "int", you would need the !!, otherwise you would lose information.
But even for b1, the gcc generates now:
40118c: 48 8b 05 7d 2e 00 00 mov 0x2e7d(%rip),%rax # 404010 <a> 401193: 48 85 c0 test %rax,%rax 401196: 0f 95 c0 setne %al
My stdbool.h contains
#define bool _Bool
And I think C99 added _Bool that makes that work.
But I didn't read the standard, and it's time for the weekend :)
-- Cheers,
David / dhildenb
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