On 18/07/2023 10:08, David Hildenbrand wrote: > On 18.07.23 10:58, Ryan Roberts wrote: >> On 17/07/2023 17:48, David Hildenbrand wrote: >>> On 17.07.23 18:01, Ryan Roberts wrote: >>>> On 17/07/2023 16:42, David Hildenbrand wrote: >>>>> On 17.07.23 16:31, Ryan Roberts wrote: >>>>>> In preparation for the introduction of large folios for anonymous >>>>>> memory, we would like to be able to split them when they have unmapped >>>>>> subpages, in order to free those unused pages under memory pressure. So >>>>>> remove the artificial requirement that the large folio needed to be at >>>>>> least PMD-sized. >>>>>> >>>>>> Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx> >>>>>> Reviewed-by: Yu Zhao <yuzhao@xxxxxxxxxx> >>>>>> Reviewed-by: Yin Fengwei <fengwei.yin@xxxxxxxxx> >>>>>> --- >>>>>> mm/rmap.c | 2 +- >>>>>> 1 file changed, 1 insertion(+), 1 deletion(-) >>>>>> >>>>>> diff --git a/mm/rmap.c b/mm/rmap.c >>>>>> index 0c0d8857dfce..2baf57d65c23 100644 >>>>>> --- a/mm/rmap.c >>>>>> +++ b/mm/rmap.c >>>>>> @@ -1430,7 +1430,7 @@ void page_remove_rmap(struct page *page, struct >>>>>> vm_area_struct *vma, >>>>>> * page of the folio is unmapped and at least one page >>>>>> * is still mapped. >>>>>> */ >>>>>> - if (folio_test_pmd_mappable(folio) && folio_test_anon(folio)) >>>>>> + if (folio_test_large(folio) && folio_test_anon(folio)) >>>>>> if (!compound || nr < nr_pmdmapped) >>>>>> deferred_split_folio(folio); >>>>> >>>>> !compound will always be true I guess, so nr_pmdmapped == 0 (which will always >>>>> be the case) will be ignored. >>>> >>>> I don't follow why !compound will always be true. This function is >>>> page_remove_rmap() (not folio_remove_rmap_range() which I add in a later >>>> patch). >>>> page_remove_rmap() can work on pmd-mapped pages where compound=true is >>>> passed in. >>> >>> I was talking about the folio_test_pmd_mappable() -> folio_test_large() change. >>> For folio_test_large() && !folio_test_pmd_mappable() I expect that we'll never >>> pass in "compound=true". >>> >> >> Sorry David, I've been staring at the code and your comment, and I still don't >> understand your point. I assumed you were trying to say that compound is always >> false and therefore "if (!compound || nr < nr_pmdmapped)" can be removed? But >> its not the case that compound is always false; it will be true when called to >> remove a pmd-mapped compound page. > > Let me try again: > > Assume, as I wrote, that we are given a folio that is "folio_test_large() && > !folio_test_pmd_mappable()". That is, a folio that is *not* pmd mappable. > > If it's not pmd-mappable, certainly, nr_pmdmapped == 0, and therefore, "nr < > nr_pmdmapped" will never ever trigger. > > The only way to have it added to the deferred split queue is, therefore "if > (!compound)". > > So *for these folios*, we will always pass "compound == false" to make that "if > (!compound)" succeed. > > > Does that make sense? Yes I agree with all of this. I thought you were pointing out an issue or proposing a change to the logic. Hence my confusion. > >> What change are you suggesting, exactly? > > Oh, I never suggested a change (I even gave you my RB). I was just thinking out > loud. >
