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David Hildenbrand wrote: > We added an early exit in thp_underused(), probably to avoid scanning > pages when there is no chance for success. > > However, assume we have max_ptes_none = 511 (default). > > Nothing should stop us from freeing all pages part of a THP that Freeing 'all pages which are part of a THP' rather? > is completely zero (512) and khugepaged will for sure not try to that is -> that are? > instantiate a THP in that case (512 shared zeropages). But if you write faulted they're not the zero page? And how are they shared? I mean be being dumb here. > > This can just trivially happen if someone writes a single 0 byte into a > PMD area, or of course, when data ends up being zero later. > > So let's remove that early exit. > > Do we want to CC stable? Hm, not sure. Probably not urgent. Surely this is worth having? > > Note that, as default, the THP shrinker is active > (/sys/kernel/mm/transparent_hugepage/shrink_underused = 1), and all > THPs are added to the deferred split lists. However, with the > max_ptes_none default we would never scan them. We would not do that. If Nit but 'we would not do that' is kinda duplicative here :) > that's not desirable, we should just disable the shrinker as default, > also not adding all THPs to the deferred split lists. > > Easy to reproduce: > > 1) Allocate some THPs filled with 0s > > > #include > #include > #include > #include > #include > > const size_t size = 1024*1024*1024; > > int main(void) > { > size_t offs; > char *area; > > area = mmap(0, size, PROT_READ | PROT_WRITE, > MAP_ANON | MAP_PRIVATE, -1, 0); > if (area == MAP_FAILED) { > printf("mmap failed\n"); > exit(-1); > } > madvise(area, size, MADV_HUGEPAGE); > > for (offs = 0; offs < size; offs += getpagesize()) > area[offs] = 0; > pause(); > } > <\prog.c> > > 2) Trigger the shrinker > > E.g., memory pressure through memhog > > 3) Observe that THPs are not getting reclaimed > > $ cat /proc/`pgrep prog`/smaps_rollup > > Would list ~1GiB of AnonHugePages. With this fix, they would get > reclaimed as expected. Yikes! > > Fixes: dafff3f4c850 ("mm: split underused THPs") > Cc: Andrew Morton > Cc: Lorenzo Stoakes > Cc: Zi Yan > Cc: Baolin Wang > Cc: "Liam R. Howlett" > Cc: Nico Pache > Cc: Ryan Roberts > Cc: Dev Jain > Cc: Barry Song > Cc: Usama Arif > Signed-off-by: David Hildenbrand LGTM, so: Reviewed-by: Lorenzo Stoakes > --- > mm/huge_memory.c | 3 --- > 1 file changed, 3 deletions(-) > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c > index 26cedfcd74189..aa3ed7a86435b 100644 > --- a/mm/huge_memory.c > +++ b/mm/huge_memory.c > @@ -4110,9 +4110,6 @@ static bool thp_underused(struct folio *folio) > void *kaddr; > int i; > > - if (khugepaged_max_ptes_none == HPAGE_PMD_NR - 1) > - return false; > - > for (i = 0; i < folio_nr_pages(folio); i++) { > kaddr = kmap_local_folio(folio, i * PAGE_SIZE); > if (!memchr_inv(kaddr, 0, PAGE_SIZE)) { > -- > 2.50.1 >