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List-Unsubscribe: On 5 Sep 2025, at 10:11, 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 > is completely zero (512) and khugepaged will for sure not try to > instantiate a THP in that case (512 shared zeropages). > > 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. > > 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 > 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. > > 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 > --- > mm/huge_memory.c | 3 --- > 1 file changed, 3 deletions(-) > LGTM. Acked-by: Zi Yan I also notice that thp_underused() checks num_zero_pages directly against khugepaged_max_ptes_none. This means mTHPs will never be regarded as underused. A similar issue you are discussing in Nico’s khugepaged mTHP support. Maybe checks against these khugepaged_max* variables should be calculated based on nr_pages of a large folio, like making these variables a ratio in other discussion. Best Regards, Yan, Zi