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Howlett" , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Usama Arif References: <20250905141137.3529867-1-david@redhat.com> From: Baolin Wang In-Reply-To: <20250905141137.3529867-1-david@redhat.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Stat-Signature: pc6um7ggpfn5gkj53ubnoxo1aergf7o6 X-Rspam-User: X-Rspamd-Queue-Id: A0BEF160002 X-Rspamd-Server: rspam05 X-HE-Tag: 1757297791-176131 X-HE-Meta: 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 fXk0tXy1 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 2025/9/5 22:11, 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 > --- Thanks for sorting this out. Make sense to me. Reviewed-by: Baolin Wang > 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)) {