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From: Lance Yang Date: Sat, 6 Sep 2025 14:39:26 +0800 X-Gmail-Original-Message-ID: X-Gm-Features: Ac12FXxJgS1tMi1cNEiT_Y8dnTFoVxumeja2536-J7u_yM_UU8Y0HYuyc3aPuek Message-ID: Subject: Re: [PATCH v1] mm/huge_memory: fix shrinking of all-zero THPs with max_ptes_none default To: David Hildenbrand Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, Andrew Morton , Lorenzo Stoakes , Zi Yan , Baolin Wang , "Liam R. Howlett" , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Usama Arif Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Migadu-Flow: FLOW_OUT X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: F30F340008 X-Stat-Signature: rxprb7bz33j6in37641n3dwyrd5sg7rw X-Rspam-User: X-HE-Tag: 1757140810-891476 X-HE-Meta: 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 +paVJim/ udBlls2A8NsJo9XVbATLk2S2yboYb63gwT7XpXgSmVVvOPSIvLBK6aV+J1XVtNZ27aw3PL4CtUZ1IBLCLOLg5iROdogDmamx8STkg2onj036gZJz00ZZcM6svS59OjbL0UBp4h8+8fAByQlz5prSmbwtRgeJAGaSFb0FveGT6LTDSJ3kUunb4suyxcF3qs+dKzG+AiOuJzo8IOFoRjK+xS4TDY0/P4O3HfhSTqoF/iSKFhsd7YUl5fk2nJzzTgHOfpEa94dbN929vkprUDrpQ4NI83ygZDBU3T80WJ94GIZoMbV1eW/845WNVZXnJd4t36pV8ABmJUfgU6aKPiOnJBAWAdwU5NcvZdEnKfGDoOWek7RsrXXSl8QGrU0XUANJnOzzoTHRxA9rzqIYmb7gEzcXI50TNiwUQ2cjTR/ZPBMfNPHvuja2KLadDEsHpVpwpCfP1cJogH2aZGvv1Ffs6V//hqkme3OUoPgF6 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 Fri, Sep 5, 2025 at 10:14=E2=80=AFPM 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 =3D 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 =3D 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 =3D 1024*1024*1024; > > int main(void) > { > size_t offs; > char *area; > > area =3D mmap(0, size, PROT_READ | PROT_WRITE, > MAP_ANON | MAP_PRIVATE, -1, 0); > if (area =3D=3D MAP_FAILED) { > printf("mmap failed\n"); > exit(-1); > } > madvise(area, size, MADV_HUGEPAGE); > > for (offs =3D 0; offs < size; offs +=3D getpagesize()) > area[offs] =3D 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 Good catch! Feel free to add: Reviewed-by: Lance Yang Cheers, Lance > --- > 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 =3D=3D HPAGE_PMD_NR - 1) > - return false; > - > for (i =3D 0; i < folio_nr_pages(folio); i++) { > kaddr =3D kmap_local_folio(folio, i * PAGE_SIZE); > if (!memchr_inv(kaddr, 0, PAGE_SIZE)) { > -- > 2.50.1 > >