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Sat, 07 Feb 2026 14:17:53 -0800 (PST) MIME-Version: 1.0 References: <20260207081613.588598-1-vernon2gm@gmail.com> <20260207081613.588598-5-vernon2gm@gmail.com> In-Reply-To: From: Barry Song <21cnbao@gmail.com> Date: Sun, 8 Feb 2026 06:17:41 +0800 X-Gm-Features: AZwV_Qh-gSOsEfSs0Y3r-5yEnjwmRauQ9mEa9C2Jg6pXLA6HDZrwCYmuNH9_Y7Y Message-ID: Subject: Re: [PATCH mm-new v7 4/5] mm: khugepaged: skip lazy-free folios To: "David Hildenbrand (Arm)" Cc: Lance Yang , Vernon Yang , akpm@linux-foundation.org, lorenzo.stoakes@oracle.com, ziy@nvidia.com, dev.jain@arm.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Vernon Yang Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspamd-Queue-Id: 711441A0002 X-Rspam-User: X-Rspamd-Server: rspam05 X-Stat-Signature: 81k4xiwm3xfxcc7much59deyir1u6ez7 X-HE-Tag: 1770502674-359353 X-HE-Meta: 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 Gp3cz8ig VPgC//d2zfUbVZi3B7bPybade4QJEMA4EOvDOmxKnM35IaDYkBx7uJ7jit1XVVvi/fjCXxXAe4eb3anHpejgnTOwJoIRBd5TpTLE0/JqMf8FRiHvsuq+hWJBxdgcWkLDNOMAIbv/55do5M514Qp7gsxX+EYMtjaU54th/43g+aF7u1x5V0/t639VNFuVV+vyclD4b1V0/cTfk4JXnFnn7dauBXrl47Z7bCGsEH8GuYVowojvNxrKFkULF/68DpUPpemQ3GfJfjzP7MMnflBz4F1rRx6Tia/pfPDg01Au/hqKhpXh//ja2Akg6njJNgh1qY4LvE/CM+ApUEkEA0VUx2WxMdv7cdSlseGdTo2LA8+CgMapq1+FlLu72eV89AF2DCUD2mk33/Negw+sexE+APy8PBCmAxKSDsUAt/6Fo7NWFdEo= 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 Sun, Feb 8, 2026 at 6:05=E2=80=AFAM David Hildenbrand (Arm) wrote: > > On 2/7/26 23:01, Barry Song wrote: > > On Sun, Feb 8, 2026 at 5:38=E2=80=AFAM David Hildenbrand (Arm) wrote: > >> > >> On 2/7/26 14:51, Lance Yang wrote: > >>> > >>> > >>> > >>> Good catch! > >>> > >> > >> Good point! > >> > >>> > >>> Right. When the VMA has VM_DROPPABLE, we would drop the lazyfree foli= o > >>> regardless of whether it (or the PTE) is dirty in try_to_unmap_one(). > >>> > >>> So, IMHO, we could go with: > >>> > >>> cc->is_khugepaged && folio_test_lazyfree(folio) && > >>> (!pte_dirty(pteval) || (vma->vm_flags & VM_DROPPABLE)) > >> > >> Hm. In a VM_DROPPABLE mapping all folios should be marked as lazy-free > >> (see folio_add_new_anon_rmap()). > >> > >> The new (collapse) folio will also be marked lazy (due to > >> folio_add_new_anon_rmap()) free and can just get dropped any time. > >> > >> So likely we should just not skip collapse for lazyfree folios in > >> VM_DROPPABLE mappings? > > > > Maybe change =E2=80=9Cjust not skip=E2=80=9D to =E2=80=9Cjust skip=E2= =80=9D? > > > > If the goal is to avoid the collapse overhead for folios that are > > about to be dropped, we might consider skipping collapse for the > > entire VMA=EF=BC=9F > If there is no memory pressure in the system, why wouldn't you just want > to collapse in a VM_DROPPABLE region? > > "about to be dropped" only applies once there is actual memory pressure. > If not, these pages stick around forever. agree. But this brings us back to the philosophy of the original patch. If there is no memory pressure, lazyfree folios won=E2=80=99t be dropped, s= o collapsing them might also be reasonable. Just collapsing fully lazyfree folios with VM_DROPPABLE while skipping partially lazyfree VMAs seems a bit confusing to me :-) Thanks Barry