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X-OriginatorOrg: arm.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 15 Sep 2025 08:12:05.9102 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 98560ecf-d98b-4e68-80a9-08ddf42f8f3a X-MS-Exchange-CrossTenant-Id: f34e5979-57d9-4aaa-ad4d-b122a662184d X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=f34e5979-57d9-4aaa-ad4d-b122a662184d;Ip=[4.158.2.129];Helo=[outbound-uk1.az.dlp.m.darktrace.com] X-MS-Exchange-CrossTenant-AuthSource: DU2PEPF00028D0D.eurprd03.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: GVXPR08MB11494 X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: 47C4EA0012 X-Stat-Signature: 5oagdnuquptgfmoq4gsgqfm48cy1an7b X-Rspam-User: X-HE-Tag: 1757923930-642673 X-HE-Meta: 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 x8yqc7gc 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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 15/09/25 1:26 pm, David Hildenbrand wrote: > On 14.09.25 09:29, Dev Jain wrote: >> >> On 14/09/25 12:33 pm, Lance Yang wrote: >>> >>> >>> On 2025/9/14 14:21, Dev Jain wrote: >>>> >>>> On 14/09/25 5:30 am, Wei Yang wrote: >>>>> When using mm_slot in ksm/khugepaged, there is code snip like: >>>>> >>>>> slot =3D mm_slot_lookup(mm_slots_hash, mm); >>>>> mm_slot =3D mm_slot_entry(slot, struct ksm_mm_slot, slot); >>>>> if (mm_slot && ..) { >>>>> } >>>>> >>>>> This is only valid when mm_slot is the first element of its wrapper >>>>> structure, otherwise a NULL slot would converted to a mm_slot with >>>>> negative value. And current code thinks it is valid and continue. >>>> >>>> Shouldn't you fix the code for the case when you can't find the slot >>>> in the hashtable, i.e slot =3D=3D NULL? Like, if (!slot) return. >>> >>> Right. For khugepaged specifically, the slot =3D=3D NULL case in >>> __khugepaged_exit() (only user of mm_slot_lookup) should probably >>> be treated as a kernel BUG for new. >>> >>> But I'm not sure if the same logic applies to KSM ;) >> >> I haven't seen the KSM analogue, but restricting the position of an >> element >> >> in a struct to make the code work should imply that the code is wrong in >> >> the first place :) > > I agree. If mm_slot_lookup() returns NULL we just just handle that > cleanly like > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c > index 6b40bdfd224c3..70a32d59d7d2f 100644 > --- a/mm/khugepaged.c > +++ b/mm/khugepaged.c > @@ -488,11 +488,13 @@ void __khugepaged_exit(struct mm_struct *mm) > > spin_lock(&khugepaged_mm_lock); > slot =3D mm_slot_lookup(mm_slots_hash, mm); > - mm_slot =3D mm_slot_entry(slot, struct khugepaged_mm_slot, slot); > - if (mm_slot && khugepaged_scan.mm_slot !=3D mm_slot) { > - hash_del(&slot->hash); > - list_del(&slot->mm_node); > - free =3D 1; > + if (slot) { > + mm_slot =3D mm_slot_entry(slot, struct > khugepaged_mm_slot, slot); > + if (mm_slot && khugepaged_scan.mm_slot !=3D mm_slot) { > + hash_del(&slot->hash); > + list_del(&slot->mm_node); > + free =3D 1; > + } > } > spin_unlock(&khugepaged_mm_lock); > > > If mm_slot_lookup() is not expected to ever return NULL, then a > VM_WARN_ON_ONCE > might be sufficient to document that this is guaranteed. > > IIUC, MMF_VM_HUGEPAGE might be set in __khugepaged_enter() in case > > (a) test_and_set_bit() succeeds > > but > > (b) mm_slot_alloc() fails > > In that case we could get NULL. > > > It is rather weird to leave the flag set in case mm_slot_alloc() > failed ... Good spot! We should move the slot allocation line before the test_and_set. IMPORTANT NOTICE: The contents of this email and any attachments are confid= ential and may also be privileged. If you are not the intended recipient, p= lease notify the sender immediately and do not disclose the contents to any= other person, use it for any purpose, or store or copy the information in = any medium. Thank you.