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X-OriginatorOrg: arm.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 14 Sep 2025 07:30:13.8929 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 358cf462-7c15-44e4-989f-08ddf3608b8a 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: DB1PEPF000509EC.eurprd03.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: AS4PR08MB7531 X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: B75F740008 X-Stat-Signature: shdiq35743wpqdgyaz1w5fxctx9hoju3 X-Rspam-User: X-HE-Tag: 1757835021-506795 X-HE-Meta: U2FsdGVkX18Ig+SnZK/Nmj7KxcnId8gALXNwc7SI8ivEos0bmtRkjs/2ViXpEEiPbm0emqRBwZPAFIVPGFdoZB/vLz88fM32dtYnGJ0ku0bXOuoBQWImVq+SUyH2TUS3zXTPsmJvgaRxK1ln7O3cKcP1+iT52whRnwwGm6qfFlW/FzfZ+P54AC1DiwPi/5rDvPHicSWGeBWr8Ex0Oqlcru5GBu4DcDqGhGQ0RKtGKxmJJiUaMhBtm53KCAYGFMNCaqPrGZkEJpm4zENIf1kAFA7PIIqbVR5KSmXKeHb3+pXoom9XZsyFv7XbYTKzc6EXUS5onCJgsrwWBGLG2wTr+YlUu7uppCetJ9N1jrMPqE7QlM/O4gBg0MXfVn21/0Bl7Z2IQJ33DCJBXo2P/ZrT3oBDxvF/N5Mw2QV6iscLWvnKdUG5Mp9B4eGT69g8lmu95DR4fOM+btoz1hAM87Wkmt2HNodqJWOi7x5E27RnwQhgLrGUTBMckgjHSg1g7I6aY4J32YpLAA5QfAVEfc4ZZahqMxSqCZjiUKAzswA9aLp55CfovpOeBJEWR6kcaot3WDju5llrx5JMaq/ZOMVF3A3AImTrNXoEgn8KZl81lqqZcQSoHa9hVETyFjWeP8mKDiV/LpW3ag7pArO/IBO/WdpeSlLOyW9mNrdlPO6/rPxFq/6zsqLLKM2eqhG1yX6YrYP3Kn8bKXdD1MndbI/c7ffe8HRH6/i4F21Uvp5gBEI2+0qHMPHRZ51WbqJ6vd8AWCub+hlMmXaWwRhHmFdN3f9TtNQBg8Fw0+6DN/c5L2farRezST+Pjhhs+ptJooKfFyye/5SBeSDVqLPA4mcOcPfG0edmuAYhsy7cZUmMbMoz+fJ2XehqNnvX141Asm/KywlKQIP9gpqRbhqgl2VeL+mFgItvH/FGjW/TQ+z2YUQzJKyfeRCVSimjLnfpZl5YcDhSgRE2Qk+ZTXroN9y hLsnA20D 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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 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 :) > >> What you are currently doing is just ensuring that if slot =3D=3D NULL, = then >> we get a legal value of mm_slot, and since the code then operates on >> the basis of mm_slot, everything will work fine. I really think this >> is a workaround to the real issue that you got slot =3D=3D NULL :) > > Anyway, an actual fix like "if (!slot) return" for khugepaged would > be better than this workaround, as Dev mentioned. > > Cheers, > Lance > >> >>> >>> Current code works since mm_slot is the first element, but make sure it >>> won't be disturbed. >>> >>> Signed-off-by: Wei Yang >>> --- >>> mm/khugepaged.c | 5 ++++- >>> mm/ksm.c | 5 ++++- >>> 2 files changed, 8 insertions(+), 2 deletions(-) >>> >>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c >>> index af5f5c80fe4e..668e74ad33b7 100644 >>> --- a/mm/khugepaged.c >>> +++ b/mm/khugepaged.c >>> @@ -108,7 +108,7 @@ struct collapse_control { >>> * @slot: hash lookup from mm to mm_slot >>> */ >>> struct khugepaged_mm_slot { >>> - struct mm_slot slot; >>> + struct mm_slot slot; /* keep it the first element */ >>> }; >>> >>> /** >>> @@ -2382,6 +2382,9 @@ static unsigned int >>> khugepaged_scan_mm_slot(unsigned int pages, int *result, >>> struct vm_area_struct *vma; >>> int progress =3D 0; >>> >>> + BUILD_BUG_ON_MSG(mm_slot_entry(NULL, struct >>> khugepaged_mm_slot, slot), >>> + "slot should be the first element"); >>> + >>> VM_BUG_ON(!pages); >>> lockdep_assert_held(&khugepaged_mm_lock); >>> *result =3D SCAN_FAIL; >>> diff --git a/mm/ksm.c b/mm/ksm.c >>> index 2ef29802a49b..0d486dbdf7d3 100644 >>> --- a/mm/ksm.c >>> +++ b/mm/ksm.c >>> @@ -124,7 +124,7 @@ typedef u8 rmap_age_t; >>> * @rmap_list: head for this mm_slot's singly-linked list of >>> rmap_items >>> */ >>> struct ksm_mm_slot { >>> - struct mm_slot slot; >>> + struct mm_slot slot; /* keep it the first element */ >>> struct ksm_rmap_item *rmap_list; >>> }; >>> >>> @@ -3842,6 +3842,9 @@ static int __init ksm_init(void) >>> struct task_struct *ksm_thread; >>> int err; >>> >>> + BUILD_BUG_ON_MSG(mm_slot_entry(NULL, struct ksm_mm_slot, slot), >>> + "slot should be the first element"); >>> + >>> /* The correct value depends on page size and endianness */ >>> zero_checksum =3D calc_checksum(ZERO_PAGE(0)); >>> /* Default to false for backwards compatibility */ >> IMPORTANT NOTICE: The contents of this email and any attachments are >> confidential and may also be privileged. If you are not the intended >> recipient, please 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. > 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.