On 2022/9/8 15:07, Michal Hocko wrote: > On Thu 08-09-22 11:25:54, Miaohe Lin wrote: >> On 2022/9/8 11:06, HORIGUCHI NAOYA(堀口 直也) wrote: >>> On Thu, Sep 08, 2022 at 10:19:03AM +0800, Miaohe Lin wrote: >>>> On 2022/9/7 20:11, Naoya Horiguchi wrote: >>> ... >>>>> >From 8a5c284df732943065d23838090d15c94cd10395 Mon Sep 17 00:00:00 2001 >>>>> From: Naoya Horiguchi <naoya.horiguchi@xxxxxxx> >>>>> Date: Wed, 7 Sep 2022 20:58:44 +0900 >>>>> Subject: [PATCH] mm/huge_memory: use pfn_to_online_page() in >>>>> split_huge_pages_all() >>>>> >>>>> NULL pointer dereference is triggered when calling thp split via debugfs >>>>> on the system with offlined memory blocks. With debug option enabled, >>>>> the following kernel messages are printed out: >>>>> >>>>> page:00000000467f4890 refcount:1 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x121c000 >>>>> flags: 0x17fffc00000000(node=0|zone=2|lastcpupid=0x1ffff) >>>>> raw: 0017fffc00000000 0000000000000000 dead000000000122 0000000000000000 >>>>> raw: 0000000000000000 0000000000000000 00000001ffffffff 0000000000000000 >>>>> page dumped because: unmovable page >>>>> page:000000007d7ab72e is uninitialized and poisoned >>>>> page dumped because: VM_BUG_ON_PAGE(PagePoisoned(p)) >>>>> ------------[ cut here ]------------ >>>>> kernel BUG at include/linux/mm.h:1248! >>>>> invalid opcode: 0000 [#1] PREEMPT SMP PTI >>>>> CPU: 16 PID: 20964 Comm: bash Tainted: G I 6.0.0-rc3-foll-numa+ #41 >>>>> ... >>>>> RIP: 0010:split_huge_pages_write+0xcf4/0xe30 >>>>> >>>>> This shows that page_to_nid() in page_zone() is unexpectedly called for an >>>>> offlined memmap. >>>>> >>>>> Use pfn_to_online_page() to get struct page in PFN walker. >>>> >>>> With changes proposed by David, this patch looks good to me. >>>> >>>> Reviewed-by: Miaohe Lin <linmiaohe@xxxxxxxxxx> >>> >>> Thank you. >>> >>>> >>>> BTW: IMHO, there might be many similar code places need to take care of memory hotremove where >>>> *pfn is operated directly* while it's not protected against memory hotremove. >>> >>> I had the similar concern, but there seems many place doing PFN walk, >>> so checking them one-by-one that offlined memory can be walked over >>> requires much effort. >> >> Yes, that will be a heavy work. We could fix them one by one if they ever occur. ;) > > Most of those whic are directly triggerable should be taken care of. It > would be still good to go through `git grep -w pfn_to_page' and evaluate > all callers. Still more than 400 callsites so not a trivial task. Agree. Even if pfn_to_online_page() is used, it might still races with the memory hotremove if there's no way to guard against it, e.g. holding an extra page refcnt. So code audit should also apply to pfn_to_online_page(). Thanks, Miaohe Lin
