On 09.10.19 11:57, Naoya Horiguchi wrote: > Hi David, > > On Wed, Oct 09, 2019 at 11:12:04AM +0200, David Hildenbrand wrote: >> There are various places where we access uninitialized memmaps, namely: >> - /proc/kpagecount >> - /proc/kpageflags >> - /proc/kpagecgroup >> - memory_failure() - which reuses stable_page_flags() from fs/proc/page.c > > Ah right, memory_failure is another victim of this bug. > >> >> We have initialized memmaps either when the section is online or when >> the page was initialized to the ZONE_DEVICE. Uninitialized memmaps contain >> garbage and in the worst case trigger kernel BUGs, especially with >> CONFIG_PAGE_POISONING. >> >> For example, not onlining a DIMM during boot and calling /proc/kpagecount >> with CONFIG_PAGE_POISONING: >> :/# cat /proc/kpagecount > tmp.test >> [ 95.600592] BUG: unable to handle page fault for address: fffffffffffffffe >> [ 95.601238] #PF: supervisor read access in kernel mode >> [ 95.601675] #PF: error_code(0x0000) - not-present page >> [ 95.602116] PGD 114616067 P4D 114616067 PUD 114618067 PMD 0 >> [ 95.602596] Oops: 0000 [#1] SMP NOPTI >> [ 95.602920] CPU: 0 PID: 469 Comm: cat Not tainted 5.4.0-rc1-next-20191004+ #11 >> [ 95.603547] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.12.1-0-ga5cab58e9a3f-prebuilt.qemu.4 >> [ 95.604521] RIP: 0010:kpagecount_read+0xce/0x1e0 >> [ 95.604917] Code: e8 09 83 e0 3f 48 0f a3 02 73 2d 4c 89 e7 48 c1 e7 06 48 03 3d ab 51 01 01 74 1d 48 8b 57 08 480 >> [ 95.606450] RSP: 0018:ffffa14e409b7e78 EFLAGS: 00010202 >> [ 95.606904] RAX: fffffffffffffffe RBX: 0000000000020000 RCX: 0000000000000000 >> [ 95.607519] RDX: 0000000000000001 RSI: 00007f76b5595000 RDI: fffff35645000000 >> [ 95.608128] RBP: 00007f76b5595000 R08: 0000000000000001 R09: 0000000000000000 >> [ 95.608731] R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000140000 >> [ 95.609327] R13: 0000000000020000 R14: 00007f76b5595000 R15: ffffa14e409b7f08 >> [ 95.609924] FS: 00007f76b577d580(0000) GS:ffff8f41bd400000(0000) knlGS:0000000000000000 >> [ 95.610599] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 >> [ 95.611083] CR2: fffffffffffffffe CR3: 0000000078960000 CR4: 00000000000006f0 >> [ 95.611686] Call Trace: >> [ 95.611906] proc_reg_read+0x3c/0x60 >> [ 95.612228] vfs_read+0xc5/0x180 >> [ 95.612505] ksys_read+0x68/0xe0 >> [ 95.612785] do_syscall_64+0x5c/0xa0 >> [ 95.613092] entry_SYSCALL_64_after_hwframe+0x49/0xbe >> >> Note that there are still two possible races as far as I can see: >> - pfn_to_online_page() succeeding but the memory getting offlined and >> removed. get_online_mems() could help once we run into this. >> - pfn_zone_device() succeeding but the memmap not being fully >> initialized yet. As the memmap is initialized outside of the memory >> hoptlug lock, get_online_mems() can't help. >> >> Let's keep the existing interfaces working with ZONE_DEVICE memory. We >> can later come back and fix these rare races and eventually speed-up the >> ZONE_DEVICE detection. > > Actually, Toshiki is writing code to refactor and optimize the pfn walking > part, where we find the pfn ranges covered by zone devices by running over > xarray pgmap_array and use the range info to reduce pointer dereferences > to speed up pfn walk. I hope he will share it soon. AFAIKT, Michal is not a friend of special-casing PFN walkers in that way. We should have a mechanism to detect if a memmap was initialized without having to go via pgmap, special-casing. See my other mail where I draft one basic approach. -- Thanks, David / dhildenb
