Re: BUG: KCSAN: data-race in folio_batch_move_lru / mpage_read_end_io

[Mirsad Todorovac <mirsad.todorovac@alu.unizg.hr>]

On 9/8/23 17:25, Matthew Wilcox wrote:
> On Thu, Aug 31, 2023 at 03:52:49PM +0100, Matthew Wilcox wrote:
>>>   read to 0xffffef9a44978bc0 of 8 bytes by task 348 on cpu 12:
>>>   folio_batch_move_lru (./include/linux/mm.h:1814 ./include/linux/mm.h:1824 ./include/linux/memcontrol.h:1636 ./include/linux/memcontrol.h:1659 mm/swap.c:216)
>>>   folio_batch_add_and_move (mm/swap.c:235)
>>>   folio_add_lru (./arch/x86/include/asm/preempt.h:95 mm/swap.c:518)
>>>   folio_add_lru_vma (mm/swap.c:538)
>>>   do_anonymous_page (mm/memory.c:4146)
>>
>> This is the part I don't understand.  The path to calling
>> folio_add_lru_vma() comes directly from vma_alloc_zeroed_movable_folio():
>>
> [snip]
>>
>> (sorry that's a lot of lines).  But there's _nowhere_ there that sets
>> PG_locked.  It's a freshly allocated page; all page flags (that are
>> actually flags; ignore the stuff up at the top) should be clear.  We
>> even check that with PAGE_FLAGS_CHECK_AT_PREP.  Plus, it doesn't
>> make sense that we'd start I/O; the page is freshly allocated, full of
>> zeroes; there's no backing store to read the page from.
>>
>> It really feels like this page was freed while it was still under I/O
>> and it's been reallocated to this victim process.
>>
>> I'm going to try a few things and see if I can figure this out.
> 
> I'm having trouble reproducing this.  Can you get it to happen reliably?
> 
> This is what I'm currently running with, and it doesn't trigger.
> I'd expect it to if we were going to hit the KCSAN bug.
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 0c5be12f9336..d22e8798c326 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -4439,6 +4439,7 @@ struct page *__alloc_pages(gfp_t gfp, unsigned int order, int preferred_nid,
>   	page = __alloc_pages_slowpath(alloc_gfp, order, &ac);
>   
>   out:
> +	VM_BUG_ON_PAGE(page && (page->flags & (PAGE_FLAGS_CHECK_AT_PREP &~ (1 << PG_head))), page);
>   	if (memcg_kmem_online() && (gfp & __GFP_ACCOUNT) && page &&
>   	    unlikely(__memcg_kmem_charge_page(page, gfp, order) != 0)) {
>   		__free_pages(page, order);

Hi,

Caught another instance of this bug involving folio_batch_move_lru: I don't seem that I can make it
happen reliably by the nature of the data racing conditions if I understood them well.

I have only found them in dmesg, I cannot determine what exactly the system was doing at that
spurious moment ...

Hope this will get some more light on the issue (6.6-rc2 vanilla torvalds tree kernel):

[  114.557937] ==================================================================
[  114.558262] BUG: KCSAN: data-race in btrfs_page_set_uptodate [btrfs] / folio_batch_move_lru

[  114.558902] write (marked) to 0xffffea0006f68f00 of 8 bytes by task 2678 on cpu 19:
[  114.558912] btrfs_page_set_uptodate (/home/marvin/linux/kernel/torvalds2/./arch/x86/include/asm/bitops.h:55 /home/marvin/linux/kernel/torvalds2/./include/asm-generic/bitops/instrumented-atomic.h:29 /home/marvin/linux/kernel/torvalds2/./include/linux/page-flags.h:741 /home/marvin/linux/kernel/torvalds2/./include/linux/page-flags.h:751 /home/marvin/linux/kernel/torvalds2/fs/btrfs/subpage.c:642) btrfs
[  114.559539] end_page_read (/home/marvin/linux/kernel/torvalds2/fs/btrfs/extent_io.c:445) btrfs
[  114.560166] end_bio_extent_readpage (/home/marvin/linux/kernel/torvalds2/fs/btrfs/extent_io.c:660) btrfs
[  114.560796] __btrfs_bio_end_io (/home/marvin/linux/kernel/torvalds2/fs/btrfs/bio.c:120) btrfs
[  114.561434] btrfs_orig_bbio_end_io (/home/marvin/linux/kernel/torvalds2/fs/btrfs/bio.c:164) btrfs
[  114.562079] btrfs_check_read_bio (/home/marvin/linux/kernel/torvalds2/fs/btrfs/bio.c:324) btrfs
[  114.562724] btrfs_end_bio_work (/home/marvin/linux/kernel/torvalds2/fs/btrfs/bio.c:359) btrfs
[  114.563360] process_one_work (/home/marvin/linux/kernel/torvalds2/kernel/workqueue.c:2630)
[  114.563371] worker_thread (/home/marvin/linux/kernel/torvalds2/kernel/workqueue.c:2697 /home/marvin/linux/kernel/torvalds2/kernel/workqueue.c:2784)
[  114.563381] kthread (/home/marvin/linux/kernel/torvalds2/kernel/kthread.c:388)
[  114.563390] ret_from_fork (/home/marvin/linux/kernel/torvalds2/arch/x86/kernel/process.c:147)
[  114.563401] ret_from_fork_asm (/home/marvin/linux/kernel/torvalds2/arch/x86/entry/entry_64.S:312)

[  114.563416] read to 0xffffea0006f68f00 of 8 bytes by task 3540 on cpu 20:
[  114.563426] folio_batch_move_lru (/home/marvin/linux/kernel/torvalds2/./include/linux/mm.h:1849 /home/marvin/linux/kernel/torvalds2/./include/linux/mm.h:1859 /home/marvin/linux/kernel/torvalds2/./include/linux/memcontrol.h:1639 /home/marvin/linux/kernel/torvalds2/./include/linux/memcontrol.h:1662 /home/marvin/linux/kernel/torvalds2/mm/swap.c:216)
[  114.563436] folio_batch_add_and_move (/home/marvin/linux/kernel/torvalds2/mm/swap.c:235)
[  114.563446] folio_add_lru (/home/marvin/linux/kernel/torvalds2/./arch/x86/include/asm/preempt.h:95 /home/marvin/linux/kernel/torvalds2/mm/swap.c:518)
[  114.563455] filemap_add_folio (/home/marvin/linux/kernel/torvalds2/mm/filemap.c:957)
[  114.563464] page_cache_ra_unbounded (/home/marvin/linux/kernel/torvalds2/mm/readahead.c:250)
[  114.563477] page_cache_ra_order (/home/marvin/linux/kernel/torvalds2/mm/readahead.c:547)
[  114.563490] ondemand_readahead (/home/marvin/linux/kernel/torvalds2/mm/readahead.c:669)
[  114.563499] page_cache_async_ra (/home/marvin/linux/kernel/torvalds2/mm/readahead.c:718)
[  114.563507] filemap_fault (/home/marvin/linux/kernel/torvalds2/mm/filemap.c:3227 /home/marvin/linux/kernel/torvalds2/mm/filemap.c:3281)
[  114.563518] __do_fault (/home/marvin/linux/kernel/torvalds2/mm/memory.c:4204)
[  114.563528] do_fault (/home/marvin/linux/kernel/torvalds2/mm/memory.c:4568 /home/marvin/linux/kernel/torvalds2/mm/memory.c:4705)
[  114.563538] __handle_mm_fault (/home/marvin/linux/kernel/torvalds2/mm/memory.c:3669 /home/marvin/linux/kernel/torvalds2/mm/memory.c:4978 /home/marvin/linux/kernel/torvalds2/mm/memory.c:5119)
[  114.563549] handle_mm_fault (/home/marvin/linux/kernel/torvalds2/mm/memory.c:5284)
[  114.563560] do_user_addr_fault (/home/marvin/linux/kernel/torvalds2/arch/x86/mm/fault.c:1413)
[  114.563572] exc_page_fault (/home/marvin/linux/kernel/torvalds2/./arch/x86/include/asm/paravirt.h:695 /home/marvin/linux/kernel/torvalds2/arch/x86/mm/fault.c:1513 /home/marvin/linux/kernel/torvalds2/arch/x86/mm/fault.c:1561)
[  114.563582] asm_exc_page_fault (/home/marvin/linux/kernel/torvalds2/./arch/x86/include/asm/idtentry.h:570)

[  114.563597] value changed: 0x0017ffffc0008101 -> 0x0017ffffc0008108

[  114.563612] Reported by Kernel Concurrency Sanitizer on:
[  114.563619] CPU: 20 PID: 3540 Comm: chrome Not tainted 6.6.0-rc2-kcsan-00003-g16819584c239-dirty #11
[  114.563630] Hardware name: ASRock X670E PG Lightning/X670E PG Lightning, BIOS 1.21 04/26/2023
[  114.563636] ==================================================================

Best regards,
Mirsad Todorovac