Re: BUG: KCSAN: data-race in folio_batch_move_lru / mpage_read_end_io
From: Mirsad Todorovac
Date: Mon Sep 18 2023 - 08:16:19 EST
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