Re: [v3 PATCH 2/2] mm: mempolicy: handle vma with unmovable pages mapped correctly in mbind

[Vlastimil Babka]

On 7/18/19 7:17 PM, Yang Shi wrote:
> When running syzkaller internally, we ran into the below bug on 4.9.x
> kernel:
> 
> kernel BUG at mm/huge_memory.c:2124!
> invalid opcode: 0000 [#1] SMP KASAN
> Dumping ftrace buffer:
>    (ftrace buffer empty)
> Modules linked in:
> CPU: 0 PID: 1518 Comm: syz-executor107 Not tainted 4.9.168+ #2
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 0.5.1 01/01/2011
> task: ffff880067b34900 task.stack: ffff880068998000
> RIP: 0010:[<ffffffff81895d6b>]  [<ffffffff81895d6b>] split_huge_page_to_list+0x8fb/0x1030 mm/huge_memory.c:2124
> RSP: 0018:ffff88006899f980  EFLAGS: 00010286
> RAX: 0000000000000000 RBX: ffffea00018f1700 RCX: 0000000000000000
> RDX: 1ffffd400031e2e7 RSI: 0000000000000001 RDI: ffffea00018f1738
> RBP: ffff88006899f9e8 R08: 0000000000000001 R09: 0000000000000000
> R10: 0000000000000000 R11: fffffbfff0d8b13e R12: ffffea00018f1400
> R13: ffffea00018f1400 R14: ffffea00018f1720 R15: ffffea00018f1401
> FS:  00007fa333996740(0000) GS:ffff88006c600000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 0000000020000040 CR3: 0000000066b9c000 CR4: 00000000000606f0
> Stack:
>  0000000000000246 ffff880067b34900 0000000000000000 ffff88007ffdc000
>  0000000000000000 ffff88006899f9e8 ffffffff812b4015 ffff880064c64e18
>  ffffea00018f1401 dffffc0000000000 ffffea00018f1700 0000000020ffd000
> Call Trace:
>  [<ffffffff818490f1>] split_huge_page include/linux/huge_mm.h:100 [inline]
>  [<ffffffff818490f1>] queue_pages_pte_range+0x7e1/0x1480 mm/mempolicy.c:538
>  [<ffffffff817ed0da>] walk_pmd_range mm/pagewalk.c:50 [inline]
>  [<ffffffff817ed0da>] walk_pud_range mm/pagewalk.c:90 [inline]
>  [<ffffffff817ed0da>] walk_pgd_range mm/pagewalk.c:116 [inline]
>  [<ffffffff817ed0da>] __walk_page_range+0x44a/0xdb0 mm/pagewalk.c:208
>  [<ffffffff817edb94>] walk_page_range+0x154/0x370 mm/pagewalk.c:285
>  [<ffffffff81844515>] queue_pages_range+0x115/0x150 mm/mempolicy.c:694
>  [<ffffffff8184f493>] do_mbind mm/mempolicy.c:1241 [inline]
>  [<ffffffff8184f493>] SYSC_mbind+0x3c3/0x1030 mm/mempolicy.c:1370
>  [<ffffffff81850146>] SyS_mbind+0x46/0x60 mm/mempolicy.c:1352
>  [<ffffffff810097e2>] do_syscall_64+0x1d2/0x600 arch/x86/entry/common.c:282
>  [<ffffffff82ff6f93>] entry_SYSCALL_64_after_swapgs+0x5d/0xdb
> Code: c7 80 1c 02 00 e8 26 0a 76 01 <0f> 0b 48 c7 c7 40 46 45 84 e8 4c
> RIP  [<ffffffff81895d6b>] split_huge_page_to_list+0x8fb/0x1030 mm/huge_memory.c:2124
>  RSP <ffff88006899f980>

...

> @@ -532,7 +531,14 @@ static int queue_pages_pte_range(pmd_t *pmd, unsigned long addr,
>  				has_unmovable |= true;
>  				break;
>  			}
> -			migrate_page_add(page, qp->pagelist, flags);
> +
> +			/*
> +			 * Do not abort immediately since there may be
> +			 * temporary off LRU pages in the range.  Still
> +			 * need migrate other LRU pages.
> +			 */
> +			if (migrate_page_add(page, qp->pagelist, flags))
> +				has_unmovable |= true;

Also = instead of |=

>  		} else
>  			break;
>  	}
> @@ -961,10 +967,21 @@ static long do_get_mempolicy(int *policy, nodemask_t *nmask,
>  /*
>   * page migration, thp tail pages can be passed.
>   */
> -static void migrate_page_add(struct page *page, struct list_head *pagelist,
> +static int migrate_page_add(struct page *page, struct list_head *pagelist,
>  				unsigned long flags)
>  {
>  	struct page *head = compound_head(page);
> +
> +	/*
> +	 * Non-movable page may reach here.  And, there may be
> +	 * temporary off LRU pages or non-LRU movable pages.
> +	 * Treat them as unmovable pages since they can't be
> +	 * isolated, so they can't be moved at the moment.  It
> +	 * should return -EIO for this case too.
> +	 */
> +	if (!PageLRU(head) && (flags & MPOL_MF_STRICT))
> +		return -EIO;

As this test is racy, why not just use the result of isolate_lru_page().

> +
>  	/*
>  	 * Avoid migrating a page that is shared with others.
>  	 */
> @@ -976,6 +993,8 @@ static void migrate_page_add(struct page *page, struct list_head *pagelist,
>  				hpage_nr_pages(head));
>  		}
>  	}
> +
> +	return 0;
>  }
>  
>  /* page allocation callback for NUMA node migration */
> @@ -1178,9 +1197,10 @@ static struct page *new_page(struct page *page, unsigned long start)
>  }
>  #else
>  
> -static void migrate_page_add(struct page *page, struct list_head *pagelist,
> +static int migrate_page_add(struct page *page, struct list_head *pagelist,
>  				unsigned long flags)
>  {
> +	return -EIO;
>  }
>  
>  int do_migrate_pages(struct mm_struct *mm, const nodemask_t *from,
>