Date:   Thu, 3 Nov 2022 16:58:01 -0700
From:   Zach O'Keefe <zokeefe@...gle.com>
To:     Yang Shi <shy828301@...il.com>
Cc:     mhocko@...e.com, akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [v2 PATCH 1/2] mm: khugepaged: allow page allocation fallback to
 eligible nodes

On Nov 03 14:36, Yang Shi wrote:
> Syzbot reported the below splat:
> 
> WARNING: CPU: 1 PID: 3646 at include/linux/gfp.h:221 __alloc_pages_node include/linux/gfp.h:221 [inline]
> WARNING: CPU: 1 PID: 3646 at include/linux/gfp.h:221 hpage_collapse_alloc_page mm/khugepaged.c:807 [inline]
> WARNING: CPU: 1 PID: 3646 at include/linux/gfp.h:221 alloc_charge_hpage+0x802/0xaa0 mm/khugepaged.c:963
> Modules linked in:
> CPU: 1 PID: 3646 Comm: syz-executor210 Not tainted 6.1.0-rc1-syzkaller-00454-ga70385240892 #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/11/2022
> RIP: 0010:__alloc_pages_node include/linux/gfp.h:221 [inline]
> RIP: 0010:hpage_collapse_alloc_page mm/khugepaged.c:807 [inline]
> RIP: 0010:alloc_charge_hpage+0x802/0xaa0 mm/khugepaged.c:963
> Code: e5 01 4c 89 ee e8 6e f9 ae ff 4d 85 ed 0f 84 28 fc ff ff e8 70 fc ae ff 48 8d 6b ff 4c 8d 63 07 e9 16 fc ff ff e8 5e fc ae ff <0f> 0b e9 96 fa ff ff 41 bc 1a 00 00 00 e9 86 fd ff ff e8 47 fc ae
> RSP: 0018:ffffc90003fdf7d8 EFLAGS: 00010293
> RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
> RDX: ffff888077f457c0 RSI: ffffffff81cd8f42 RDI: 0000000000000001
> RBP: ffff888079388c0c R08: 0000000000000001 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
> R13: dffffc0000000000 R14: 0000000000000000 R15: 0000000000000000
> FS:  00007f6b48ccf700(0000) GS:ffff8880b9b00000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00007f6b48a819f0 CR3: 00000000171e7000 CR4: 00000000003506e0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> Call Trace:
>  <TASK>
>  collapse_file+0x1ca/0x5780 mm/khugepaged.c:1715
>  hpage_collapse_scan_file+0xd6c/0x17a0 mm/khugepaged.c:2156
>  madvise_collapse+0x53a/0xb40 mm/khugepaged.c:2611
>  madvise_vma_behavior+0xd0a/0x1cc0 mm/madvise.c:1066
>  madvise_walk_vmas+0x1c7/0x2b0 mm/madvise.c:1240
>  do_madvise.part.0+0x24a/0x340 mm/madvise.c:1419
>  do_madvise mm/madvise.c:1432 [inline]
>  __do_sys_madvise mm/madvise.c:1432 [inline]
>  __se_sys_madvise mm/madvise.c:1430 [inline]
>  __x64_sys_madvise+0x113/0x150 mm/madvise.c:1430
>  do_syscall_x64 arch/x86/entry/common.c:50 [inline]
>  do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
>  entry_SYSCALL_64_after_hwframe+0x63/0xcd
> RIP: 0033:0x7f6b48a4eef9
> Code: 28 00 00 00 75 05 48 83 c4 28 c3 e8 b1 15 00 00 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 b8 ff ff ff f7 d8 64 89 01 48
> RSP: 002b:00007f6b48ccf318 EFLAGS: 00000246 ORIG_RAX: 000000000000001c
> RAX: ffffffffffffffda RBX: 00007f6b48af0048 RCX: 00007f6b48a4eef9
> RDX: 0000000000000019 RSI: 0000000000600003 RDI: 0000000020000000
> RBP: 00007f6b48af0040 R08: 0000000000000000 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000246 R12: 00007f6b48aa53a4
> R13: 00007f6b48bffcbf R14: 00007f6b48ccf400 R15: 0000000000022000
>  </TASK>
> 
> The khugepaged code would pick up the node with the most hit as the preferred
> node, and also tries to do some balance if several nodes have the same
> hit record.  Basically it does conceptually:
>     * If the target_node <= last_target_node, then iterate from
> last_target_node + 1 to MAX_NUMNODES (1024 on default config)
>     * If the max_value == node_load[nid], then target_node = nid
> 
> But there is a corner case, paritucularly for MADV_COLLAPSE, that the
> non-existing node may be returned as preferred node.
> 
> Assuming the system has 2 nodes, the target_node is 0 and the
> last_target_node is 1, if MADV_COLLAPSE path is hit, the max_value may
> be 0, then it may return 2 for target_node, but it is actually not
> existing (offline), so the warn is triggered.
> 
> The node balance was introduced by commit 9f1b868a13ac ("mm: thp:
> khugepaged: add policy for finding target node") to satisfy
> "numactl --interleave=all".  But interleaving is a mere hint rather than
> something that has hard requirements.
> 
> So use nodemask to record the nodes which have the same hit record, the
> hugepage allocation could fallback to those nodes.  And remove
> __GFP_THISNODE since it does disallow fallback.  And if nodemask is
> empty (no node is set), it means there is one single node has the most
> hist record, the nodemask approach actually behaves like __GFP_THISNODE.
> 
> Reported-by: syzbot+0044b22d177870ee974f@...kaller.appspotmail.com
> Suggested-by: Zach O'Keefe <zokeefe@...gle.com>
> Suggested-by: Michal Hocko <mhocko@...e.com>
> Signed-off-by: Yang Shi <shy828301@...il.com>

Reviewed-by: Zach O'Keefe <zokeefe@...gel.com>

> ---
>  mm/khugepaged.c | 32 ++++++++++++++------------------
>  1 file changed, 14 insertions(+), 18 deletions(-)
> 
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index ea0d186bc9d4..572ce7dbf4b0 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -97,8 +97,8 @@ struct collapse_control {
>  	/* Num pages scanned per node */
>  	u32 node_load[MAX_NUMNODES];
>  
> -	/* Last target selected in hpage_collapse_find_target_node() */
> -	int last_target_node;
> +	/* nodemask for allocation fallback */
> +	nodemask_t alloc_nmask;
>  };
>  
>  /**
> @@ -734,7 +734,6 @@ static void khugepaged_alloc_sleep(void)
>  
>  struct collapse_control khugepaged_collapse_control = {
>  	.is_khugepaged = true,
> -	.last_target_node = NUMA_NO_NODE,
>  };
>  
>  static bool hpage_collapse_scan_abort(int nid, struct collapse_control *cc)
> @@ -783,16 +782,11 @@ static int hpage_collapse_find_target_node(struct collapse_control *cc)
>  			target_node = nid;
>  		}
>  
> -	/* do some balance if several nodes have the same hit record */
> -	if (target_node <= cc->last_target_node)
> -		for (nid = cc->last_target_node + 1; nid < MAX_NUMNODES;
> -		     nid++)
> -			if (max_value == cc->node_load[nid]) {
> -				target_node = nid;
> -				break;
> -			}
> +	for_each_online_node(nid) {
> +		if (max_value == cc->node_load[nid])
> +			node_set(nid, cc->alloc_nmask);
> +	}
>  
> -	cc->last_target_node = target_node;
>  	return target_node;
>  }
>  #else
> @@ -802,9 +796,10 @@ static int hpage_collapse_find_target_node(struct collapse_control *cc)
>  }
>  #endif
>  
> -static bool hpage_collapse_alloc_page(struct page **hpage, gfp_t gfp, int node)
> +static bool hpage_collapse_alloc_page(struct page **hpage, gfp_t gfp, int node,
> +				      nodemask_t *nmask)
>  {
> -	*hpage = __alloc_pages_node(node, gfp, HPAGE_PMD_ORDER);
> +	*hpage = __alloc_pages(gfp, HPAGE_PMD_ORDER, node, nmask);
>  	if (unlikely(!*hpage)) {
>  		count_vm_event(THP_COLLAPSE_ALLOC_FAILED);
>  		return false;
> @@ -955,12 +950,11 @@ static int __collapse_huge_page_swapin(struct mm_struct *mm,
>  static int alloc_charge_hpage(struct page **hpage, struct mm_struct *mm,
>  			      struct collapse_control *cc)
>  {
> -	/* Only allocate from the target node */
>  	gfp_t gfp = (cc->is_khugepaged ? alloc_hugepage_khugepaged_gfpmask() :
> -		     GFP_TRANSHUGE) | __GFP_THISNODE;
> +		     GFP_TRANSHUGE);
>  	int node = hpage_collapse_find_target_node(cc);
>  
> -	if (!hpage_collapse_alloc_page(hpage, gfp, node))
> +	if (!hpage_collapse_alloc_page(hpage, gfp, node, &cc->alloc_nmask))
>  		return SCAN_ALLOC_HUGE_PAGE_FAIL;
>  	if (unlikely(mem_cgroup_charge(page_folio(*hpage), mm, gfp)))
>  		return SCAN_CGROUP_CHARGE_FAIL;
> @@ -1144,6 +1138,7 @@ static int hpage_collapse_scan_pmd(struct mm_struct *mm,
>  		goto out;
>  
>  	memset(cc->node_load, 0, sizeof(cc->node_load));
> +	nodes_clear(cc->alloc_nmask);
>  	pte = pte_offset_map_lock(mm, pmd, address, &ptl);
>  	for (_address = address, _pte = pte; _pte < pte + HPAGE_PMD_NR;
>  	     _pte++, _address += PAGE_SIZE) {
> @@ -2078,6 +2073,7 @@ static int hpage_collapse_scan_file(struct mm_struct *mm, unsigned long addr,
>  	present = 0;
>  	swap = 0;
>  	memset(cc->node_load, 0, sizeof(cc->node_load));
> +	nodes_clear(cc->alloc_nmask);
>  	rcu_read_lock();
>  	xas_for_each(&xas, page, start + HPAGE_PMD_NR - 1) {
>  		if (xas_retry(&xas, page))
> @@ -2581,7 +2577,6 @@ int madvise_collapse(struct vm_area_struct *vma, struct vm_area_struct **prev,
>  	if (!cc)
>  		return -ENOMEM;
>  	cc->is_khugepaged = false;
> -	cc->last_target_node = NUMA_NO_NODE;
>  
>  	mmgrab(mm);
>  	lru_add_drain_all();
> @@ -2607,6 +2602,7 @@ int madvise_collapse(struct vm_area_struct *vma, struct vm_area_struct **prev,
>  		}
>  		mmap_assert_locked(mm);
>  		memset(cc->node_load, 0, sizeof(cc->node_load));
> +		nodes_clear(cc->alloc_nmask);
>  		if (IS_ENABLED(CONFIG_SHMEM) && vma->vm_file) {
>  			struct file *file = get_file(vma->vm_file);
>  			pgoff_t pgoff = linear_page_index(vma, addr);
> -- 
> 2.26.3
> 

Thanks for the patch, Yang! Looks good. khugepaged selftest is good too.

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