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From: Chen Huang <>
To: Greg Kroah-Hartman <>
Cc: Roman Gushchin <>,
	Muchun Song <>,
	"Wang Hai" <>, <>,
	<>, <>,
	Andrew Morton <>,
	Alexei Starovoitov <>
Subject: Re: [PATCH 5.10.y 01/11] mm: memcontrol: Use helpers to read page's memcg data
Date: Tue, 17 Aug 2021 09:45:00 +0800	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <YRppmvYOftjAAl/>

在 2021/8/16 21:35, Greg Kroah-Hartman 写道:
> On Mon, Aug 16, 2021 at 09:21:11PM +0800, Chen Huang wrote:
>> 在 2021/8/16 16:34, Greg Kroah-Hartman 写道:
>>> On Mon, Aug 16, 2021 at 07:21:37AM +0000, Chen Huang wrote:
>>>> From: Roman Gushchin <>
>>> What is the git commit id of this patch in Linus's tree?
>>>> Patch series "mm: allow mapping accounted kernel pages to userspace", v6.
>>>> Currently a non-slab kernel page which has been charged to a memory cgroup
>>>> can't be mapped to userspace.  The underlying reason is simple: PageKmemcg
>>>> flag is defined as a page type (like buddy, offline, etc), so it takes a
>>>> bit from a page->mapped counter.  Pages with a type set can't be mapped to
>>>> userspace.
>>>> But in general the kmemcg flag has nothing to do with mapping to
>>>> userspace.  It only means that the page has been accounted by the page
>>>> allocator, so it has to be properly uncharged on release.
>>>> Some bpf maps are mapping the vmalloc-based memory to userspace, and their
>>>> memory can't be accounted because of this implementation detail.
>>>> This patchset removes this limitation by moving the PageKmemcg flag into
>>>> one of the free bits of the page->mem_cgroup pointer.  Also it formalizes
>>>> accesses to the page->mem_cgroup and page->obj_cgroups using new helpers,
>>>> adds several checks and removes a couple of obsolete functions.  As the
>>>> result the code became more robust with fewer open-coded bit tricks.
>>>> This patch (of 4):
>>>> Currently there are many open-coded reads of the page->mem_cgroup pointer,
>>>> as well as a couple of read helpers, which are barely used.
>>>> It creates an obstacle on a way to reuse some bits of the pointer for
>>>> storing additional bits of information.  In fact, we already do this for
>>>> slab pages, where the last bit indicates that a pointer has an attached
>>>> vector of objcg pointers instead of a regular memcg pointer.
>>>> This commits uses 2 existing helpers and introduces a new helper to
>>>> converts all read sides to calls of these helpers:
>>>>   struct mem_cgroup *page_memcg(struct page *page);
>>>>   struct mem_cgroup *page_memcg_rcu(struct page *page);
>>>>   struct mem_cgroup *page_memcg_check(struct page *page);
>>>> page_memcg_check() is intended to be used in cases when the page can be a
>>>> slab page and have a memcg pointer pointing at objcg vector.  It does
>>>> check the lowest bit, and if set, returns NULL.  page_memcg() contains a
>>>> VM_BUG_ON_PAGE() check for the page not being a slab page.
>>>> To make sure nobody uses a direct access, struct page's
>>>> mem_cgroup/obj_cgroups is converted to unsigned long memcg_data.
>>>> Signed-off-by: Roman Gushchin <>
>>>> Signed-off-by: Andrew Morton <>
>>>> Signed-off-by: Alexei Starovoitov <>
>>>> Reviewed-by: Shakeel Butt <>
>>>> Acked-by: Johannes Weiner <>
>>>> Acked-by: Michal Hocko <>
>>>> Link:
>>>> Link:
>>>> Link:
>>>> Conflicts:
>>>> 	mm/memcontrol.c
>>> The "Conflicts:" lines should be removed.
>>> Please fix up the patch series and resubmit.  But note, this seems
>>> really intrusive, are you sure these are all needed?
>> OK,I will resend the patchset.
>> Roman Gushchin's patchset formalize accesses to the page->mem_cgroup and
>> page->obj_cgroups. But for LRU pages and most other raw memcg, they may
>> pin to a memcg cgroup pointer, which should always point to an object cgroup
>> pointer. That's the problem I met. And Muchun Song's patchset fix this.
>> So I think these are all needed.
> What in-tree driver causes this to happen and under what workload?
>>> What UIO driver are you using that is showing problems like this?
>> The UIO driver is my own driver, and it's creation likes this:
>> First, we register a device
>> 	pdev = platform_device_register_simple("uio_driver,0, NULL, 0);
>> and use uio_info to describe the UIO driver, the page is alloced and used
>> for uio_vma_fault
>> 	info->mem[0].addr = (phys_addr_t) kzalloc(PAGE_SIZE, GFP_ATOMIC);
> That is not a physical address, and is not what the uio api is for at
> all.  Please do not abuse it that way.
>> then we register the UIO driver.
>> 	uio_register_device(&pdev->dev, info)
> So no in-tree drivers are having problems with the existing code, only
> fake ones?

Yes, but the nullptr porblem may not just about uio driver. For now, page struct
has a union
union {
	struct mem_cgroup *mem_cgroup;
	struct obj_cgroup **obj_cgroups;
For the slab pages, the union info should belong to obj_cgroups. And for user
pages, it should belong to mem_cgroup. When a slab page changes its obj_cgroups,
then another user page which is in the same compound page of that slab page will
gets the wrong mem_cgroup in __mod_lruvec_page_state(), and will trigger nullptr
in mem_cgroup_lruvec(). Correct me if I'm wrong. Thanks!

static inline void __mod_lruvec_page_state(struct page *page,
                                           enum node_stat_item idx, int val)
        struct page *head = compound_head(page); /* rmap on tail pages */
        pg_data_t *pgdat = page_pgdat(page);
        struct lruvec *lruvec;

        /* Untracked pages have no memcg, no lruvec. Update only the node */
        if (!head->mem_cgroup) {
                __mod_node_page_state(pgdat, idx, val);

        lruvec = mem_cgroup_lruvec(head->mem_cgroup, pgdat);
        __mod_lruvec_state(lruvec, idx, val);

> thanks,
> greg k-h
> .

  reply	other threads:[~2021-08-17  1:45 UTC|newest]

Thread overview: 20+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-16  7:21 [PATCH 5.10.y 00/11] mm: memcontrol: fix nullptr in __mod_lruvec_page_state() Chen Huang
2021-08-16  7:21 ` [PATCH 5.10.y 01/11] mm: memcontrol: Use helpers to read page's memcg data Chen Huang
2021-08-16  8:34   ` Greg Kroah-Hartman
2021-08-16 13:21     ` Chen Huang
2021-08-16 13:35       ` Greg Kroah-Hartman
2021-08-17  1:45         ` Chen Huang [this message]
2021-08-17  6:14           ` Greg Kroah-Hartman
2021-08-19 11:43             ` Chen Huang
2021-08-19 14:55               ` Greg Kroah-Hartman
2021-08-18  2:02       ` Roman Gushchin
2021-08-16  7:21 ` [PATCH 5.10.y 02/11] mm: memcontrol/slab: Use helpers to access slab page's memcg_data Chen Huang
2021-08-16  7:21 ` [PATCH 5.10.y 03/11] mm: Introduce page memcg flags Chen Huang
2021-08-16  7:21 ` [PATCH 5.10.y 04/11] mm: Convert page kmemcg type to a page memcg flag Chen Huang
2021-08-16  7:21 ` [PATCH 5.10.y 05/11] mm: memcontrol: introduce obj_cgroup_{un}charge_pages Chen Huang
2021-08-16  7:21 ` [PATCH 5.10.y 06/11] mm: memcontrol: directly access page->memcg_data in mm/page_alloc.c Chen Huang
2021-08-16  7:21 ` [PATCH 5.10.y 07/11] mm: memcontrol: change ug->dummy_page only if memcg changed Chen Huang
2021-08-16  7:21 ` [PATCH 5.10.y 08/11] mm: memcontrol: use obj_cgroup APIs to charge kmem pages Chen Huang
2021-08-16  7:21 ` [PATCH 5.10.y 09/11] mm: memcontrol: inline __memcg_kmem_{un}charge() into obj_cgroup_{un}charge_pages() Chen Huang
2021-08-16  7:21 ` [PATCH 5.10.y 10/11] mm: memcontrol: move PageMemcgKmem to the scope of CONFIG_MEMCG_KMEM Chen Huang
2021-08-16  7:21 ` [PATCH 5.10.y 11/11] mm/memcg: fix NULL pointer dereference in memcg_slab_free_hook() Chen Huang

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