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Subject: Re: [PATCH 5.10.y 01/11] mm: memcontrol: Use helpers to read page's
 memcg data
To:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Roman Gushchin <guro@fb.com>
CC:     Muchun Song <songmuchun@bytedance.com>,
        Wang Hai <wanghai38@huawei.com>,
        <linux-kernel@vger.kernel.org>, <linux-mm@kvack.org>,
        <stable@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        "Alexei Starovoitov" <ast@kernel.org>
References: <20210816072147.3481782-1-chenhuang5@huawei.com>
 <20210816072147.3481782-2-chenhuang5@huawei.com> <YRojDsTAjSnw0jIh@kroah.com>
 <a4c545a8-fff0-38bb-4749-3483c9334daa@huawei.com>
 <YRppmvYOftjAAl/R@kroah.com>
 <0d3c6aa4-be05-3c93-bdcd-ac30788d82bd@huawei.com>
 <YRtT4obiu86hp6z3@kroah.com>
From:   Chen Huang <chenhuang5@huawei.com>
Message-ID: <9e946879-8a6e-6b86-9d8b-54a17976c6be@huawei.com>
Date:   Thu, 19 Aug 2021 19:43:37 +0800
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在 2021/8/17 14:14, Greg Kroah-Hartman 写道:
> On Tue, Aug 17, 2021 at 09:45:00AM +0800, Chen Huang wrote:
>>
>>
>> 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 <guro@fb.com>
>>>>>
>>>>> 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 <guro@fb.com>
>>>>>> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
>>>>>> Signed-off-by: Alexei Starovoitov <ast@kernel.org>
>>>>>> Reviewed-by: Shakeel Butt <shakeelb@google.com>
>>>>>> Acked-by: Johannes Weiner <hannes@cmpxchg.org>
>>>>>> Acked-by: Michal Hocko <mhocko@suse.com>
>>>>>> Link: https://lkml.kernel.org/r/20201027001657.3398190-1-guro@fb.com
>>>>>> Link: https://lkml.kernel.org/r/20201027001657.3398190-2-guro@fb.com
>>>>>> Link: https://lore.kernel.org/bpf/20201201215900.3569844-2-guro@fb.com
>>>>>>
>>>>>> 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!
> 
> And how can that be triggered by a user in the 5.10.y kernel tree at the
> moment?
> 
> I'm all for fixing problems, but this one does not seem like it is an
> actual issue for the 5.10 tree right now.  Am I missing something?
> 
> thanks,
> 
Sorry, it maybe just the problem of my own driver.
Please ignore the patchset.

Thanks!
> greg k-h
> 
> .
>