linux-kernel - Re: [PATCH 5.10.y 01/11] mm: memcontrol: Use helpers to read page's memcg data

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Date:   Mon, 16 Aug 2021 21:21:11 +0800
From:   Chen Huang <chenhuang5@...wei.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
CC:     Roman Gushchin <guro@...com>,
        Muchun Song <songmuchun@...edance.com>,
        "Wang Hai" <wanghai38@...wei.com>, <linux-kernel@...r.kernel.org>,
        <linux-mm@...ck.org>, <stable@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexei Starovoitov <ast@...nel.org>
Subject: Re: [PATCH 5.10.y 01/11] mm: memcontrol: Use helpers to read page's
 memcg data



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@...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@...com>
>> Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
>> Signed-off-by: Alexei Starovoitov <ast@...nel.org>
>> Reviewed-by: Shakeel Butt <shakeelb@...gle.com>
>> Acked-by: Johannes Weiner <hannes@...xchg.org>
>> Acked-by: Michal Hocko <mhocko@...e.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 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);
then we register the UIO driver.
	uio_register_device(&pdev->dev, info)

Thanks!

> thanks,
> 
> greg k-h
> 
> .
>