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

From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To: Chen Huang <chenhuang5@huawei.com>
Cc: Roman Gushchin <guro@fb.com>,
	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>
Subject: Re: [PATCH 5.10.y 01/11] mm: memcontrol: Use helpers to read page's memcg data
Date: Mon, 16 Aug 2021 15:35:22 +0200	[thread overview]
Message-ID: <YRppmvYOftjAAl/R@kroah.com> (raw)
In-Reply-To: <a4c545a8-fff0-38bb-4749-3483c9334daa@huawei.com>

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?

thanks,

greg k-h