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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: 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? > > Sorry for jumping in late. I agree that the patchset is quite intrusive and I really doubt we need to backport it. The main goal of my patchset was to enable memory accounting for bpf maps (which can be mmaped to userspace). I don't see why we need it otherwise. Muchun's patchset unifies the treatment of non-slab kernel objects (e.g. large kmallocs) with slab objects and prevents them to pin dying memory cgroups. However the problem existed for years and I doubt we need it in 5.10 so badly. > > 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. Can you, please, be more specific here? Thanks!