On Mon, Nov 23, 2020 at 3:40 PM Michal Hocko <mhocko@xxxxxxxx> wrote: > > On Fri 20-11-20 23:44:26, Muchun Song wrote: > > On Fri, Nov 20, 2020 at 9:11 PM Michal Hocko <mhocko@xxxxxxxx> wrote: > > > > > > On Fri 20-11-20 20:40:46, Muchun Song wrote: > > > > On Fri, Nov 20, 2020 at 4:42 PM Michal Hocko <mhocko@xxxxxxxx> wrote: > > > > > > > > > > On Fri 20-11-20 14:43:04, Muchun Song wrote: > > > > > [...] > > > > > > > > > > Thanks for improving the cover letter and providing some numbers. I have > > > > > only glanced through the patchset because I didn't really have more time > > > > > to dive depply into them. > > > > > > > > > > Overall it looks promissing. To summarize. I would prefer to not have > > > > > the feature enablement controlled by compile time option and the kernel > > > > > command line option should be opt-in. I also do not like that freeing > > > > > the pool can trigger the oom killer or even shut the system down if no > > > > > oom victim is eligible. > > > > > > > > Hi Michal, > > > > > > > > I have replied to you about those questions on the other mail thread. > > > > > > > > Thanks. > > > > > > > > > > > > > > One thing that I didn't really get to think hard about is what is the > > > > > effect of vmemmap manipulation wrt pfn walkers. pfn_to_page can be > > > > > invalid when racing with the split. How do we enforce that this won't > > > > > blow up? > > > > > > > > This feature depends on the CONFIG_SPARSEMEM_VMEMMAP, > > > > in this case, the pfn_to_page can work. The return value of the > > > > pfn_to_page is actually the address of it's struct page struct. > > > > I can not figure out where the problem is. Can you describe the > > > > problem in detail please? Thanks. > > > > > > struct page returned by pfn_to_page might get invalid right when it is > > > returned because vmemmap could get freed up and the respective memory > > > released to the page allocator and reused for something else. See? > > > > If the HugeTLB page is already allocated from the buddy allocator, > > the struct page of the HugeTLB can be freed? Does this exist? > > Nope, struct pages only ever get deallocated when the respective memory > (they describe) is hotremoved via hotplug. > > > If yes, how to free the HugeTLB page to the buddy allocator > > (cannot access the struct page)? > > But I do not follow how that relates to my concern above. Sorry. I shouldn't understand your concerns. vmemmap pages page frame +-----------+ mapping to +-----------+ | | -------------> | 0 | +-----------+ +-----------+ | | -------------> | 1 | +-----------+ +-----------+ | | -------------> | 2 | +-----------+ +-----------+ | | -------------> | 3 | +-----------+ +-----------+ | | -------------> | 4 | +-----------+ +-----------+ | | -------------> | 5 | +-----------+ +-----------+ | | -------------> | 6 | +-----------+ +-----------+ | | -------------> | 7 | +-----------+ +-----------+ In this patch series, we will free the page frame 2-7 to the buddy allocator. You mean that pfn_to_page can return invalid value when the pfn is the page frame 2-7? Thanks. > > -- > Michal Hocko > SUSE Labs -- Yours, Muchun
