On Mon, Nov 23, 2020 at 7:32 PM Michal Hocko <mhocko@xxxxxxxx> wrote: > > On Mon 23-11-20 19:16:18, Muchun Song wrote: > > On Mon, Nov 23, 2020 at 6:43 PM Michal Hocko <mhocko@xxxxxxxx> wrote: > > > > > > On Mon 23-11-20 18:36:33, Muchun Song wrote: > > > > On Mon, Nov 23, 2020 at 5:43 PM Michal Hocko <mhocko@xxxxxxxx> wrote: > > > > > > > > > > On Mon 23-11-20 16:53:53, Muchun Song wrote: > > > > > > 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. > > > > > > > > > > No I really mean that pfn_to_page will give you a struct page pointer > > > > > from pages which you release from the vmemmap page tables. Those pages > > > > > might get reused as soon sa they are freed to the page allocator. > > > > > > > > We will remap vmemmap pages 2-7 (virtual addresses) to page > > > > frame 1. And then we free page frame 2-7 to the buddy allocator. > > > > > > And this doesn't really happen in an atomic fashion from the pfn walker > > > POV, right? So it is very well possible that > > > > Yeah, you are right. But it may not be a problem for HugeTLB pages. > > Because in most cases, we only read the tail struct page and get the > > head struct page through compound_head() when the pfn is within > > a HugeTLB range. Right? > > Many pfn walkers would encounter the head page first and then skip over > the rest. Those should be reasonably safe. But there is no guarantee and > the fact that you need a valid page->compound_head which might get > scribbled over once you have the struct page makes this extremely > subtle. In this patch series, we can guarantee that the page->compound_head is always valid. Because we reuse the first tail page. Maybe you need to look closer at this series. Thanks. > > -- > > SUSE Labs -- Yours, Muchun
