On Tue, Feb 8, 2022 at 1:46 PM Muchun Song <songmuchun@xxxxxxxxxxxxx> wrote: > > The feature of minimizing overhead of struct page associated with each > HugeTLB page aims to free its vmemmap pages (used as struct page) to > save memory, where is ~14GB/16GB per 1TB HugeTLB pages (2MB/1GB type). > In short, when a HugeTLB page is allocated or freed, the vmemmap array > representing the range associated with the page will need to be remapped. > When a page is allocated, vmemmap pages are freed after remapping. > When a page is freed, previously discarded vmemmap pages must be > allocated before remapping. More implementations and details can be > found here [1]. > > The preparation of freeing vmemmap pages associated with each HugeTLB > page is ready, so we can support this feature for arm64 now. The > flush_dcache_page() need to be adapted to operate on the head page's > flags since the tail vmemmap pages are mapped with read-only after > the feature is enabled (clear operation is not permitted). > > There was some discussions about this in the thread [2], but there was > no conclusion in the end. And I copied the concern proposed by Anshuman > to here. > > 1st concern: > ''' > But what happens when a hot remove section's vmemmap area (which is > being teared down) is nearby another vmemmap area which is either created > or being destroyed for HugeTLB alloc/free purpose. As you mentioned > HugeTLB pages inside the hot remove section might be safe. But what about > other HugeTLB areas whose vmemmap area shares page table entries with > vmemmap entries for a section being hot removed ? Massive HugeTLB alloc > /use/free test cycle using memory just adjacent to a memory hotplug area, > which is always added and removed periodically, should be able to expose > this problem. > ''' > > Answer: At the time memory is removed, all HugeTLB pages either have been > migrated away or dissolved. So there is no race between memory hot remove > and free_huge_page_vmemmap(). Therefore, HugeTLB pages inside the hot > remove section is safe. Let's talk your question "what about other > HugeTLB areas whose vmemmap area shares page table entries with vmemmap > entries for a section being hot removed ?", the question is not > established. The minimal granularity size of hotplug memory 128MB (on > arm64, 4k base page), any HugeTLB smaller than 128MB is within a section, > then, there is no share PTE page tables between HugeTLB in this section > and ones in other sections and a HugeTLB page could not cross two > sections. In this case, the section cannot be freed. Any HugeTLB bigger > than 128MB (section size) whose vmemmap pages is an integer multiple of > 2MB (PMD-mapped). As long as: > > 1) HugeTLBs are naturally aligned, power-of-two sizes > 2) The HugeTLB size >= the section size > 3) The HugeTLB size >= the vmemmap leaf mapping size > > Then a HugeTLB will not share any leaf page table entries with *anything > else*, but will share intermediate entries. In this case, at the time memory > is removed, all HugeTLB pages either have been migrated away or dissolved. > So there is also no race between memory hot remove and > free_huge_page_vmemmap(). > > 2nd concern: > ''' > differently, not sure if ptdump would require any synchronization. > > Dumping an wrong value is probably okay but crashing because a page table > entry is being freed after ptdump acquired the pointer is bad. On arm64, > ptdump() is protected against hotremove via [get|put]_online_mems(). > ''' > > Answer: The ptdump should be fine since vmemmap_remap_free() only exchanges > PTEs or split the PMD entry (which means allocating a PTE page table). Both > operations do not free any page tables (PTE), so ptdump cannot run into a > UAF on any page tables. The wrost case is just dumping an wrong value. > > [1] https://lore.kernel.org/all/20210510030027.56044-1-songmuchun@xxxxxxxxxxxxx/ > [2] https://lore.kernel.org/all/20210518091826.36937-1-songmuchun@xxxxxxxxxxxxx/ > > Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx> Hi Mark, I have updated the commit suggested from you in the previous version, do you (or other maintainers) have any comments on this? Thanks.
