On 3/31/22 12:26, Muchun Song 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 infrastructure of freeing vmemmap pages associated with each HugeTLB > page is already there, we can easily enable HUGETLB_PAGE_FREE_VMEMMAP > for arm64, the only thing to be fixed is flush_dcache_page() . > > 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 and explain why those concern is superfluous. It is safe to > enable it for x86_64 as well as arm64. > > 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 splits 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 worst 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> > --- > v4: > - Introduce ARCH_WANT_HUGETLB_PAGE_FREE_VMEMMAP (implemented in the previous > patch) to enable this feature for arm64. > > v3: > - Rework patch's subject. > - Clarify the feature of HUGETLB_PAGE_FREE_VMEMMAP is already there in the > current code and easyly be enabled for arm64 into commit log. > - Add hugetlb_free_vmemmap_enabled() check into flush_dcache_page(). > > Thanks for Barry's suggestions. > > v2: > - Update commit message (Mark Rutland). > - Fix flush_dcache_page(). > > arch/arm64/Kconfig | 1 + > arch/arm64/mm/flush.c | 13 +++++++++++++ > 2 files changed, 14 insertions(+) > > diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig > index c842878f8133..37f72e3a75d0 100644 > --- a/arch/arm64/Kconfig > +++ b/arch/arm64/Kconfig > @@ -94,6 +94,7 @@ config ARM64 > select ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT > select ARCH_WANT_FRAME_POINTERS > select ARCH_WANT_HUGE_PMD_SHARE if ARM64_4K_PAGES || (ARM64_16K_PAGES && !ARM64_VA_BITS_36) > + select ARCH_WANT_HUGETLB_PAGE_FREE_VMEMMAP > select ARCH_WANT_LD_ORPHAN_WARN > select ARCH_WANTS_NO_INSTR > select ARCH_HAS_UBSAN_SANITIZE_ALL > diff --git a/arch/arm64/mm/flush.c b/arch/arm64/mm/flush.c > index 2aaf950b906c..c67c1ca856c2 100644 > --- a/arch/arm64/mm/flush.c > +++ b/arch/arm64/mm/flush.c > @@ -68,6 +68,19 @@ EXPORT_SYMBOL_GPL(__sync_icache_dcache); > */ > void flush_dcache_page(struct page *page) > { > + /* > + * Only the head page's flags of HugeTLB can be cleared since the tail > + * vmemmap pages associated with each HugeTLB page are mapped with > + * read-only when CONFIG_HUGETLB_PAGE_FREE_VMEMMAP is enabled (more > + * details can refer to vmemmap_remap_pte()). Although > + * __sync_icache_dcache() only set PG_dcache_clean flag on the head > + * page struct, some tail page structs still can be seen the flag is > + * set since the head vmemmap page frame is reused (more details can > + * refer to the comments above page_fixed_fake_head()). > + */ > + if (hugetlb_free_vmemmap_enabled() && PageHuge(page)) > + page = compound_head(page); > + > if (test_bit(PG_dcache_clean, &page->flags)) > clear_bit(PG_dcache_clean, &page->flags); > } With restructuring above code comment inside flush_dcache_page(), Reviewed-by: Anshuman Khandual <anshuman.khandual@xxxxxxx>
