On Wed, Apr 13, 2022 at 04:03:33PM +0530, Anshuman Khandual wrote: > > > 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> > Thanks for your review. Will update soon.
