[PATCH v2 2/2] arm64: mm: hugetlb: add support for free vmemmap pages of HugeTLB

[Muchun Song]

On Tue, Feb 8, 2022 at 1:46 PM Muchun Song <songmuchun at bytedance.com> 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@bytedance.com/
> [2] https://lore.kernel.org/all/20210518091826.36937-1-songmuchun@bytedance.com/
>
> Signed-off-by: Muchun Song <songmuchun at bytedance.com>

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.