Re: [PATCH v2 1/2] arm64: mm: vmemmap populate to page level if not section aligned

[Zhenhua Huang <quic_zhenhuah@xxxxxxxxxxx>]

On 2025/1/2 11:51, Anshuman Khandual wrote:
> On 12/30/24 13:18, Zhenhua Huang wrote:
> > Hi Anshuman,
> >
> > On 2024/12/27 15:49, Anshuman Khandual wrote:
> > > On 12/24/24 19:39, Catalin Marinas wrote:
> > > > On Tue, Dec 24, 2024 at 05:32:06PM +0800, Zhenhua Huang wrote:
> > > > > Thanks Catalin for review!
> > > > > Merry Christmas.
> > > >
> > > > Merry Christmas to you too!
> > > >
> > > > > On 2024/12/21 2:30, Catalin Marinas wrote:
> > > > > > On Mon, Dec 09, 2024 at 05:42:26PM +0800, Zhenhua Huang wrote:
> > > > > > > Fixes: c1cc1552616d ("arm64: MMU initialisation")
> > > > > >
> > > > > > I wouldn't add a fix for the first commit adding arm64 support, we did
> > > > > > not even have memory hotplug at the time (added later in 5.7 by commit
> > > > > > bbd6ec605c0f ("arm64/mm: Enable memory hot remove")). IIUC, this hasn't
> > > > > > been a problem until commit ba72b4c8cf60 ("mm/sparsemem: support
> > > > > > sub-section hotplug"). That commit broke some arm64 assumptions.
> > > > >
> > > > > Shall we add ba72b4c8cf60 ("mm/sparsemem: support sub-section hotplug")
> > > > > because it broke arm64 assumptions ?
> > > >
> > > > Yes, I think that would be better. And a cc stable to 5.4 (the above
> > > > commit appeared in 5.3).
> > >
> > > Agreed. This is a problem which needs fixing but not sure if proposed patch
> > > here fixes that problem.
> > >
> > > > > > > diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> > > > > > > index e2739b69e11b..fd59ee44960e 100644
> > > > > > > --- a/arch/arm64/mm/mmu.c
> > > > > > > +++ b/arch/arm64/mm/mmu.c
> > > > > > > @@ -1177,7 +1177,9 @@ int __meminit vmemmap_populate(unsigned long start, unsigned long end, int node,
> > > > > > >     {
> > > > > > >         WARN_ON((start < VMEMMAP_START) || (end > VMEMMAP_END));
> > > > > > > -    if (!IS_ENABLED(CONFIG_ARM64_4K_PAGES))
> > > > > > > +    if (!IS_ENABLED(CONFIG_ARM64_4K_PAGES) ||
> > > > > > > +    !IS_ALIGNED(page_to_pfn((struct page *)start), PAGES_PER_SECTION) ||
> > > > > > > +    !IS_ALIGNED(page_to_pfn((struct page *)end), PAGES_PER_SECTION))
> > > > > > >             return vmemmap_populate_basepages(start, end, node, altmap);
> > > > > > >         else
> > > > > > >             return vmemmap_populate_hugepages(start, end, node, altmap);
> > > > > >
> > > > > > An alternative would be to fix unmap_hotplug_pmd_range() etc. to avoid
> > > > > > nuking the whole vmemmap pmd section if it's not empty. Not sure how
> > > > > > easy that is, whether we have the necessary information (I haven't
> > > > > > looked in detail).
> > > > > >
> > > > > > A potential issue - can we hotplug 128MB of RAM and only unplug 2MB? If
> > > > > > that's possible, the problem isn't solved by this patch.
> > >
> > > Believe this is possible after sub-section hotplug and hotremove support.
> > >
> > > > > Indeed, seems there is no guarantee that plug size must be equal to unplug
> > > > > size...
> > > > >
> > > > > I have two ideas:
> > > > > 1. Completely disable this PMD mapping optimization since there is no
> > > > > guarantee we must align 128M memory for hotplug ..
> > > >
> > > > I'd be in favour of this, at least if CONFIG_MEMORY_HOTPLUG is enabled.
> > > > I think the only advantage here is that we don't allocate a full 2MB
> > > > block for vmemmap when only plugging in a sub-section.
> > >
> > > Agreed, that will be the right fix for the problem which can be back ported.
> > > We will have to prevent PMD/PUD/CONT mappings for both linear and as well as
> >
> > Thanks Anshuman, yeah.. we must handle linear mapping as well.
> >
> > > vmemmap for all non-boot memory sections, that can be hot-unplugged.
> > >
> > > Something like the following ? [untested]
> > >
> > > diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> > > index 216519663961..56b9c6891f46 100644
> > > --- a/arch/arm64/mm/mmu.c
> > > +++ b/arch/arm64/mm/mmu.c
> > > @@ -1171,9 +1171,15 @@ int __meminit vmemmap_check_pmd(pmd_t *pmdp, int node,
> > >    int __meminit vmemmap_populate(unsigned long start, unsigned long end, int node,
> > >                   struct vmem_altmap *altmap)
> > >    {
> > > +       unsigned long start_pfn;
> > > +       struct mem_section *ms;
> > > +
> > >           WARN_ON((start < VMEMMAP_START) || (end > VMEMMAP_END));
> > >    -       if (!IS_ENABLED(CONFIG_ARM64_4K_PAGES))
> > > +       start_pfn = page_to_pfn((struct page *)start);
> > > +       ms = __pfn_to_section(start_pfn);
> > > +
> > > +       if (!IS_ENABLED(CONFIG_ARM64_4K_PAGES) || !early_section(ms))
> >
> > LGTM. I will follow your and Catalin's suggestion to prepare further patches, Thanks!
> >
> > >                   return vmemmap_populate_basepages(start, end, node, altmap);
> > >           else
> > >                   return vmemmap_populate_hugepages(start, end, node, altmap);
> > > @@ -1334,10 +1340,15 @@ struct range arch_get_mappable_range(void)
> > >    int arch_add_memory(int nid, u64 start, u64 size,
> > >                       struct mhp_params *params)
> > >    {
> > > +       unsigned long start_pfn = page_to_pfn((struct page *)start);
> > > +       struct mem_section *ms = __pfn_to_section(start_pfn);
> > >           int ret, flags = NO_EXEC_MAPPINGS;
> > >             VM_BUG_ON(!mhp_range_allowed(start, size, true));
> > >    +       if (!early_section(ms))
> > > +               flags |= NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS;
> >
> > However, here comes another doubt, given that the subsection size is 2M, shouldn't we have ability to support PMD SECTION MAPPING if CONFIG_ARM64_4K_PAGES? This might be the optimization we want to maintain?>
> > Should we remove NO_BLOCK_MAPPINGS and add more constraints to avoid pud_set_huge if CONFIG_ARM64_4K_PAGES ?
>
> I guess this has been covered on the thread.

Yeah, the example I shared on separated thread, we can wrap up the 
discussion here :)

> > > +
> > >           if (can_set_direct_map())
> > >                   flags |= NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS;
> > >   
> > > > > 2. If we want to take this optimization.
> > > > > I propose adding one argument to vmemmap_free to indicate if the entire
> > > > > section is freed(based on subsection map). Vmemmap_free is a common function
> > > > > and might affect other architectures... The process would be:
> > > > > vmemmap_free
> > > > >      unmap_hotplug_range //In unmap_hotplug_pmd_range() as you mentioned：if
> > > > > whole section is freed, proceed as usual. Otherwise, *just clear out struct
> > > > > page content but do not free*.
> > > > >      free_empty_tables // will be called only if entire section is freed
> > > > >
> > > > > On the populate side,
> > > > > else if (vmemmap_check_pmd(pmd, node, addr, next)) //implement this function
> > > > >      continue;    //Buffer still exists, just abort..
> > > > >
> > > > > Could you please comment further whether #2 is feasible ?
> > > >
> > > > vmemmap_free() already gets start/end, so it could at least check the
> > > > alignment and avoid freeing if it's not unplugging a full section. It
> > >
> > > unmap_hotplug_pmd_range()
> > > {
> > >      do {
> > >          if (pmd_sect(pmd)) {
> > >              pmd_clear(pmdp);
> > >              flush_tlb_kernel_range(addr, addr + PAGE_SIZE);
> > >                           if (free_mapped)
> > >                                   free_hotplug_page_range(pmd_page(pmd),
> > >                                                           PMD_SIZE, altmap);
> > >          }
> > >      } while ()
> > > }
> > >
> > > Do you mean clearing the PMD entry but not freeing the mapped page for vmemmap ?
> > > In that case should the hot-unplug fail or not ? If we free the pfns (successful
> > > hot-unplug), then leaving behind entire PMD entry for covering the remaining sub
> > > sections, is going to be problematic as it still maps the removed pfns as well !
> >
> > Could you please help me to understand in which scenarios this might cause issue? I assume we won't touch these struct page further?
>
> Regardless of whether the non-present pfns are accessed or not from the cpu, having
> page table mappings covering them might probably create corresponding TLB entries ?
> IIUC from an arch perspective, this seems undesirable and possibly some what risky.

Got it.. I know it's tricky indeed.

> > > > does leave a 2MB vmemmap block in place when freeing the last subsection
> > > > but it's safer than freeing valid struct page entries. In addition, it
> > > > could query the memory hotplug state with something like
> > > > find_memory_block() and figure out whether the section is empty.
> > >
> > > I guess there are two potential solutions, if unmap_hotplug_pmd_range() were to
> > > handle sub-section removal.
> > >
> > > 1) Skip pmd_clear() when entire section is not covered
> > >
> > > a. pmd_clear() only if all but the current subsection have been removed earlier
> > >      via is_subsection_map_empty() or something similar.
> > >
> > > b. Skip pmd_clear() if the entire section covering that PMD is not being removed
> > >      but that might be problematic, as it still maps potentially unavailable pfns,
> > >      which are now hot-unplugged out.
> > >
> > > 2) Break PMD into base pages
> > >
> > > a. pmd_clear() only if all but the current subsection have been removed earlier
> > >      via is_subsection_map_empty() or something similar.
> > >
> > > b. Break entire PMD into base page mappings and remove entries corresponding to
> > >      the subsection being removed. Although the BBM sequence needs to be followed
> > >      while making sure that no other part of the kernel is accessing subsections,
> > >      that are mapped via the erstwhile PMD but currently not being removed.
> > >
> > > > Anyway, I'll be off until the new year, maybe I get other ideas by then.