Re: [PATCH -next] mm/hotplug: skip bad PFNs from pfn_to_online_page()

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On 6/14/19 9:52 PM, Dan Williams wrote:
On Fri, Jun 14, 2019 at 9:18 AM Aneesh Kumar K.V
<aneesh.kumar@xxxxxxxxxxxxx> wrote:

On 6/14/19 9:05 PM, Oscar Salvador wrote:
On Fri, Jun 14, 2019 at 02:28:40PM +0530, Aneesh Kumar K.V wrote:
Can you check with this change on ppc64.  I haven't reviewed this series yet.
I did limited testing with change . Before merging this I need to go
through the full series again. The vmemmap poplulate on ppc64 needs to
handle two translation mode (hash and radix). With respect to vmemap
hash doesn't setup a translation in the linux page table. Hence we need
to make sure we don't try to setup a mapping for a range which is
arleady convered by an existing mapping.

diff --git a/arch/powerpc/mm/init_64.c b/arch/powerpc/mm/init_64.c
index a4e17a979e45..15c342f0a543 100644
--- a/arch/powerpc/mm/init_64.c
+++ b/arch/powerpc/mm/init_64.c
@@ -88,16 +88,23 @@ static unsigned long __meminit vmemmap_section_start(unsigned long page)
    * which overlaps this vmemmap page is initialised then this page is
    * initialised already.
    */
-static int __meminit vmemmap_populated(unsigned long start, int page_size)
+static bool __meminit vmemmap_populated(unsigned long start, int page_size)
   {
      unsigned long end = start + page_size;
      start = (unsigned long)(pfn_to_page(vmemmap_section_start(start)));

-    for (; start < end; start += (PAGES_PER_SECTION * sizeof(struct page)))
-            if (pfn_valid(page_to_pfn((struct page *)start)))
-                    return 1;
+    for (; start < end; start += (PAGES_PER_SECTION * sizeof(struct page))) {

-    return 0;
+            struct mem_section *ms;
+            unsigned long pfn = page_to_pfn((struct page *)start);
+
+            if (pfn_to_section_nr(pfn) >= NR_MEM_SECTIONS)
+                    return 0;

I might be missing something, but is this right?
Having a section_nr above NR_MEM_SECTIONS is invalid, but if we return 0 here,
vmemmap_populate will go on and populate it.

I should drop that completely. We should not hit that condition at all.
I will send a final patch once I go through the full patch series making
sure we are not breaking any ppc64 details.

Wondering why we did the below

#if defined(ARCH_SUBSECTION_SHIFT)
#define SUBSECTION_SHIFT (ARCH_SUBSECTION_SHIFT)
#elif defined(PMD_SHIFT)
#define SUBSECTION_SHIFT (PMD_SHIFT)
#else
/*
   * Memory hotplug enabled platforms avoid this default because they
   * either define ARCH_SUBSECTION_SHIFT, or PMD_SHIFT is a constant, but
   * this is kept as a backstop to allow compilation on
   * !ARCH_ENABLE_MEMORY_HOTPLUG archs.
   */
#define SUBSECTION_SHIFT 21
#endif

why not

#if defined(ARCH_SUBSECTION_SHIFT)
#define SUBSECTION_SHIFT (ARCH_SUBSECTION_SHIFT)
#else
#define SUBSECTION_SHIFT  SECTION_SHIFT

That should be SECTION_SIZE_SHIFT

#endif

ie, if SUBSECTION is not supported by arch we have one sub-section per
section?

A couple comments:

The only reason ARCH_SUBSECTION_SHIFT exists is because PMD_SHIFT on
PowerPC was a non-constant value. However, I'm planning to remove the
distinction in the next rev of the patches. Jeff rightly points out
that having a variable subsection size per arch will lead to
situations where persistent memory namespaces are not portable across
archs. So I plan to just make SUBSECTION_SHIFT 21 everywhere.



persistent memory namespaces are not portable across archs because they have PAGE_SIZE dependency. Then we have dependencies like the page size with which we map the vmemmap area. Why not let the arch arch decide the SUBSECTION_SHIFT and default to one subsection per section if arch is not enabled to work with subsection.

-aneesh




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