[Crash-utility] [PATCH] Speed up "kmem -[sS]" by optimizing is_page_ptr()

[Dave Anderson]

Hi Kasuhito,

Just as a follow-up review of this part of your original patch:

  +#ifdef VMEMMAP_VADDR
  +               nr = nr_mem_sections;
  +               if (machdep->flags & VMEMMAP)
  +                       nr = pfn_to_section_nr((addr - VMEMMAP_VADDR) / SIZE(page));
  +               else if (readmem(addr + OFFSET(page_flags), KVADDR, &flags,
  +                       sizeof(ulong), "page.flags", RETURN_ON_ERROR|QUIET))
  +                       nr = (flags >> (SIZE(page_flags)*8 - SECTIONS_SHIFT())
  +                               & ((1UL << SECTIONS_SHIFT()) - 1));
  +
  +               if (nr < nr_mem_sections) {
  +#else
                  for (nr = 0; nr < nr_mem_sections ; nr++) {
  +#endif
  
One of my original concerns was associated with the backwards-compatiblity
of the non-VMEMMAP page->flags usage, primarily because it has changed 
over the years.  Perhaps the SECTIONS_SHIFT part has remained the same, 
but depending upon its future stability in this function still worries me.

But more importantly, the VMEMMAP section of your patch fails on 
architectures like ARM64 which have a phys_offset that needs to be
recognized w/respect to physical addresses.  For example, here's an 
ARM64 system whose physical addressing starts at 0x4000000000:

  crash> help -m | grep phys_offset
             phys_offset: 4000000000
  crash> kmem -p | head
        PAGE        PHYSICAL      MAPPING       INDEX CNT FLAGS
  ffff7e0000000000 4000000000                0        0  0 0
  ffff7e0000000040 4000001000                0        0  0 0
  ffff7e0000000080 4000002000                0        0  0 0
  ffff7e00000000c0 4000003000                0        0  0 0
  ffff7e0000000100 4000004000                0        0  0 0
  ffff7e0000000140 4000005000                0        0  0 0
  ffff7e0000000180 4000006000                0        0  0 0
  ffff7e00000001c0 4000007000                0        0  0 0
  ffff7e0000000200 4000008000                0        0  0 0
  crash>

Your patch presumes that the pfn can calculated by using 
(addr - VMEMMAP_VADDR) / SIZE(page), which does not take the 
phys_offset into account.  Therefore, with your patch, any call
to is_page_ptr() will fail:
  
  crash> kmem -S dentry
  CACHE            NAME                 OBJSIZE  ALLOCATED     TOTAL  SLABS  SSIZE
  kmem: page_to_nid: invalid page: ffff7e0004821500
  kmem: dentry: cannot gather relevant slab data
  ffff8003ed151200 dentry                   304          ?         ?      ?     8k
  crash> kmem -p | grep ffff7e0004821500
  ffff7e0004821500 4120854000                0        0  1 3fffe000008100 slab,head
  crash>

Perhaps the existing nr_mem_sections iteration loop could be transformed
into a binary search loop?  Just a thought...

Thanks,
  Dave