!heap –p –a VS !heap –x - windbg

Question

I have for years used the !heap –p –a for various tasks.
Now I’m starting to debug on Win8 using the WinDbg 6.2.9200 found in the latest Win8 sdk.
Here I have found that the !heap –p –a does not always work, and that the output from
!address “advertise” usage of !heap –x (see below) .
After reading the !heap -? , I can’t understand the difference!
Anyone who knows the difference?
Which command do you use to see the details of a heap block ?
0:008> !address 335168f8
<cut cut>
Usage: Heap
Base Address: 32b43000
End Address: 33540000
Region Size: 009fd000
State: 00001000 MEM_COMMIT
Protect: 00000004 PAGE_READWRITE
Type: 00020000 MEM_PRIVATE
Allocation Base: 32570000
Allocation Protect: 00000004 PAGE_READWRITE
More info: heap owning the address: !heap 0xa80000
More info: heap segment
More info: heap entry containing the address: !heap -x 0x335168f8
0:008> !heap -x 0x335168f8
Entry User Heap Segment Size PrevSize Unused Flags
-----------------------------------------------------------------------------
335168f0 335168f8 00a80000 32570000 30 30 1c busy extra fill
0:008> !heap -p -a 0x335168f8
0:008> .echo "nothing !!"
nothing !!

Answer 1

Windbg uses a different mechanism for looking up the heap information depending on which flag you use.
The -p flag tells it that you have enabled Page Heap via gflags.exe or similar. When Page Heap is enabled, Windows keeps a separate set of structures (_DPH_HEAP_ROOT and co) for tracking allocations. If PageHeap is not on, there won't be any such structures, so you will get no output. I also expect that -p -a will just search backward from the address to try to find the _DPH_HEAP_BLOCK which describes the allocation.
The -x flag tells Windbg to walk the the _HEAP/_HEAP_ENTRY structures which Windows uses for keeping track of allocations. This set of structures describe all active allocations which have gone through the standard allocators (e.g., malloc, new, LocalAlloc,HeapAlloc`, etc).
There are a few great papers on the internals of Windows' heap allocators. I really like the paper Chris Valasek (#nudehaberdasher) did a few years ago on the Low Fragmentation Heap which was implemented in Windows 7 (and the principles still apply in Win8).