[RFC] mm: Enable generic pfn_valid() to handle early sections with memmap holes

[Anshuman Khandual]

On 3/8/21 2:25 PM, Mike Rapoport wrote:
> Hi Anshuman,
> 
> On Mon, Mar 08, 2021 at 08:57:53AM +0530, Anshuman Khandual wrote:
>> Platforms like arm and arm64 have redefined pfn_valid() because their early
>> memory sections might have contained memmap holes caused by memblock areas
>> tagged with MEMBLOCK_NOMAP, which should be skipped while validating a pfn
>> for struct page backing. This scenario could be captured with a new option
>> CONFIG_HAVE_EARLY_SECTION_MEMMAP_HOLES and then generic pfn_valid() can be
>> improved to accommodate such platforms. This reduces overall code footprint
>> and also improves maintainability.
> 
> I wonder whether arm64 would still need to free parts of its memmap after

free_unused_memmap() is applicable when CONFIG_SPARSEMEM_VMEMMAP is not enabled.
I am not sure whether there still might be some platforms or boards which would
benefit from this. Hence lets just keep this unchanged for now.

> the section size was reduced. Maybe the pain of arm64::pfn_valid() is not
> worth the memory savings anymore?

arm64 pfn_valid() special case was primarily because of MEMBLOCK_NOMAP tagged
memory areas, which are reserved by the firmware.

> 
>> Commit 4f5b0c178996 ("arm, arm64: move free_unused_memmap() to generic mm")
>> had used CONFIG_HAVE_ARCH_PFN_VALID to gate free_unused_memmap(), which in
>> turn had expanded its scope to new platforms like arc and m68k. Rather lets
>> restrict back the scope for free_unused_memmap() to arm and arm64 platforms
>> using this new config option i.e CONFIG_HAVE_EARLY_SECTION_MEMMAP.
> 
> The whole point of 4f5b0c178996 was to let arc and m68k to free unused
> memory map with FLATMEM so they won't need DISCONTIGMEM or SPARSEMEM. So
> whatever implementation there will be for arm/arm64, please keep arc and
> m68k functionally intact.

Okay. Will protect free_unused_memmap() on HAVE_EARLY_SECTION_MEMMAP_HOLES
config as well.

diff --git a/mm/memblock.c b/mm/memblock.c
index d9fa2e62ab7a..11b624e94127 100644
--- a/mm/memblock.c
+++ b/mm/memblock.c
@@ -1927,8 +1927,11 @@ static void __init free_unused_memmap(void)
        unsigned long start, end, prev_end = 0;
        int i;
 
-       if (!IS_ENABLED(CONFIG_HAVE_EARLY_SECTION_MEMMAP_HOLES) ||
-           IS_ENABLED(CONFIG_SPARSEMEM_VMEMMAP))
+       if (IS_ENABLED(CONFIG_SPARSEMEM_VMEMMAP))
+               return;
+
+       if (!IS_ENABLED(CONFIG_HAVE_EARLY_SECTION_MEMMAP_HOLES) &&
+           !IS_ENABLED(CONFIG_HAVE_ARCH_PFN_VALID))
                return;