Re: [PATCH v4 7/7] mm/mm_init: Use for_each_valid_pfn() in init_unavailable_range()

From: David Hildenbrand
Date: Fri Apr 25 2025 - 16:13:05 EST


On 25.04.25 21:08, David Woodhouse wrote:
On 25 April 2025 17:17:25 BST, David Hildenbrand <david@xxxxxxxxxx> wrote:
On 23.04.25 15:33, David Woodhouse wrote:
From: David Woodhouse <dwmw@xxxxxxxxxxxx>

Currently, memmap_init initializes pfn_hole with 0 instead of
ARCH_PFN_OFFSET. Then init_unavailable_range will start iterating each
page from the page at address zero to the first available page, but it
won't do anything for pages below ARCH_PFN_OFFSET because pfn_valid
won't pass.

If ARCH_PFN_OFFSET is very large (e.g., something like 2^64-2GiB if the
kernel is used as a library and loaded at a very high address), the
pointless iteration for pages below ARCH_PFN_OFFSET will take a very
long time, and the kernel will look stuck at boot time.

Use for_each_valid_pfn() to skip the pointless iterations.

Reported-by: Ruihan Li <lrh2000@xxxxxxxxxx>
Suggested-by: Mike Rapoport <rppt@xxxxxxxxxx>
Signed-off-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
Reviewed-by: Mike Rapoport (Microsoft) <rppt@xxxxxxxxxx>
Tested-by: Ruihan Li <lrh2000@xxxxxxxxxx>
---
mm/mm_init.c | 6 +-----
1 file changed, 1 insertion(+), 5 deletions(-)

diff --git a/mm/mm_init.c b/mm/mm_init.c
index 41884f2155c4..0d1a4546825c 100644
--- a/mm/mm_init.c
+++ b/mm/mm_init.c
@@ -845,11 +845,7 @@ static void __init init_unavailable_range(unsigned long spfn,
unsigned long pfn;
u64 pgcnt = 0;
- for (pfn = spfn; pfn < epfn; pfn++) {
- if (!pfn_valid(pageblock_start_pfn(pfn))) {
- pfn = pageblock_end_pfn(pfn) - 1;
- continue;
- }

So, if the first pfn in a pageblock is not valid, we skip the whole pageblock ...

+ for_each_valid_pfn(pfn, spfn, epfn) {
__init_single_page(pfn_to_page(pfn), pfn, zone, node);
__SetPageReserved(pfn_to_page(pfn));
pgcnt++;

but here, we would process further pfns inside such a pageblock?


Is it not the case that either *all*, or *none*, of the PFNs within a given pageblock will be valid?

Hmm, good point. I was thinking about sub-sections, but all early sections are fully valid.

(Also, at least on x86, the subsection size should match the pageblock size; might not be the case on other architectures, like arm64 with 64K base pages ...)


I assumed that was *why* it had that skip, as an attempt at the kind of optimisation that for_each_valid_pfn() now gives us?

But it's interesting in this code that we didn't optimize for "if the first pfn is valid, all the remaining ones are valid". We would still check each PFN.

In any case, trying to figure out why Lorenzo ran into an issue ... if it's nit because of the pageblock, maybe something in for_each_valid_pfn with sparsemem is still shaky.

--
Cheers,

David / dhildenb