[PATCH v2] kho: add support for deferred struct page init

[Michal Clapinski]

When `CONFIG_DEFERRED_STRUCT_PAGE_INIT` is enabled, struct page
initialization is deferred to parallel kthreads that run later
in the boot process.

During KHO restoration, `deserialize_bitmap()` writes metadata for
each preserved memory region. However, if the struct page has not been
initialized, this write targets uninitialized memory, potentially
leading to errors like:
```
BUG: unable to handle page fault for address: ...
```

Fix this by introducing `kho_get_preserved_page()`,  which ensures
all struct pages in a preserved region are initialized by calling
`init_deferred_page()` which is a no-op when deferred init is disabled
or when the struct page is already initialized.

Signed-off-by: Evangelos Petrongonas <epetron@xxxxxxxxx>
Signed-off-by: Michal Clapinski <mclapinski@xxxxxxxxxx>
---
v2: updated a comment

I think we can't initialize those struct pages in kho_restore_page.
I encountered this stack:
  page_zone(start_page)
  __pageblock_pfn_to_page
  set_zone_contiguous
  page_alloc_init_late

So, at the end of page_alloc_init_late struct pages are expected to be
already initialized. set_zone_contiguous() looks at the first and last
struct page of each pageblock in each populated zone to figure out if
the zone is contiguous. If a kho page lands on a pageblock boundary,
this will lead to access of an uninitialized struct page.
There is also page_ext_init that invokes pfn_to_nid, which calls
page_to_nid for each section-aligned page.
There might be other places that do something similar. Therefore, it's
a good idea to initialize all struct pages by the end of deferred
struct page init. That's why I'm resending Evangelos's patch.

I also tried to implement Pratyush's idea, i.e. iterate over zones,
then get node from zone. I didn't notice any performance difference
even with 8GB of kho.

I repeated Evangelos's testing:
In order to test the fix, I modified the KHO selftest, to allocate more
memory and do so from higher memory to trigger the incompatibility. The
branch with those changes can be found in:
https://git.infradead.org/?p=users/vpetrog/linux.git;a=shortlog;h=refs/heads/kho-deferred-struct-page-init
---
 kernel/liveupdate/Kconfig          |  2 --
 kernel/liveupdate/kexec_handover.c | 23 ++++++++++++++++++++++-
 2 files changed, 22 insertions(+), 3 deletions(-)

diff --git a/kernel/liveupdate/Kconfig b/kernel/liveupdate/Kconfig
index 1a8513f16ef7..c13af38ba23a 100644
--- a/kernel/liveupdate/Kconfig
+++ b/kernel/liveupdate/Kconfig
@@ -1,12 +1,10 @@
 # SPDX-License-Identifier: GPL-2.0-only
 
 menu "Live Update and Kexec HandOver"
-	depends on !DEFERRED_STRUCT_PAGE_INIT
 
 config KEXEC_HANDOVER
 	bool "kexec handover"
 	depends on ARCH_SUPPORTS_KEXEC_HANDOVER && ARCH_SUPPORTS_KEXEC_FILE
-	depends on !DEFERRED_STRUCT_PAGE_INIT
 	select MEMBLOCK_KHO_SCRATCH
 	select KEXEC_FILE
 	select LIBFDT
diff --git a/kernel/liveupdate/kexec_handover.c b/kernel/liveupdate/kexec_handover.c
index b851b09a8e99..26bb45b25809 100644
--- a/kernel/liveupdate/kexec_handover.c
+++ b/kernel/liveupdate/kexec_handover.c
@@ -457,6 +457,27 @@ static int kho_mem_serialize(struct kho_out *kho_out)
 	return err;
 }
 
+/*
+ * With CONFIG_DEFERRED_STRUCT_PAGE_INIT, struct pages in higher memory regions
+ * may not be initialized yet at the time KHO deserializes preserved memory.
+ * KHO uses the struct page to store metadata and a later initialization would
+ * overwrite it.
+ * Ensure all the struct pages in the preservation are
+ * initialized. deserialize_bitmap() marks the reservation as noinit to make
+ * sure they don't get re-initialized later.
+ */
+static struct page *__init kho_get_preserved_page(phys_addr_t phys,
+						  unsigned int order)
+{
+	unsigned long pfn = PHYS_PFN(phys);
+	int nid = early_pfn_to_nid(pfn);
+
+	for (int i = 0; i < (1 << order); i++)
+		init_deferred_page(pfn + i, nid);
+
+	return pfn_to_page(pfn);
+}
+
 static void __init deserialize_bitmap(unsigned int order,
 				      struct khoser_mem_bitmap_ptr *elm)
 {
@@ -467,7 +488,7 @@ static void __init deserialize_bitmap(unsigned int order,
 		int sz = 1 << (order + PAGE_SHIFT);
 		phys_addr_t phys =
 			elm->phys_start + (bit << (order + PAGE_SHIFT));
-		struct page *page = phys_to_page(phys);
+		struct page *page = kho_get_preserved_page(phys, order);
 		union kho_page_info info;
 
 		memblock_reserve(phys, sz);
-- 
2.53.0.rc2.204.g2597b5adb4-goog