kho: fix restoring of contiguous ranges of order-0 pages

When contiguous ranges of order-0 pages are restored, kho_restore_page()
calls prep_compound_page() with the first page in the range and order as
parameters and then kho_restore_pages() calls split_page() to make sure
all pages in the range are order-0.

However, since split_page() is not intended to split compound pages and
with VM_DEBUG enabled it will trigger a VM_BUG_ON_PAGE().

Update kho_restore_page() so that it will use prep_compound_page() when it
restores a folio and make sure it properly sets page count for both large
folios and ranges of order-0 pages.

Link: https://lkml.kernel.org/r/20251125110917.843744-3-rppt@kernel.org
Fixes: a667300bd53f ("kho: add support for preserving vmalloc allocations")
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Reported-by: Pratyush Yadav <pratyush@kernel.org>
Cc: Alexander Graf <graf@amazon.com>
Cc: Pasha Tatashin <pasha.tatashin@soleen.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/kernel/liveupdate/kexec_handover.c b/kernel/liveupdate/kexec_handover.c
index 096b7db28baf784ac3af6ff32d01fb4b198eabb4..9dc51fab604f11bcb9ae68bffd500e5c7b8816b6 100644 (file)
--- a/kernel/liveupdate/kexec_handover.c
+++ b/kernel/liveupdate/kexec_handover.c
@@ -219,11 +219,11 @@ static int __kho_preserve_order(struct kho_mem_track *track, unsigned long pfn,
        return 0;
 }
 
-static struct page *kho_restore_page(phys_addr_t phys)
+static struct page *kho_restore_page(phys_addr_t phys, bool is_folio)
 {
        struct page *page = pfn_to_online_page(PHYS_PFN(phys));
+       unsigned int nr_pages, ref_cnt;
        union kho_page_info info;
-       unsigned int nr_pages;
 
        if (!page)
                return NULL;
@@ -243,11 +243,16 @@ static struct page *kho_restore_page(phys_addr_t phys)
        /* Head page gets refcount of 1. */
        set_page_count(page, 1);
 
-       /* For higher order folios, tail pages get a page count of zero. */
+       /*
+        * For higher order folios, tail pages get a page count of zero.
+        * For physically contiguous order-0 pages every pages gets a page
+        * count of 1
+        */
+       ref_cnt = is_folio ? 0 : 1;
        for (unsigned int i = 1; i < nr_pages; i++)
-               set_page_count(page + i, 0);
+               set_page_count(page + i, ref_cnt);
 
-       if (info.order > 0)
+       if (is_folio && info.order)
                prep_compound_page(page, info.order);
 
        adjust_managed_page_count(page, nr_pages);
@@ -262,7 +267,7 @@ static struct page *kho_restore_page(phys_addr_t phys)
  */
 struct folio *kho_restore_folio(phys_addr_t phys)
 {
-       struct page *page = kho_restore_page(phys);
+       struct page *page = kho_restore_page(phys, true);
 
        return page ? page_folio(page) : NULL;
 }
@@ -287,11 +292,10 @@ struct page *kho_restore_pages(phys_addr_t phys, unsigned int nr_pages)
        while (pfn < end_pfn) {
                const unsigned int order =
                        min(count_trailing_zeros(pfn), ilog2(end_pfn - pfn));
-               struct page *page = kho_restore_page(PFN_PHYS(pfn));
+               struct page *page = kho_restore_page(PFN_PHYS(pfn), false);
 
                if (!page)
                        return NULL;
-               split_page(page, order);
                pfn += 1 << order;
        }