linux-kernel - [PATCH 1/2] kho: fix unpreservation of higher-order vmalloc preservations

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Message-ID: <20251103180235.71409-2-pratyush@kernel.org>
Date: Mon,  3 Nov 2025 19:02:31 +0100
From: Pratyush Yadav <pratyush@...nel.org>
To: Andrew Morton <akpm@...ux-foundation.org>,
	Baoquan He <bhe@...hat.com>,
	Alexander Graf <graf@...zon.com>,
	Mike Rapoport <rppt@...nel.org>,
	Pasha Tatashin <pasha.tatashin@...een.com>,
	Pratyush Yadav <pratyush@...nel.org>
Cc: kexec@...ts.infradead.org,
	linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH 1/2] kho: fix unpreservation of higher-order vmalloc preservations

kho_vmalloc_unpreserve_chunk() calls __kho_unpreserve() with end_pfn as
pfn + 1. This happens to work for 0-order pages, but leaks higher order
pages.

For example, say order 2 pages back the allocation. During preservation,
they get preserved in the order 2 bitmaps, but
kho_vmalloc_unpreserve_chunk() would try to unpreserve them from the
order 0 bitmaps, which should not have these bits set anyway, leaving
the order 2 bitmaps untouched. This results in the pages being carried
over to the next kernel. Nothing will free those pages in the next boot,
leaking them.

Fix this by taking the order into account when calculating the end PFN
for __kho_unpreserve().

Fixes: a667300bd53f2 ("kho: add support for preserving vmalloc allocations")
Signed-off-by: Pratyush Yadav <pratyush@...nel.org>
---

Notes:
    When Pasha's patch [0] to add kho_unpreserve_pages() is merged, maybe it
    would be a better idea to use kho_unpreserve_pages() here? But that is
    something for later I suppose.
    
    [0] https://lore.kernel.org/linux-mm/20251101142325.1326536-4-pasha.tatashin@soleen.com/

 kernel/kexec_handover.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/kernel/kexec_handover.c b/kernel/kexec_handover.c
index cc5aaa738bc50..c2bcbb10918ce 100644
--- a/kernel/kexec_handover.c
+++ b/kernel/kexec_handover.c
@@ -862,7 +862,8 @@ static struct kho_vmalloc_chunk *new_vmalloc_chunk(struct kho_vmalloc_chunk *cur
 	return NULL;
 }
 
-static void kho_vmalloc_unpreserve_chunk(struct kho_vmalloc_chunk *chunk)
+static void kho_vmalloc_unpreserve_chunk(struct kho_vmalloc_chunk *chunk,
+					 unsigned short order)
 {
 	struct kho_mem_track *track = &kho_out.ser.track;
 	unsigned long pfn = PHYS_PFN(virt_to_phys(chunk));
@@ -871,7 +872,7 @@ static void kho_vmalloc_unpreserve_chunk(struct kho_vmalloc_chunk *chunk)
 
 	for (int i = 0; i < ARRAY_SIZE(chunk->phys) && chunk->phys[i]; i++) {
 		pfn = PHYS_PFN(chunk->phys[i]);
-		__kho_unpreserve(track, pfn, pfn + 1);
+		__kho_unpreserve(track, pfn, pfn + (1 << order));
 	}
 }
 
@@ -882,7 +883,7 @@ static void kho_vmalloc_free_chunks(struct kho_vmalloc *kho_vmalloc)
 	while (chunk) {
 		struct kho_vmalloc_chunk *tmp = chunk;
 
-		kho_vmalloc_unpreserve_chunk(chunk);
+		kho_vmalloc_unpreserve_chunk(chunk, kho_vmalloc->order);
 
 		chunk = KHOSER_LOAD_PTR(chunk->hdr.next);
 		free_page((unsigned long)tmp);
-- 
2.47.3