[PATCH v5 4/5] kdump: wait for DMA to finish when using CMA

Jiri Bohac
2025-06-12T10:18:42Z

When re-using the CMA area for kdump there is a risk of pending DMA
into pinned user pages in the CMA area.

Pages residing in CMA areas can usually not get long-term pinned and
are instead migrated away from the CMA area, so long-term pinning is
typically not a concern. (BUGs in the kernel might still lead to
long-term pinning of such pages if everything goes wrong.)

Pages pinned without FOLL_LONGTERM remain in the CMA and may possibly
be the source or destination of a pending DMA transfer.

Although there is no clear specification how long a page may be pinned
without FOLL_LONGTERM, pinning without the flag shows an intent of the
caller to only use the memory for short-lived DMA transfers, not a transfer
initiated by a device asynchronously at a random time in the future.

Add a delay of CMA_DMA_TIMEOUT_SEC seconds before starting the kdump
kernel, giving such short-lived DMA transfers time to finish before
the CMA memory is re-used by the kdump kernel.

Set CMA_DMA_TIMEOUT_SEC to 10 seconds - chosen arbitrarily as both
a huge margin for a DMA transfer, yet not increasing the kdump time
too significantly.

Signed-off-by: Jiri Bohac <jbohac@suse.cz>
Acked-by: David Hildenbrand <david@redhat.com>

---
Changes since v4:
- reworded the paragraph about long-term pinning
- simplified crash_cma_clear_pending_dma()
- dropped cma_dma_timeout_sec variable

---
Changes since v3:
- renamed CMA_DMA_TIMEOUT_SEC to CMA_DMA_TIMEOUT_MSEC, change delay to 10 seconds
- introduce a cma_dma_timeout_sec initialized to CMA_DMA_TIMEOUT_SEC
to make the timeout trivially tunable if needed in the future

---
kernel/crash_core.c | 15 +++++++++++++++
1 file changed, 15 insertions(+)

diff --git a/kernel/crash_core.c b/kernel/crash_core.c
index 335b8425dd4b..a4ef79591eb2 100644
--- a/kernel/crash_core.c
+++ b/kernel/crash_core.c
@@ -21,6 +21,7 @@
#include <linux/reboot.h>
#include <linux/btf.h>
#include <linux/objtool.h>
+#include <linux/delay.h>

#include <asm/page.h>
#include <asm/sections.h>
@@ -33,6 +34,11 @@
/* Per cpu memory for storing cpu states in case of system crash. */
note_buf_t __percpu *crash_notes;

+/* time to wait for possible DMA to finish before starting the kdump kernel
+ * when a CMA reservation is used
+ */
+#define CMA_DMA_TIMEOUT_SEC 10
+
#ifdef CONFIG_CRASH_DUMP

int kimage_crash_copy_vmcoreinfo(struct kimage *image)
@@ -97,6 +103,14 @@ int kexec_crash_loaded(void)
}
EXPORT_SYMBOL_GPL(kexec_crash_loaded);

+static void crash_cma_clear_pending_dma(void)
+{
+ if (!crashk_cma_cnt)
+ return;
+
+ mdelay(CMA_DMA_TIMEOUT_SEC * 1000);
+}
+
/*
* No panic_cpu check version of crash_kexec(). This function is called
* only when panic_cpu holds the current CPU number; this is the only CPU
@@ -119,6 +133,7 @@ void __noclone __crash_kexec(struct pt_regs *regs)
crash_setup_regs(&fixed_regs, regs);
crash_save_vmcoreinfo();
machine_crash_shutdown(&fixed_regs);
+ crash_cma_clear_pending_dma();
machine_kexec(kexec_crash_image);
}
kexec_unlock();

--
Jiri Bohac <jbohac@suse.cz>
SUSE Labs, Prague, Czechia