Re: [PATCH v4 4/5] kdump: wait for DMA to finish when using CMA

[Jiri Bohac <jbohac@suse.cz>]

On Tue, Jun 03, 2025 at 06:25:57PM +0200, David Hildenbrand wrote:
> On 03.06.25 17:59, Jiri Bohac wrote:
> I'd phrase it more like "Pages residing in CMA areas can usually not get
> long-term pinned, 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."

...

> > If you want, I have no problem changing this to:
> > +	mdelay(cma_dma_timeout_sec * 1000);
> 
> Probably good enough. Or just hard-code 10s and call it a day. :)

Thanks for your comments, David. This would be the v5 of this
patch:

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

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>

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

---
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

---
 include/linux/crash_core.h |  3 +++
 kernel/crash_core.c        | 15 +++++++++++++++
 2 files changed, 18 insertions(+)

diff --git a/include/linux/crash_core.h b/include/linux/crash_core.h
index 44305336314e..805a07042c96 100644
--- a/include/linux/crash_core.h
+++ b/include/linux/crash_core.h
@@ -56,6 +56,9 @@ static inline unsigned int crash_get_elfcorehdr_size(void) { return 0; }
 /* Alignment required for elf header segment */
 #define ELF_CORE_HEADER_ALIGN   4096
 
+/* Default value for cma_dma_timeout_sec */
+#define CMA_DMA_TIMEOUT_SEC 10
+
 extern int crash_exclude_mem_range(struct crash_mem *mem,
 				   unsigned long long mstart,
 				   unsigned long long mend);
diff --git a/kernel/crash_core.c b/kernel/crash_core.c
index 335b8425dd4b..540fd75a4a0d 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
+ */
+unsigned int cma_dma_timeout_sec = CMA_DMA_TIMEOUT_SEC;
+
 #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