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a=rsa-sha256; c=relaxed/relaxed; d=suse.cz; s=susede2_rsa; t=1749022829; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=56cQPDkweYJVMIO8Oci9vsmGNfnHiTtULArO7sVXI0E=; b=OxVEDYUsfzYZ8xLXAyKnAep5+KO8k1PveIDBEEmCWHTQ4eS8oThZT0+ho+B5Wb1NmHnGW/ MH6K6tJXSf730OJJUAsmL1esZW6+iYIjlDSn1Gb0NBGikfEClqbPI8KDiECcHXFoLYIeGN RenGbSrvnINQqRQdV9GAd3eCOvFhYEk= DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=suse.cz; s=susede2_ed25519; t=1749022829; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=56cQPDkweYJVMIO8Oci9vsmGNfnHiTtULArO7sVXI0E=; b=UZIaHJ3RHQIdgYLYuU4/PIRCCWguUqwjEreL+bp2SSgkwji2YmH80JvHeYeKO8HT6ILLiv wndEVd0nFMHoSfDg== Date: Wed, 4 Jun 2025 09:40:29 +0200 From: Jiri Bohac To: David Hildenbrand Cc: Baoquan He , Vivek Goyal , Dave Young , kexec@lists.infradead.org, Philipp Rudo , Donald Dutile , Pingfan Liu , Tao Liu , linux-kernel@vger.kernel.org, David Hildenbrand , Michal Hocko Subject: Re: [PATCH v4 4/5] kdump: wait for DMA to finish when using CMA Message-ID: References: Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Spam-Flag: NO X-Spam-Score: -4.30 X-Spamd-Result: default: False [-4.30 / 50.00]; BAYES_HAM(-3.00)[100.00%]; NEURAL_HAM_LONG(-1.00)[-1.000]; NEURAL_HAM_SHORT(-0.20)[-1.000]; MIME_GOOD(-0.10)[text/plain]; MISSING_XM_UA(0.00)[]; FROM_HAS_DN(0.00)[]; MIME_TRACE(0.00)[0:+]; RCPT_COUNT_TWELVE(0.00)[12]; ARC_NA(0.00)[]; RCVD_COUNT_ZERO(0.00)[0]; MID_RHS_MATCH_FROMTLD(0.00)[]; DKIM_SIGNED(0.00)[suse.cz:s=susede2_rsa,suse.cz:s=susede2_ed25519]; FROM_EQ_ENVFROM(0.00)[]; TO_DN_SOME(0.00)[]; TO_MATCH_ENVRCPT_ALL(0.00)[]; FUZZY_BLOCKED(0.00)[rspamd.com]; DBL_BLOCKED_OPENRESOLVER(0.00)[suse.cz:email] X-Spam-Level: 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 --- 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 #include #include +#include #include #include @@ -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 SUSE Labs, Prague, Czechia