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SubjectRe: [RFC PATCH] vmcore: Add a kernel cmdline device_dump_limit
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Hi Kairui,

Thanks for the patch. Please see my comments in-line:

On 05/10/2019 03:50 PM, Kairui Song wrote:
> Device dump allow drivers to add device related dump data to vmcore as
> they want. This have a potential issue, the data is stored in memory,
> drivers may append too much data and use too much memory. The vmcore is
> typically used in a kdump kernel which runs in a pre-reserved small
> chunk of memory. So as a result it will make kdump unusable at all due
> to OOM issues.
>
> So introduce new device_dump_limit= kernel parameter, and set the
> default limit to 0, so device dump is not enabled unless user specify
> the accetable maxiam

^^^^ acceptable maximum

> memory usage for device dump data. In this way user
> will also have the chance to adjust the kdump reserved memory
> accordingly.

Hmmm., this doesn't give much confidence with the
PROC_VMCORE_DEVICE_DUMP feature in its current shape. Rather shouldn't
we be enabling config PROC_VMCORE_DEVICE_DUMP only under EXPERT mode for
now, considering that this feature needs further thrashing and testing
with real setups including platforms where drivers append large amounts
of data to vmcore:

diff --git a/fs/proc/Kconfig b/fs/proc/Kconfig
index 817c02b13b1d..c47a12cf7fc0 100644
--- a/fs/proc/Kconfig
+++ b/fs/proc/Kconfig
@@ -45,7 +45,7 @@ config PROC_VMCORE
Exports the dump image of crashed kernel in ELF format.

config PROC_VMCORE_DEVICE_DUMP
- bool "Device Hardware/Firmware Log Collection"
+ bool "Device Hardware/Firmware Log Collection" if EXPERT
depends on PROC_VMCORE
default n
help
@@ -59,6 +59,12 @@ config PROC_VMCORE_DEVICE_DUMP
If you say Y here, the collected device dumps will be added
as ELF notes to /proc/vmcore.

+ Considering that there can be device drivers which append
+ large amounts of data to vmcore, you should say N here unless
+ you are reserving a large chunk of memory for crashdump
+ kernel, because otherwise the crashdump kernel might become
+ unusable due to OOM issues.
+

May be you can add a 'Fixes:' tag here.

> Signed-off-by: Kairui Song <kasong@redhat.com>
> ---
> fs/proc/vmcore.c | 20 ++++++++++++++++++++
> 1 file changed, 20 insertions(+)
>
> diff --git a/fs/proc/vmcore.c b/fs/proc/vmcore.c
> index 3fe90443c1bb..e28695ef2439 100644
> --- a/fs/proc/vmcore.c
> +++ b/fs/proc/vmcore.c
> @@ -53,6 +53,9 @@ static struct proc_dir_entry *proc_vmcore;
> /* Device Dump list and mutex to synchronize access to list */
> static LIST_HEAD(vmcoredd_list);
> static DEFINE_MUTEX(vmcoredd_mutex);
> +
> +/* Device Dump Limit */
> +static size_t vmcoredd_limit;
> #endif /* CONFIG_PROC_VMCORE_DEVICE_DUMP */
>
> /* Device Dump Size */
> @@ -1465,6 +1468,11 @@ int vmcore_add_device_dump(struct vmcoredd_data *data)
> data_size = roundup(sizeof(struct vmcoredd_header) + data->size,
> PAGE_SIZE);
>
> + if (vmcoredd_orig_sz + data_size >= vmcoredd_limit) {
> + ret = -ENOMEM;

Should we be adding a WARN() here to let the user know that the device
dump data will not be available in vmcore?

> + goto out_err;
> + }
> +
> /* Allocate buffer for driver's to write their dumps */
> buf = vmcore_alloc_buf(data_size);
> if (!buf) {
> @@ -1502,6 +1510,18 @@ int vmcore_add_device_dump(struct vmcoredd_data *data)
> return ret;
> }
> EXPORT_SYMBOL(vmcore_add_device_dump);
> +
> +static int __init parse_vmcoredd_limit(char *arg)
> +{
> + char *end;
> +
> + if (!arg)
> + return -EINVAL;
> + vmcoredd_limit = memparse(arg, &end);
> + return end > arg ? 0 : -EINVAL;
> +
> +}
> +__setup("device_dump_limit=", parse_vmcoredd_limit);

We should be adding this boot argument and its description to
'Documentation/admin-guide/kernel-parameters.txt'

> #endif /* CONFIG_PROC_VMCORE_DEVICE_DUMP */
>
> /* Free all dumps in vmcore device dump list */
>

Thanks,
Bhupesh

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