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Subject: Re: [RFC PATCH] vmcore: Add a kernel cmdline device_dump_limit
To:     Kairui Song <kasong@redhat.com>, linux-kernel@vger.kernel.org
Cc:     "kexec@lists.infradead.org" <kexec@lists.infradead.org>,
        "David S . Miller" <davem@davemloft.net>,
        Rahul Lakkireddy <rahul.lakkireddy@chelsio.com>,
        Eric Biederman <ebiederm@xmission.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Dave Young <dyoung@redhat.com>,
        Alexey Dobriyan <adobriyan@gmail.com>,
        Ganesh Goudar <ganeshgr@chelsio.com>
References: <20190510102051.25647-1-kasong@redhat.com>
From:   Bhupesh Sharma <bhsharma@redhat.com>
Message-ID: <4f453ec6-67a6-2c8f-2aab-acb54ae55645@redhat.com>
Date:   Fri, 10 May 2019 16:47:13 +0530
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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