Date:   Mon, 20 May 2019 14:20:58 +0800
From:   Kairui Song <kasong@...hat.com>
To:     Bhupesh Sharma <bhsharma@...hat.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "kexec@...ts.infradead.org" <kexec@...ts.infradead.org>,
        "David S . Miller" <davem@...emloft.net>,
        Rahul Lakkireddy <rahul.lakkireddy@...lsio.com>,
        Eric Biederman <ebiederm@...ssion.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dave Young <dyoung@...hat.com>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Ganesh Goudar <ganeshgr@...lsio.com>
Subject: Re: [RFC PATCH] vmcore: Add a kernel cmdline device_dump_limit

On Mon, May 20, 2019 at 1:55 PM Bhupesh Sharma <bhsharma@...hat.com> wrote:
>
> On 05/16/2019 01:49 PM, Kairui Song wrote:
> > On Fri, May 10, 2019 at 7:17 PM Bhupesh Sharma <bhsharma@...hat.com> wrote:
> >>
> >> 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
> >
> > Will fix this typo.
>
> Ok.
>
> >>> 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:
> >
> > I think no need to move it to expert mode, just leave it disabled by
> > default should be better, that should be enough to make sure driver
> > won't append that much memory and cause OOM, while it could still be
> > enabled without changing the kernel, so this feature won't bring extra
> > risk, and could be enabled anytime easily.
>
> I have seen some arm64 users report issues on mailing lists with
> PROC_VMCORE_DEVICE_DUMP enabled as this causes frequent OOM in the arm64
> crash dump kernel.
>
> I think they are using this infrastructure to extend/enable device
> driver debugging on some arm64 platforms and finding issues with the
> crash dump kernel.
>
> I will do some analysis later-on (when I get some spare time) and post a
> patch (if needed) to put the same under EXPERT mode for now.
>
> >> 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.
> >
> > Problem is previous commit seems not broken, just bring extra memory
> > stress. Is "Fixes:" tag suitable for this commit?
>
> I think since the earlier patch causes an OOM, it would be better to
> atleast mention it in the git log (for easier git bisect later on).
>
> If not the 'Fixes:' tag may be we can use a 'Since commit ..' like
> wording in the commit log.
>
> >>> Signed-off-by: Kairui Song <kasong@...hat.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?
> >
> > Yes, that could be very helpful. How about pr_err_once? WARN is too
> > noise, just give a hint to the user that device dump is disabled
> > should be enough, so user will know why device dump data is not
> > present and will just enable it.
>
> Sure, pr_err() should be OK as well.
>
> >>> +             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'
> >
> > Good suggestion, will update the document.
> >
> >>
> >>>    #endif /* CONFIG_PROC_VMCORE_DEVICE_DUMP */
> >>>
> >>>    /* Free all dumps in vmcore device dump list */
> >>>
>
> Thanks,
> Bhupesh

Thanks for the reply, I've updated the patch accordingly and sent V2.

-- 
Best Regards,
Kairui Song

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