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Subject: Re: [RFC PATCH v3 0/3] pmem memmap dump support
To:     kexec@lists.infradead.org, nvdimm@lists.linux.dev
Cc:     linux-kernel@vger.kernel.org, dan.j.williams@intel.com,
        bhe@redhat.com, ruansy.fnst@fujitsu.com, y-goto@fujitsu.com,
        yangx.jy@fujitsu.com, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Dave Jiang <dave.jiang@intel.com>,
        Dave Young <dyoung@redhat.com>,
        Eric Biederman <ebiederm@xmission.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>,
        Ira Weiny <ira.weiny@intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Vishal Verma <vishal.l.verma@intel.com>,
        Vivek Goyal <vgoyal@redhat.com>, x86@kernel.org
References: <20230602102656.131654-1-lizhijian@fujitsu.com>
From:   "Li, Zhijian" <lizhijian@fujitsu.com>
Message-ID: <a866e326-cc51-1971-5351-b8338e04be5f@fujitsu.com>
Date:   Sun, 25 Jun 2023 18:27:13 +0800
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kindly ping, I do need your feedback especially some voices from the 
nvdimm. If there are any clarifications needed or if my initial email 
requires further details, please do not hesitate to let me know. I am 
more than willing to provide more additional information.

Thanks
Zhijian

on 6/2/2023 6:26 PM, Li Zhijian wrote:
> Hello folks,
>
> After sending out the previous version of the patch set, we received some comments,
> and we really appreciate your input. However, as you can see, the current patch
> set is still in its early stages, especially in terms of the solution selection,
> which may still undergo changes.
>
> Changes in V3:
> Mainly based on the understanding from the first version, I implemented the proposal
> suggested by Dan. In the kdump kernel, the device's superblock is read through
> a device file interface to calculate the metadata range. In the second version,
> the first kernel writes the metadata range to vmcoreinfo, and after kdump occurs,
> the kdump kernel can directly read it from /proc/vmcore.
>
> Comparing these two approaches, the advantage of Version 3 is fewer kernel
> modifications, but the downside is the introduction of a new external library,
> libndctl, to search for each namespace, which introduces a higher level of
> coupling with ndctl.
>
> One important thing to note about both V2 and V3 is the introduction of a new
> ELF flag, PF_DEV, to indicate whether a range is on a device. I'm not sure if
> there are better alternatives or if we can use this flag internally without
> exposing it in elf.h.
>
> We greatly appreciate your feedback and would like to hear your response.
>
> In RFC stage, I folded these 3 projects in this same cover letter for reviewing convenience.
> kernel(3):
>    nvdimm: set force_raw=1 in kdump kernel
>    x86/crash: Add pmem region into PT_LOADs of vmcore
>    kernel/kexec_file: Mark pmem region with new flag PF_DEV
> kexec-tools(1):
>    kexec: Add and mark pmem region into PT_LOADs
> makedumpfile(3):
>    elf_info.c: Introduce is_pmem_pt_load_range
>    makedumpfile.c: Exclude all pmem pages
>    makedumpfile: get metadata boundaries from pmem's infoblock
>
> Currently, this RFC has already implemented to supported case D*. And the case A&B is disabled
> deliberately in makedumpfile.
> ---
>
> pmem memmap can also be called pmem metadata here.
>
> ### Background and motivate overview ###
> ---
> Crash dump is an important feature for trouble shooting of kernel. It is the final way to chase what
> happened at the kernel panic, slowdown, and so on. It is the most important tool for customer support.
> However, a part of data on pmem is not included in crash dump, it may cause difficulty to analyze
> trouble around pmem (especially Filesystem-DAX).
>
> A pmem namespace in "fsdax" or "devdax" mode requires allocation of per-page metadata[1]. The allocation
> can be drawn from either mem(system memory) or dev(pmem device), see `ndctl help create-namespace` for
> more details. In fsdax, struct page array becomes very important, it is one of the key data to find
> status of reverse map.
>
> So, when metadata was stored in pmem, even pmem's per-page metadata will not be dumped. That means
> troubleshooters are unable to check more details about pmem from the dumpfile.
>
> ### Make pmem memmap dump support ###
> ---
> Our goal is that whether metadata is stored on mem or pmem, its metadata can be dumped and then the
> crash-utilities can read more details about the pmem. Of course, this feature can be enabled/disabled.
>
> First, based on our previous investigation, according to the location of metadata and the scope of
> dump, we can divide it into the following four cases: A, B, C, D.
> It should be noted that although we mentioned case A&B below, we do not want these two cases to be
> part of this feature, because dumping the entire pmem will consume a lot of space, and more importantly,
> it may contain user sensitive data.
>
> +-------------+----------+------------+
> |\+--------+\     metadata location   |
> |            ++-----------------------+
> | dump scope  |  mem     |   PMEM     |
> +-------------+----------+------------+
> | entire pmem |     A    |     B      |
> +-------------+----------+------------+
> | metadata    |     C    |     D      |
> +-------------+----------+------------+
>
> ### Testing ###
> Only x86_64 are tested. Please note that we have to disable the 2nd kernel's libnvdimm to ensure the
> metadata in 2nd kernel will not be touched again.
>
> below 2 commits use sha256 to check the metadata in 1st kernel during panic and makedumpfile in 2nd kernel.
> https://github.com/zhijianli88/makedumpfile/commit/91a135be6980e6e87b9e00b909aaaf8ef9566ec0
> https://github.com/zhijianli88/linux/commit/55bef07f8f0b2e587737b796e73b92f242947e5a
>
> ### TODO ###
> Only x86 are fully supported for both kexec_load(2) and kexec_file_load(2)
> kexec_file_load(2) on other architectures are TODOs.
> ---
> [1] Pmem region layout:
>     ^<--namespace0.0---->^<--namespace0.1------>^
>     |                    |                      |
>     +--+m----------------+--+m------------------+---------------------+-+a
>     |++|e                |++|e                  |                     |+|l
>     |++|t                |++|t                  |                     |+|i
>     |++|a                |++|a                  |                     |+|g
>     |++|d  namespace0.0  |++|d  namespace0.1    |     un-allocated    |+|n
>     |++|a    fsdax       |++|a     devdax       |                     |+|m
>     |++|t                |++|t                  |                     |+|e
>     +--+a----------------+--+a------------------+---------------------+-+n
>     |                                                                   |t
>     v<-----------------------pmem region------------------------------->v
>
> [2] https://lore.kernel.org/linux-mm/70F971CF-1A96-4D87-B70C-B971C2A1747C@roc.cs.umass.edu/T/
> [3] https://lore.kernel.org/linux-mm/3c752fc2-b6a0-2975-ffec-dba3edcf4155@fujitsu.com/
>
> ### makedumpfile output in case B ####
> kdump.sh[224]: makedumpfile: version 1.7.2++ (released on 20 Oct 2022)
> kdump.sh[224]: command line: makedumpfile -l --message-level 31 -d 31 /proc/vmcore /sysroot/var/crash/127.0.0.1-2023-04-21-02:50:57//vmcore-incomplete
> kdump.sh[224]: sadump: does not have partition header
> kdump.sh[224]: sadump: read dump device as unknown format
> kdump.sh[224]: sadump: unknown format
> kdump.sh[224]:                phys_start         phys_end       virt_start         virt_end  is_pmem
> kdump.sh[224]: LOAD[ 0]          1000000          3c26000 ffffffff81000000 ffffffff83c26000    false
> kdump.sh[224]: LOAD[ 1]           100000         7f000000 ffff888000100000 ffff88807f000000    false
> kdump.sh[224]: LOAD[ 2]         bf000000         bffd7000 ffff8880bf000000 ffff8880bffd7000    false
> kdump.sh[224]: LOAD[ 3]        100000000        140000000 ffff888100000000 ffff888140000000    false
> kdump.sh[224]: LOAD[ 4]        140000000        23e200000 ffff888140000000 ffff88823e200000     true
> kdump.sh[224]: Linux kdump
> kdump.sh[224]: VMCOREINFO   :
> kdump.sh[224]:   OSRELEASE=6.3.0-rc3-pmem-bad+
> kdump.sh[224]:   BUILD-ID=0546bd82db93706799d3eea38194ac648790aa85
> kdump.sh[224]:   PAGESIZE=4096
> kdump.sh[224]: page_size    : 4096
> kdump.sh[224]:   SYMBOL(init_uts_ns)=ffffffff82671300
> kdump.sh[224]:   OFFSET(uts_namespace.name)=0
> kdump.sh[224]:   SYMBOL(node_online_map)=ffffffff826bbe08
> kdump.sh[224]:   SYMBOL(swapper_pg_dir)=ffffffff82446000
> kdump.sh[224]:   SYMBOL(_stext)=ffffffff81000000
> kdump.sh[224]:   SYMBOL(vmap_area_list)=ffffffff82585fb0
> kdump.sh[224]:   SYMBOL(devm_memmap_vmcore_head)=ffffffff825603c0
> kdump.sh[224]:   SIZE(devm_memmap_vmcore)=40
> kdump.sh[224]:   OFFSET(devm_memmap_vmcore.entry)=0
> kdump.sh[224]:   OFFSET(devm_memmap_vmcore.start)=16
> kdump.sh[224]:   OFFSET(devm_memmap_vmcore.end)=24
> kdump.sh[224]:   SYMBOL(mem_section)=ffff88813fff4000
> kdump.sh[224]:   LENGTH(mem_section)=2048
> kdump.sh[224]:   SIZE(mem_section)=16
> kdump.sh[224]:   OFFSET(mem_section.section_mem_map)=0
> ...
> kdump.sh[224]: STEP [Checking for memory holes  ] : 0.012699 seconds
> kdump.sh[224]: STEP [Excluding unnecessary pages] : 0.538059 seconds
> kdump.sh[224]: STEP [Copying data               ] : 0.995418 seconds
> kdump.sh[224]: STEP [Copying data               ] : 0.000067 seconds
> kdump.sh[224]: Writing erase info...
> kdump.sh[224]: offset_eraseinfo: 5d02266, size_eraseinfo: 0
> kdump.sh[224]: Original pages  : 0x00000000001c0cfd
> kdump.sh[224]:   Excluded pages   : 0x00000000001a58d2
> kdump.sh[224]:     Pages filled with zero  : 0x0000000000006805
> kdump.sh[224]:     Non-private cache pages : 0x0000000000019e93
> kdump.sh[224]:     Private cache pages     : 0x0000000000077572
> kdump.sh[224]:     User process data pages : 0x0000000000002c3b
> kdump.sh[224]:     Free pages              : 0x0000000000010e8d
> kdump.sh[224]:     Hwpoison pages          : 0x0000000000000000
> kdump.sh[224]:     Offline pages           : 0x0000000000000000
> kdump.sh[224]:     pmem metadata pages     : 0x0000000000000000
> kdump.sh[224]:     pmem userdata pages     : 0x00000000000fa200
> kdump.sh[224]:   Remaining pages  : 0x000000000001b42b
> kdump.sh[224]:   (The number of pages is reduced to 6%.)
> kdump.sh[224]: Memory Hole     : 0x000000000007d503
> kdump.sh[224]: --------------------------------------------------
> kdump.sh[224]: Total pages     : 0x000000000023e200
> kdump.sh[224]: Write bytes     : 97522590
> kdump.sh[224]: Cache hit: 191669, miss: 292, hit rate: 99.8%
> kdump.sh[224]: The dumpfile is saved to /sysroot/var/crash/127.0.0.1-2023-04-21-02:50:57//vmcore-incomplete.
> kdump.sh[224]: makedumpfile Completed.
>
> CC: Baoquan He <bhe@redhat.com>
> CC: Borislav Petkov <bp@alien8.de>
> CC: Dan Williams <dan.j.williams@intel.com>
> CC: Dave Hansen <dave.hansen@linux.intel.com>
> CC: Dave Jiang <dave.jiang@intel.com>
> CC: Dave Young <dyoung@redhat.com>
> CC: Eric Biederman <ebiederm@xmission.com>
> CC: "H. Peter Anvin" <hpa@zytor.com>
> CC: Ingo Molnar <mingo@redhat.com>
> CC: Ira Weiny <ira.weiny@intel.com>
> CC: Thomas Gleixner <tglx@linutronix.de>
> CC: Vishal Verma <vishal.l.verma@intel.com>
> CC: Vivek Goyal <vgoyal@redhat.com>
> CC: x86@kernel.org
> CC: kexec@lists.infradead.org
> CC: nvdimm@lists.linux.dev
>