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Date:   Mon, 5 Jun 2023 20:12:53 -0700
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Subject: Re: [RFC v3 07/21] mm: PKRAM: introduce super block
Content-Language: en-US
To:     Coiby Xu <coxu@redhat.com>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, x86@kernel.org,
        hpa@zytor.com, dave.hansen@linux.intel.com, luto@kernel.org,
        peterz@infradead.org, rppt@kernel.org, akpm@linux-foundation.org,
        ebiederm@xmission.com, keescook@chromium.org, graf@amazon.com,
        jason.zeng@intel.com, lei.l.li@intel.com,
        steven.sistare@oracle.com, fam.zheng@bytedance.com,
        mgalaxy@akamai.com, kexec@lists.infradead.org
References: <1682554137-13938-1-git-send-email-anthony.yznaga@oracle.com>
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From:   Anthony Yznaga <anthony.yznaga@oracle.com>
In-Reply-To: <aee5eximmv33ke26xrodm7tlqwwojalf2lz3zrgkt75bxeuupe@y3t2sfgdcy7k>
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On 6/5/23 7:55 PM, Coiby Xu wrote:
> On Mon, Jun 05, 2023 at 07:01:56PM -0700, Anthony Yznaga wrote:
>> Hi Coiby,
>>
>> On 6/4/23 7:40 PM, Coiby Xu wrote:
>>> Hi Anthony,
>>>
>>> On Wed, Apr 26, 2023 at 05:08:43PM -0700, Anthony Yznaga wrote:
>>>> The PKRAM super block is the starting point for restoring preserved
>>>> memory. By providing the super block to the new kernel at boot time,
>>>> preserved memory can be reserved and made available to be restored.
>>>> To point the kernel to the location of the super block, one passes
>>>> its pfn via the 'pkram' boot param.
>>>
>>> I'm curious to ask how will the 'pkram' boot param be passed. It 
>>> seems I
>>> can't find the answer in this patch set.
>>
>> The pfn of the super block read from /sys/kernel/pkram is passed to
>>
>> the next kernel by adding the boot parameter, pkram=<super block pfn>.
>>
>> The next kernel picks it up through the early_param("pkram", 
>> parse_pkram_sb_pfn)
>>
>> in this patch below.
>
> Thanks for the explanation! Sorry I didn't make my question clear. I
> should have asked who is going to read /sys/kernel/pkram how this pkram
> boot parameter will be added for the next kernel.

I have not specified this in the patches. One way would be to write a script

as wrapper around kexec that determines and appends the appropriate boot

parameter.


Anthony

>
>>
>>
>> Anthony
>>
>>
>>>
>>>
>>>> For that purpose, the pkram super
>>>> block pfn is exported via /sys/kernel/pkram. If none is passed, any
>>>> preserved memory will not be kept, and a new super block will be
>>>> allocated.
>>>>
>>>> Originally-by: Vladimir Davydov <vdavydov.dev@gmail.com>
>>>> Signed-off-by: Anthony Yznaga <anthony.yznaga@oracle.com>
>>>> ---
>>>> mm/pkram.c | 102 
>>>> +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++--
>>>> 1 file changed, 100 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/mm/pkram.c b/mm/pkram.c
>>>> index da166cb6afb7..c66b2ae4d520 100644
>>>> --- a/mm/pkram.c
>>>> +++ b/mm/pkram.c
>>>> @@ -5,15 +5,18 @@
>>>> #include <linux/init.h>
>>>> #include <linux/io.h>
>>>> #include <linux/kernel.h>
>>>> +#include <linux/kobject.h>
>>>> #include <linux/list.h>
>>>> #include <linux/mm.h>
>>>> #include <linux/module.h>
>>>> #include <linux/mutex.h>
>>>> #include <linux/notifier.h>
>>>> +#include <linux/pfn.h>
>>>> #include <linux/pkram.h>
>>>> #include <linux/reboot.h>
>>>> #include <linux/sched.h>
>>>> #include <linux/string.h>
>>>> +#include <linux/sysfs.h>
>>>> #include <linux/types.h>
>>>>
>>>> #include "internal.h"
>>>> @@ -82,12 +85,38 @@ struct pkram_node {
>>>> #define PKRAM_ACCMODE_MASK    3
>>>>
>>>> /*
>>>> + * The PKRAM super block contains data needed to restore the 
>>>> preserved memory
>>>> + * structure on boot. The pointer to it (pfn) should be passed via 
>>>> the 'pkram'
>>>> + * boot param if one wants to restore preserved data saved by the 
>>>> previously
>>>> + * executing kernel. For that purpose the kernel exports the pfn via
>>>> + * /sys/kernel/pkram. If none is passed, preserved memory if any 
>>>> will not be
>>>> + * preserved and a new clean page will be allocated for the super 
>>>> block.
>>>> + *
>>>> + * The structure occupies a memory page.
>>>> + */
>>>> +struct pkram_super_block {
>>>> +    __u64    node_pfn;        /* first element of the node list */
>>>> +};
>>>> +
>>>> +static unsigned long pkram_sb_pfn __initdata;
>>>> +static struct pkram_super_block *pkram_sb;
>>>> +
>>>> +/*
>>>>  * For convenience sake PKRAM nodes are kept in an auxiliary 
>>>> doubly-linked list
>>>>  * connected through the lru field of the page struct.
>>>>  */
>>>> static LIST_HEAD(pkram_nodes);            /* linked through 
>>>> page::lru */
>>>> static DEFINE_MUTEX(pkram_mutex);        /* serializes open/close */
>>>>
>>>> +/*
>>>> + * The PKRAM super block pfn, see above.
>>>> + */
>>>> +static int __init parse_pkram_sb_pfn(char *arg)
>>>> +{
>>>> +    return kstrtoul(arg, 16, &pkram_sb_pfn);
>>>> +}
>>>> +early_param("pkram", parse_pkram_sb_pfn);
>>>> +
>>>> static inline struct page *pkram_alloc_page(gfp_t gfp_mask)
>>>> {
>>>>     return alloc_page(gfp_mask);
>>>> @@ -270,6 +299,7 @@ static void pkram_stream_init(struct 
>>>> pkram_stream *ps,
>>>>  * @gfp_mask specifies the memory allocation mask to be used when 
>>>> saving data.
>>>>  *
>>>>  * Error values:
>>>> + *    %ENODEV: PKRAM not available
>>>>  *    %ENAMETOOLONG: name len >= PKRAM_NAME_MAX
>>>>  *    %ENOMEM: insufficient memory available
>>>>  *    %EEXIST: node with specified name already exists
>>>> @@ -285,6 +315,9 @@ int pkram_prepare_save(struct pkram_stream *ps, 
>>>> const char *name, gfp_t gfp_mask
>>>>     struct pkram_node *node;
>>>>     int err = 0;
>>>>
>>>> +    if (!pkram_sb)
>>>> +        return -ENODEV;
>>>> +
>>>>     if (strlen(name) >= PKRAM_NAME_MAX)
>>>>         return -ENAMETOOLONG;
>>>>
>>>> @@ -404,6 +437,7 @@ void pkram_discard_save(struct pkram_stream *ps)
>>>>  * Returns 0 on success, -errno on failure.
>>>>  *
>>>>  * Error values:
>>>> + *    %ENODEV: PKRAM not available
>>>>  *    %ENOENT: node with specified name does not exist
>>>>  *    %EBUSY: save to required node has not finished yet
>>>>  *
>>>> @@ -414,6 +448,9 @@ int pkram_prepare_load(struct pkram_stream *ps, 
>>>> const char *name)
>>>>     struct pkram_node *node;
>>>>     int err = 0;
>>>>
>>>> +    if (!pkram_sb)
>>>> +        return -ENODEV;
>>>> +
>>>>     mutex_lock(&pkram_mutex);
>>>>     node = pkram_find_node(name);
>>>>     if (!node) {
>>>> @@ -825,6 +862,13 @@ static void __pkram_reboot(void)
>>>>         node->node_pfn = node_pfn;
>>>>         node_pfn = page_to_pfn(page);
>>>>     }
>>>> +
>>>> +    /*
>>>> +     * Zero out pkram_sb completely since it may have been passed 
>>>> from
>>>> +     * the previous boot.
>>>> +     */
>>>> +    memset(pkram_sb, 0, PAGE_SIZE);
>>>> +    pkram_sb->node_pfn = node_pfn;
>>>> }
>>>>
>>>> static int pkram_reboot(struct notifier_block *notifier,
>>>> @@ -832,7 +876,8 @@ static int pkram_reboot(struct notifier_block 
>>>> *notifier,
>>>> {
>>>>     if (val != SYS_RESTART)
>>>>         return NOTIFY_DONE;
>>>> -    __pkram_reboot();
>>>> +    if (pkram_sb)
>>>> +        __pkram_reboot();
>>>>     return NOTIFY_OK;
>>>> }
>>>>
>>>> @@ -840,9 +885,62 @@ static int pkram_reboot(struct notifier_block 
>>>> *notifier,
>>>>     .notifier_call = pkram_reboot,
>>>> };
>>>>
>>>> +static ssize_t show_pkram_sb_pfn(struct kobject *kobj,
>>>> +        struct kobj_attribute *attr, char *buf)
>>>> +{
>>>> +    unsigned long pfn = pkram_sb ? PFN_DOWN(__pa(pkram_sb)) : 0;
>>>> +
>>>> +    return sprintf(buf, "%lx\n", pfn);
>>>> +}
>>>> +
>>>> +static struct kobj_attribute pkram_sb_pfn_attr =
>>>> +    __ATTR(pkram, 0444, show_pkram_sb_pfn, NULL);
>>>> +
>>>> +static struct attribute *pkram_attrs[] = {
>>>> +    &pkram_sb_pfn_attr.attr,
>>>> +    NULL,
>>>> +};
>>>> +
>>>> +static struct attribute_group pkram_attr_group = {
>>>> +    .attrs = pkram_attrs,
>>>> +};
>>>> +
>>>> +/* returns non-zero on success */
>>>> +static int __init pkram_init_sb(void)
>>>> +{
>>>> +    unsigned long pfn;
>>>> +    struct pkram_node *node;
>>>> +
>>>> +    if (!pkram_sb) {
>>>> +        struct page *page;
>>>> +
>>>> +        page = pkram_alloc_page(GFP_KERNEL | __GFP_ZERO);
>>>> +        if (!page) {
>>>> +            pr_err("PKRAM: Failed to allocate super block\n");
>>>> +            return 0;
>>>> +        }
>>>> +        pkram_sb = page_address(page);
>>>> +    }
>>>> +
>>>> +    /*
>>>> +     * Build auxiliary doubly-linked list of nodes connected through
>>>> +     * page::lru for convenience sake.
>>>> +     */
>>>> +    pfn = pkram_sb->node_pfn;
>>>> +    while (pfn) {
>>>> +        node = pfn_to_kaddr(pfn);
>>>> +        pkram_insert_node(node);
>>>> +        pfn = node->node_pfn;
>>>> +    }
>>>> +    return 1;
>>>> +}
>>>> +
>>>> static int __init pkram_init(void)
>>>> {
>>>> -    register_reboot_notifier(&pkram_reboot_notifier);
>>>> +    if (pkram_init_sb()) {
>>>> + register_reboot_notifier(&pkram_reboot_notifier);
>>>> +        sysfs_update_group(kernel_kobj, &pkram_attr_group);
>>>> +    }
>>>>     return 0;
>>>> }
>>>> module_init(pkram_init);
>>>> -- 
>>>> 1.9.4
>>>>
>>>>
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>>>>
>>>
>>
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