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Date:   Tue, 6 Jun 2023 10:55:06 +0800
From:   Coiby Xu <coxu@redhat.com>
To:     Anthony Yznaga <anthony.yznaga@oracle.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
Subject: Re: [RFC v3 07/21] mm: PKRAM: introduce super block
Message-ID: <aee5eximmv33ke26xrodm7tlqwwojalf2lz3zrgkt75bxeuupe@y3t2sfgdcy7k>
References: <1682554137-13938-1-git-send-email-anthony.yznaga@oracle.com>
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 <f287334c-4054-525e-52ea-a0bbf077b653@oracle.com>
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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.

>
>
>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
>>>
>>>
>>>_______________________________________________
>>>kexec mailing list
>>>kexec@lists.infradead.org
>>>http://lists.infradead.org/mailman/listinfo/kexec
>>>
>>
>

-- 
Best regards,
Coiby