Re: [RFC v3 07/21] mm: PKRAM: introduce super block

[Coiby Xu <coxu@redhat.com>]

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
>>>
>>>
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>>>
>>
>

-- 
Best regards,
Coiby


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