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Date: Tue, 7 Aug 2018 10:35:47 +0530
From: Bhupesh Sharma <bhsharma@...hat.com>
To: Omar Sandoval <osandov@...ndov.com>, linux-kernel@...r.kernel.org,
linux-fsdevel@...r.kernel.org,
Andrew Morton <akpm@...ux-foundation.org>
Cc: Alexey Dobriyan <adobriyan@...il.com>,
Eric Biederman <ebiederm@...ssion.com>,
James Morse <james.morse@....com>, kernel-team@...com
Subject: Re: [PATCH v4 1/9] proc/kcore: don't grab lock for kclist_add()
Hello Omar,
On 07/26/2018 05:29 AM, Omar Sandoval wrote:
> From: Omar Sandoval <osandov@...com>
>
> kclist_add() is only called at init time, so there's no point in
> grabbing any locks. We're also going to replace the rwlock with a rwsem,
> which we don't want to try grabbing during early boot.
>
> While we're here, mark kclist_add() with __init so that we'll get a
> warning if it's called from non-init code.
>
> Reviewed-by: Andrew Morton <akpm@...ux-foundation.org>
> Signed-off-by: Omar Sandoval <osandov@...com>
> ---
> fs/proc/kcore.c | 7 +++----
> include/linux/kcore.h | 2 +-
> 2 files changed, 4 insertions(+), 5 deletions(-)
>
> diff --git a/fs/proc/kcore.c b/fs/proc/kcore.c
> index 66c373230e60..b0b9a76f28d6 100644
> --- a/fs/proc/kcore.c
> +++ b/fs/proc/kcore.c
> @@ -62,16 +62,15 @@ static LIST_HEAD(kclist_head);
> static DEFINE_RWLOCK(kclist_lock);
> static int kcore_need_update = 1;
>
> -void
> -kclist_add(struct kcore_list *new, void *addr, size_t size, int type)
> +/* This doesn't grab kclist_lock, so it should only be used at init time. */
> +void __init kclist_add(struct kcore_list *new, void *addr, size_t size,
> + int type)
> {
> new->addr = (unsigned long)addr;
> new->size = size;
> new->type = type;
>
> - write_lock(&kclist_lock);
> list_add_tail(&new->list, &kclist_head);
> - write_unlock(&kclist_lock);
> }
>
> static size_t get_kcore_size(int *nphdr, size_t *elf_buflen)
> diff --git a/include/linux/kcore.h b/include/linux/kcore.h
> index 8de55e4b5ee9..c20f296438fb 100644
> --- a/include/linux/kcore.h
> +++ b/include/linux/kcore.h
> @@ -35,7 +35,7 @@ struct vmcoredd_node {
> };
>
> #ifdef CONFIG_PROC_KCORE
> -extern void kclist_add(struct kcore_list *, void *, size_t, int type);
> +void __init kclist_add(struct kcore_list *, void *, size_t, int type);
> #else
> static inline
> void kclist_add(struct kcore_list *new, void *addr, size_t size, int type)
>
I have been looking at a problem on arm64 platforms where we are trying
to get access to PHYS_OFFSET symbol (which indicates the start of
physical RAM) in user-space for determining the start of physical RAM in
user-space utilities like 'kexec-tools' (please see [1] and [2] for
details).
Now, I have a 'kexec-tools' implementation available which can read the
PHYS_OFFSET from the VMCOREINFO inside '/proc/kcore', which I plan to
publish soon on my github tree.
I also see that 'readelf' and 'crash' can read the VMCOREINFO from the
'/proc/kcore' properly after this patch:
# readelf -a --wide /proc/kcore
Displaying notes found at file offset 0x00000778 with length 0x000019b4:
Owner Data size Description
CORE 0x00000188 NT_PRSTATUS (prstatus structure)
CORE 0x00000088 NT_PRPSINFO (prpsinfo structure)
CORE 0x00001040 NT_TASKSTRUCT (task structure)
VMCOREINFO 0x00000710 Unknown note type: (0x00000000)
<..snip..>
# crash /root/git/linux/vmlinux vmcore -d31
Elf64_Nhdr:
n_namesz: 11 ("VMCOREINFO")
n_descsz: 1829
n_type: 0 (unused)
OSRELEASE=4.18.0-rc7+
PAGESIZE=65536
SYMBOL(init_uts_ns)=ffff5493078a5428
SYMBOL(node_online_map)=ffff54930789d1c8
SYMBOL(swapper_pg_dir)=ffff549308380000
SYMBOL(_stext)=ffff549306681000
SYMBOL(vmap_area_list)=ffff549307944ee0
SYMBOL(mem_section)=ffff92047fffe400
LENGTH(mem_section)=64
SIZE(mem_section)=16
OFFSET(mem_section.section_mem_map)=0
SIZE(page)=64
SIZE(pglist_data)=6656
SIZE(zone)=1728
SIZE(free_area)=88
SIZE(list_head)=16
SIZE(nodemask_t)=8
OFFSET(page.flags)=0
OFFSET(page._refcount)=52
OFFSET(page.mapping)=24
OFFSET(page.lru)=8
OFFSET(page._mapcount)=48
OFFSET(page.private)=40
OFFSET(page.compound_dtor)=16
OFFSET(page.compound_order)=17
OFFSET(page.compound_head)=8
OFFSET(pglist_data.node_zones)=0
OFFSET(pglist_data.nr_zones)=5984
OFFSET(pglist_data.node_start_pfn)=5992
OFFSET(pglist_data.node_spanned_pages)=6008
OFFSET(pglist_data.node_id)=6016
OFFSET(zone.free_area)=192
OFFSET(zone.vm_stat)=1536
OFFSET(zone.spanned_pages)=96
OFFSET(free_area.free_list)=0
OFFSET(list_head.next)=0
OFFSET(list_head.prev)=8
OFFSET(vmap_area.va_start)=0
OFFSET(vmap_area.list)=48
LENGTH(zone.free_area)=14
SYMBOL(log_buf)=ffff5493078ddc30
SYMBOL(log_buf_len)=ffff5493078ddc28
SYMBOL(log_first_idx)=ffff5493081054ac
SYMBOL(clear_idx)=ffff5493081054b8
SYMBOL(log_next_idx)=ffff5493081054a8
SIZE(printk_log)=16
OFFSET(printk_log.ts_nsec)=0
OFFSET(printk_log.len)=8
OFFSET(printk_log.text_len)=10
OFFSET(printk_log.dict_len)=12
LENGTH(free_area.free_list)=5
NUMBER(NR_FREE_PAGES)=0
NUMBER(PG_lru)=5
NUMBER(PG_private)=12
NUMBER(PG_swapcache)=9
NUMBER(PG_swapbacked)=18
NUMBER(PG_slab)=8
NUMBER(PG_hwpoison)=21
NUMBER(PG_head_mask)=32768
NUMBER(PAGE_BUDDY_MAPCOUNT_VALUE)=-129
NUMBER(HUGETLB_PAGE_DTOR)=2
NUMBER(VA_BITS)=48
NUMBER(kimage_voffset)=0xffff5492f5c00000
NUMBER(PHYS_OFFSET)=0xffffee1380000000
CRASHTIME=1532965574
So, for what it is worth:
Reviewed-by and Tested-by: Bhupesh Sharma <bhsharma@...hat.com>
Thanks,
Bhupesh
[1] https://www.spinics.net/lists/kexec/msg20842.html
[2] https://www.spinics.net/lists/kexec/msg20618.html
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