Re: [PATCH v1] mm: zero remaining unavailable struct pages (Re: kernel panic in reading /proc/kpageflags when enabling RAM-simulated PMEM)

[Oscar Salvador <osalvador@techadventures.net>]

On Wed, Jun 13, 2018 at 05:41:08AM +0000, Naoya Horiguchi wrote:
> Hi everyone, 
> 
> I wrote a patch for this issue.
> There was a discussion about prechecking approach, but I finally found
> out it's hard to make change on memblock after numa_init, so I take
> another apporach (see patch description).
> 
> I'm glad if you check that it works for you.
> 
> Thanks,
> Naoya Horiguchi
> ---
> From: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
> Date: Wed, 13 Jun 2018 12:43:27 +0900
> Subject: [PATCH] mm: zero remaining unavailable struct pages
> 
> There is a kernel panic that is triggered when reading /proc/kpageflags
> on the kernel booted with kernel parameter 'memmap=nn[KMG]!ss[KMG]':
> 
>   BUG: unable to handle kernel paging request at fffffffffffffffe
>   PGD 9b20e067 P4D 9b20e067 PUD 9b210067 PMD 0
>   Oops: 0000 [#1] SMP PTI
>   CPU: 2 PID: 1728 Comm: page-types Not tainted 4.17.0-rc6-mm1-v4.17-rc6-180605-0816-00236-g2dfb086ef02c+ #160
>   Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.11.0-2.fc28 04/01/2014
>   RIP: 0010:stable_page_flags+0x27/0x3c0
>   Code: 00 00 00 0f 1f 44 00 00 48 85 ff 0f 84 a0 03 00 00 41 54 55 49 89 fc 53 48 8b 57 08 48 8b 2f 48 8d 42 ff 83 e2 01 48 0f 44 c7 <48> 8b 00 f6 c4 01 0f 84 10 03 00 00 31 db 49 8b 54 24 08 4c 89 e7
>   RSP: 0018:ffffbbd44111fde0 EFLAGS: 00010202
>   RAX: fffffffffffffffe RBX: 00007fffffffeff9 RCX: 0000000000000000
>   RDX: 0000000000000001 RSI: 0000000000000202 RDI: ffffed1182fff5c0
>   RBP: ffffffffffffffff R08: 0000000000000001 R09: 0000000000000001
>   R10: ffffbbd44111fed8 R11: 0000000000000000 R12: ffffed1182fff5c0
>   R13: 00000000000bffd7 R14: 0000000002fff5c0 R15: ffffbbd44111ff10
>   FS:  00007efc4335a500(0000) GS:ffff93a5bfc00000(0000) knlGS:0000000000000000
>   CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>   CR2: fffffffffffffffe CR3: 00000000b2a58000 CR4: 00000000001406e0
>   Call Trace:
>    kpageflags_read+0xc7/0x120
>    proc_reg_read+0x3c/0x60
>    __vfs_read+0x36/0x170
>    vfs_read+0x89/0x130
>    ksys_pread64+0x71/0x90
>    do_syscall_64+0x5b/0x160
>    entry_SYSCALL_64_after_hwframe+0x44/0xa9
>   RIP: 0033:0x7efc42e75e23
>   Code: 09 00 ba 9f 01 00 00 e8 ab 81 f4 ff 66 2e 0f 1f 84 00 00 00 00 00 90 83 3d 29 0a 2d 00 00 75 13 49 89 ca b8 11 00 00 00 0f 05 <48> 3d 01 f0 ff ff 73 34 c3 48 83 ec 08 e8 db d3 01 00 48 89 04 24
> 
> According to kernel bisection, this problem became visible due to commit
> f7f99100d8d9 which changes how struct pages are initialized.
> 
> Memblock layout affects the pfn ranges covered by node/zone. Consider
> that we have a VM with 2 NUMA nodes and each node has 4GB memory, and
> the default (no memmap= given) memblock layout is like below:
> 
>   MEMBLOCK configuration:
>    memory size = 0x00000001fff75c00 reserved size = 0x000000000300c000
>    memory.cnt  = 0x4
>    memory[0x0]     [0x0000000000001000-0x000000000009efff], 0x000000000009e000 bytes on node 0 flags: 0x0
>    memory[0x1]     [0x0000000000100000-0x00000000bffd6fff], 0x00000000bfed7000 bytes on node 0 flags: 0x0
>    memory[0x2]     [0x0000000100000000-0x000000013fffffff], 0x0000000040000000 bytes on node 0 flags: 0x0
>    memory[0x3]     [0x0000000140000000-0x000000023fffffff], 0x0000000100000000 bytes on node 1 flags: 0x0
>    ...
> 
> If you give memmap=1G!4G (so it just covers memory[0x2]),
> the range [0x100000000-0x13fffffff] is gone:
> 
>   MEMBLOCK configuration:
>    memory size = 0x00000001bff75c00 reserved size = 0x000000000300c000
>    memory.cnt  = 0x3
>    memory[0x0]     [0x0000000000001000-0x000000000009efff], 0x000000000009e000 bytes on node 0 flags: 0x0
>    memory[0x1]     [0x0000000000100000-0x00000000bffd6fff], 0x00000000bfed7000 bytes on node 0 flags: 0x0
>    memory[0x2]     [0x0000000140000000-0x000000023fffffff], 0x0000000100000000 bytes on node 1 flags: 0x0
>    ...
> 
> This causes shrinking node 0's pfn range because it is calculated by
> the address range of memblock.memory. So some of struct pages in the
> gap range are left uninitialized.
> 
> We have a function zero_resv_unavail() which does zeroing the struct
> pages outside memblock.memory, but currently it covers only the reserved
> unavailable range (i.e. memblock.memory && !memblock.reserved).
> This patch extends it to cover all unavailable range, which fixes
> the reported issue.
> 
> Fixes: f7f99100d8d9 ("mm: stop zeroing memory during allocation in vmemmap")
> Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
> ---
>  include/linux/memblock.h | 16 ----------------
>  mm/page_alloc.c          | 33 ++++++++++++++++++++++++---------
>  2 files changed, 24 insertions(+), 25 deletions(-)
> 
> diff --git a/include/linux/memblock.h b/include/linux/memblock.h
> index ca59883c8364..f191e51c5d2a 100644
> --- a/include/linux/memblock.h
> +++ b/include/linux/memblock.h
> @@ -236,22 +236,6 @@ void __next_mem_pfn_range(int *idx, int nid, unsigned long *out_start_pfn,
>  	for_each_mem_range_rev(i, &memblock.memory, &memblock.reserved,	\
>  			       nid, flags, p_start, p_end, p_nid)
>  
> -/**
> - * for_each_resv_unavail_range - iterate through reserved and unavailable memory
> - * @i: u64 used as loop variable
> - * @flags: pick from blocks based on memory attributes
> - * @p_start: ptr to phys_addr_t for start address of the range, can be %NULL
> - * @p_end: ptr to phys_addr_t for end address of the range, can be %NULL
> - *
> - * Walks over unavailable but reserved (reserved && !memory) areas of memblock.
> - * Available as soon as memblock is initialized.
> - * Note: because this memory does not belong to any physical node, flags and
> - * nid arguments do not make sense and thus not exported as arguments.
> - */
> -#define for_each_resv_unavail_range(i, p_start, p_end)			\
> -	for_each_mem_range(i, &memblock.reserved, &memblock.memory,	\
> -			   NUMA_NO_NODE, MEMBLOCK_NONE, p_start, p_end, NULL)
> -
>  static inline void memblock_set_region_flags(struct memblock_region *r,
>  					     unsigned long flags)
>  {
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 1772513358e9..098f7c2c127b 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -6487,25 +6487,40 @@ void __paginginit free_area_init_node(int nid, unsigned long *zones_size,
>   * struct pages which are reserved in memblock allocator and their fields
>   * may be accessed (for example page_to_pfn() on some configuration accesses
>   * flags). We must explicitly zero those struct pages.
> + *
> + * This function also addresses a similar issue where struct pages are left
> + * uninitialized because the physical address range is not covered by
> + * memblock.memory or memblock.reserved. That could happen when memblock
> + * layout is manually configured via memmap=.
>   */
>  void __paginginit zero_resv_unavail(void)
>  {
>  	phys_addr_t start, end;
>  	unsigned long pfn;
>  	u64 i, pgcnt;
> +	phys_addr_t next = 0;
>  
>  	/*
> -	 * Loop through ranges that are reserved, but do not have reported
> -	 * physical memory backing.
> +	 * Loop through unavailable ranges not covered by memblock.memory.
>  	 */
>  	pgcnt = 0;
> -	for_each_resv_unavail_range(i, &start, &end) {
> -		for (pfn = PFN_DOWN(start); pfn < PFN_UP(end); pfn++) {
> -			if (!pfn_valid(ALIGN_DOWN(pfn, pageblock_nr_pages)))
> -				continue;
> -			mm_zero_struct_page(pfn_to_page(pfn));
> -			pgcnt++;
> +	for_each_mem_range(i, &memblock.memory, NULL,
> +			NUMA_NO_NODE, MEMBLOCK_NONE, &start, &end, NULL) {
> +		if (next < start) {
> +			for (pfn = PFN_DOWN(next); pfn < PFN_UP(start); pfn++) {
> +				if (!pfn_valid(ALIGN_DOWN(pfn, pageblock_nr_pages)))
> +					continue;
> +				mm_zero_struct_page(pfn_to_page(pfn));
> +				pgcnt++;
> +			}
>  		}
> +		next = end;
> +	}
> +	for (pfn = PFN_DOWN(next); pfn < max_pfn; pfn++) {
> +		if (!pfn_valid(ALIGN_DOWN(pfn, pageblock_nr_pages)))
> +			continue;
> +		mm_zero_struct_page(pfn_to_page(pfn));
> +		pgcnt++;
>  	}

Hi Naoya,

Is the second loop really needed?

AFAIK, max_pfn is set to the latest pfn of E820_TYPE_RAM type, and since
you are going through all memory ranges within memblock.memory, and then assigning next = end,
I think that at the time we are done with the first loop, next will always point
to max_pfn (I only checked it for x86).
Am I right o did I overlooked something?

Besides that, I did some tests and I can no longer reproduce the error.
So feel free to add: 

Tested-by: Oscar Salvador <osalvador@suse.de>

>  
>  	/*
> @@ -6516,7 +6531,7 @@ void __paginginit zero_resv_unavail(void)
>  	 * this code can be removed.
>  	 */
>  	if (pgcnt)
> -		pr_info("Reserved but unavailable: %lld pages", pgcnt);
> +		pr_info("Zeroed struct page in unavailable ranges: %lld pages", pgcnt);
>  }
>  #endif /* CONFIG_HAVE_MEMBLOCK */
>  
> -- 
> 2.7.4
> 

Thanks

Best Regards
Oscar Salvador