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

/*
@@ -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
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