[RFC PATCH v2] mm: initialize struct pages reserved by ZONE_DEVICE driver.

[Toshiki Fukasawa <t-fukasawa@vx.jp.nec.com>]

A kernel panic is observed during reading
/proc/kpage{cgroup,count,flags} for first few pfns allocated by
pmem namespace:

BUG: unable to handle page fault for address: fffffffffffffffe
[  114.495280] #PF: supervisor read access in kernel mode
[  114.495738] #PF: error_code(0x0000) - not-present page
[  114.496203] PGD 17120e067 P4D 17120e067 PUD 171210067 PMD 0
[  114.496713] Oops: 0000 [#1] SMP PTI
[  114.497037] CPU: 9 PID: 1202 Comm: page-types Not tainted 5.3.0-rc1
[  114.497621] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.11.0-0-g63451fca13-prebuilt.qemu-project.org 04/01/2014
[  114.498706] RIP: 0010:stable_page_flags+0x27/0x3f0
[  114.499142] Code: 82 66 90 66 66 66 66 90 48 85 ff 0f 84 d1 03 00 00 41 54 55 48 89 fd 53 48 8b 57 08 48 8b 1f 48 8d 42 ff 83 e2 01 48 0f 44 c7 <48> 8b 00 f6 c4 02 0f 84 57 03 00 00 45 31 e4 48 8b 55 08 48 89 ef
[  114.500788] RSP: 0018:ffffa5e601a0fe60 EFLAGS: 00010202
[  114.501373] RAX: fffffffffffffffe RBX: ffffffffffffffff RCX: 0000000000000000
[  114.502009] RDX: 0000000000000001 RSI: 00007ffca13a7310 RDI: ffffd07489000000
[  114.502637] RBP: ffffd07489000000 R08: 0000000000000001 R09: 0000000000000000
[  114.503270] R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000240000
[  114.503896] R13: 0000000000080000 R14: 00007ffca13a7310 R15: ffffa5e601a0ff08
[  114.504530] FS:  00007f0266c7f540(0000) GS:ffff962dbbac0000(0000) knlGS:0000000000000000
[  114.505245] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  114.505754] CR2: fffffffffffffffe CR3: 000000023a204000 CR4: 00000000000006e0
[  114.506401] Call Trace:
[  114.506660]  kpageflags_read+0xb1/0x130
[  114.507051]  proc_reg_read+0x39/0x60
[  114.507387]  vfs_read+0x8a/0x140
[  114.507686]  ksys_pread64+0x61/0xa0
[  114.508021]  do_syscall_64+0x5f/0x1a0
[  114.508372]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
[  114.508844] RIP: 0033:0x7f0266ba426b

The first few pages of ZONE_DEVICE expressed as the range
(altmap->base_pfn) to (altmap->base_pfn + altmap->reserve) are
skipped by struct page initialization. Some pfn walkers like
/proc/kpage{cgroup, count, flags} can't handle these uninitialized
struct pages, which causes the error.

In previous discussion, Dan seemed to have concern that the struct
page area of some pages indicated by vmem_altmap->reserve may not
be allocated. (See https://lore.kernel.org/lkml/CAPcyv4i5FjTOnPbXNcTzvt+e6RQYow0JRQwSFuxaa62LSuvzHQ@mail.gmail.com/)
However, arch_add_memory() called by devm_memremap_pages() allocates
struct page area for pages containing addresses in the range
(res.start) to (res.start + resource_size(res)), which include the
pages indicated by vmem_altmap->reserve. If I read correctly, it is
allocated as requested at least on x86_64. Also, memmap_init_zone()
initializes struct pages in the same range.
So I think the struct pages should be initialized.

Signed-off-by: Toshiki Fukasawa <t-fukasawa@vx.jp.nec.com>
Cc: stable@vger.kernel.org
---
Changes since rev 1:
 Instead of avoiding uninitialized pages on the pfn walker side,
 we initialize struct pages.

mm/page_alloc.c | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 9c91949..6d180ae 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -5846,8 +5846,7 @@ void __meminit memmap_init_zone(unsigned long size, int nid, unsigned long zone,
 
 #ifdef CONFIG_ZONE_DEVICE
 	/*
-	 * Honor reservation requested by the driver for this ZONE_DEVICE
-	 * memory. We limit the total number of pages to initialize to just
+	 * We limit the total number of pages to initialize to just
 	 * those that might contain the memory mapping. We will defer the
 	 * ZONE_DEVICE page initialization until after we have released
 	 * the hotplug lock.
@@ -5856,8 +5855,6 @@ void __meminit memmap_init_zone(unsigned long size, int nid, unsigned long zone,
 		if (!altmap)
 			return;
 
-		if (start_pfn == altmap->base_pfn)
-			start_pfn += altmap->reserve;
 		end_pfn = altmap->base_pfn + vmem_altmap_offset(altmap);
 	}
 #endif
-- 
1.8.3.1