[PATCH] mm: idle-page: fix oops because end_pfn is larger than max_pfn

[Colin King]

From: Colin Ian King <colin.king@xxxxxxxxxxxxx>

Currently the calcuation of end_pfn can round up the pfn number to
more than the actual maximum number of pfns, causing an Oops. Fix
this by ensuring end_pfn is never more than max_pfn.

This can be easily triggered when on systems where the end_pfn gets
rounded up to more than max_pfn using the idle-page stress-ng
stress test:

sudo stress-ng --idle-page 0

[ 3812.222790] BUG: unable to handle kernel paging request at 00000000000020d8
[ 3812.224341] #PF error: [normal kernel read fault]
[ 3812.225144] PGD 0 P4D 0
[ 3812.225626] Oops: 0000 [#1] SMP PTI
[ 3812.226264] CPU: 1 PID: 11039 Comm: stress-ng-idle- Not tainted 5.0.0-5-generic #6-Ubuntu
[ 3812.227643] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1ubuntu1 04/01/2014
[ 3812.229286] RIP: 0010:page_idle_get_page+0xc8/0x1a0
[ 3812.230173] Code: 0f b1 0a 75 7d 48 8b 03 48 89 c2 48 c1 e8 33 83 e0 07 48 c1 ea 36 48 8d 0c 40 4c 8d 24 88 49 c1 e4 07 4c 03 24 d5 00 89 c3 be <49> 8b 44 24 58 48 8d b8 80 a1 02 00 e8 07 d5 77 00 48 8b 53 08 48
[ 3812.234641] RSP: 0018:ffffafd7c672fde8 EFLAGS: 00010202
[ 3812.235792] RAX: 0000000000000005 RBX: ffffe36341fff700 RCX: 000000000000000f
[ 3812.237739] RDX: 0000000000000284 RSI: 0000000000000275 RDI: 0000000001fff700
[ 3812.239225] RBP: ffffafd7c672fe00 R08: ffffa0bc34056410 R09: 0000000000000276
[ 3812.241027] R10: ffffa0bc754e9b40 R11: ffffa0bc330f6400 R12: 0000000000002080
[ 3812.242555] R13: ffffe36341fff700 R14: 0000000000080000 R15: ffffa0bc330f6400
[ 3812.244073] FS: 00007f0ec1ea5740(0000) GS:ffffa0bc7db00000(0000) knlGS:0000000000000000
[ 3812.245968] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 3812.247162] CR2: 00000000000020d8 CR3: 0000000077d68000 CR4: 00000000000006e0
[ 3812.249045] Call Trace:
[ 3812.249625] page_idle_bitmap_write+0x8c/0x140
[ 3812.250567] sysfs_kf_bin_write+0x5c/0x70
[ 3812.251406] kernfs_fop_write+0x12e/0x1b0
[ 3812.252282] __vfs_write+0x1b/0x40
[ 3812.253002] vfs_write+0xab/0x1b0
[ 3812.253941] ksys_write+0x55/0xc0
[ 3812.254660] __x64_sys_write+0x1a/0x20
[ 3812.255446] do_syscall_64+0x5a/0x110
[ 3812.256254] entry_SYSCALL_64_after_hwframe+0x44/0xa9

Fixes: 33c3fc71c8cf ("mm: introduce idle page tracking")
Signed-off-by: Colin Ian King <colin.king@xxxxxxxxxxxxx>
---
 mm/page_idle.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/page_idle.c b/mm/page_idle.c
index 0b39ec0c945c..295512465065 100644
--- a/mm/page_idle.c
+++ b/mm/page_idle.c
@@ -136,7 +136,7 @@ static ssize_t page_idle_bitmap_read(struct file *file, struct kobject *kobj,
 
 	end_pfn = pfn + count * BITS_PER_BYTE;
 	if (end_pfn > max_pfn)
-		end_pfn = ALIGN(max_pfn, BITMAP_CHUNK_BITS);
+		end_pfn = max_pfn;
 
 	for (; pfn < end_pfn; pfn++) {
 		bit = pfn % BITMAP_CHUNK_BITS;
@@ -181,7 +181,7 @@ static ssize_t page_idle_bitmap_write(struct file *file, struct kobject *kobj,
 
 	end_pfn = pfn + count * BITS_PER_BYTE;
 	if (end_pfn > max_pfn)
-		end_pfn = ALIGN(max_pfn, BITMAP_CHUNK_BITS);
+		end_pfn = max_pfn;
 
 	for (; pfn < end_pfn; pfn++) {
 		bit = pfn % BITMAP_CHUNK_BITS;
-- 
2.20.1