Patch series "mm: fix max_pfn not falling on section boundary", v2.

Playing with different memory sizes for a x86-64 guest, I discovered that
some memmaps (highest section if max_mem does not fall on the section
boundary) are marked as being valid and online, but contain garbage.  We
have to properly initialize these memmaps.

Looking at /proc/kpageflags and friends, I found some more issues,
partially related to this.

This patch (of 3):

If max_pfn is not aligned to a section boundary, we can easily run into
BUGs.  This can e.g., be triggered on x86-64 under QEMU by specifying a
memory size that is not a multiple of 128MB (e.g., 4097MB, but also
4160MB).  I was told that on real HW, we can easily have this scenario
(esp., one of the main reasons sub-section hotadd of devmem was added).

The issue is, that we have a valid memmap (pfn_valid()) for the whole
section, and the whole section will be marked "online".
pfn_to_online_page() will succeed, but the memmap contains garbage.

E.g., doing a "./page-types -r -a 0x144001" when QEMU was started with "-m
4160M" - (see tools/vm/page-types.c):

[  200.476376] BUG: unable to handle page fault for address: fffffffffffffffe
[  200.477500] #PF: supervisor read access in kernel mode
[  200.478334] #PF: error_code(0x0000) - not-present page
[  200.479076] PGD 59614067 P4D 59614067 PUD 59616067 PMD 0
[  200.479557] Oops: 0000 [#4] SMP NOPTI
[  200.479875] CPU: 0 PID: 603 Comm: page-types Tainted: G      D W         5.5.0-rc1-next-20191209 #93
[  200.480646] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.12.0-59-gc9ba5276e321-prebuilt.qemu4
[  200.481648] RIP: 0010:stable_page_flags+0x4d/0x410
[  200.482061] Code: f3 ff 41 89 c0 48 b8 00 00 00 00 01 00 00 00 45 84 c0 0f 85 cd 02 00 00 48 8b 53 08 48 8b 2b 48f
[  200.483644] RSP: 0018:ffffb139401cbe60 EFLAGS: 00010202
[  200.484091] RAX: fffffffffffffffe RBX: fffffbeec5100040 RCX: 0000000000000000
[  200.484697] RDX: 0000000000000001 RSI: ffffffff9535c7cd RDI: 0000000000000246
[  200.485313] RBP: ffffffffffffffff R08: 0000000000000000 R09: 0000000000000000
[  200.485917] R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000144001
[  200.486523] R13: 00007ffd6ba55f48 R14: 00007ffd6ba55f40 R15: ffffb139401cbf08
[  200.487130] FS:  00007f68df717580(0000) GS:ffff9ec77fa00000(0000) knlGS:0000000000000000
[  200.487804] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  200.488295] CR2: fffffffffffffffe CR3: 0000000135d48000 CR4: 00000000000006f0
[  200.488897] Call Trace:
[  200.489115]  kpageflags_read+0xe9/0x140
[  200.489447]  proc_reg_read+0x3c/0x60
[  200.489755]  vfs_read+0xc2/0x170
[  200.490037]  ksys_pread64+0x65/0xa0
[  200.490352]  do_syscall_64+0x5c/0xa0
[  200.490665]  entry_SYSCALL_64_after_hwframe+0x49/0xbe

But it can be triggered much easier via "cat /proc/kpageflags > /dev/null"
after cold/hot plugging a DIMM to such a system:

[root@localhost ~]# cat /proc/kpageflags > /dev/null
[  111.517275] BUG: unable to handle page fault for address: fffffffffffffffe
[  111.517907] #PF: supervisor read access in kernel mode
[  111.518333] #PF: error_code(0x0000) - not-present page
[  111.518771] PGD a240e067 P4D a240e067 PUD a2410067 PMD 0

This patch fixes that by at least zero-ing out that memmap (so e.g.,
page_to_pfn() will not crash).  Commit 907ec5fc ("mm: zero remaining
unavailable struct pages") tried to fix a similar issue, but forgot to
consider this special case.

After this patch, there are still problems to solve.  E.g., not all of
these pages falling into a memory hole will actually get initialized later
and set PageReserved - they are only zeroed out - but at least the
immediate crashes are gone.  A follow-up patch will take care of this.

Link: http://lkml.kernel.org/r/20191211163201.17179-2-david@redhat.com
Fixes: f7f99100 ("mm: stop zeroing memory during allocation in vmemmap")
Signed-off-by: David Hildenbrand <david@redhat.com>
Tested-by: Daniel Jordan <daniel.m.jordan@oracle.com>
Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc: Pavel Tatashin <pasha.tatashin@oracle.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Steven Sistare <steven.sistare@oracle.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Daniel Jordan <daniel.m.jordan@oracle.com>
Cc: Bob Picco <bob.picco@oracle.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: <stable@vger.kernel.org>	[4.15+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>