mm/pagewalk: fix usage of pmd_leaf()/pud_leaf() without present check
pmd_leaf()/pud_leaf() only implies a pmd_present()/pud_present() check on
some architectures. We really should check for
pmd_present()/pud_present() first.
This should explain the report we got on ppc64 (which has
CONFIG_PGTABLE_HAS_HUGE_LEAVES set in the config) that triggered:
VM_WARN_ON_ONCE(pmd_leaf(pmdp_get_lockless(pmdp)));
Likely we had a PMD migration entry for which pmd_leaf() did not trigger.
We raced with restoring the PMD migration entry, and suddenly saw a
pmd_leaf(). In this case, pte_offset_map_lock() saved us from more
trouble, because it rechecks the PMD value, but we would not have
processed the migration entry -- which is not too bad because the only
user of FW_MIGRATION is KSM for unsharing, and KSM only applies to small
folios.
Further, we shouldn't re-read the PMD/PUD value for our warning, the
primary purpose of the VM_WARN_ON_ONCE() is to find spurious use of
pmd_leaf()/pud_leaf() without CONFIG_PGTABLE_HAS_HUGE_LEAVES.
As a side note, we are currently not implementing FW_MIGRATION support for
PUD migration entries, which likely should exist due to hugetlb. Add a
TODO so this won't fall through the cracks if more FW_MIGRATION users get
added.
Was able to write a quick reproducer and verify that the issue no longer triggers with this fix.
https://gitlab.com/davidhildenbrand/scratchspace/-/blob/main/reproducers/move-pages-pmd-leaf.c
Without this fix after a couple of seconds in a VM with 2 NUMA nodes:
[ 54.333753] ------------[ cut here ]------------
[ 54.334901] WARNING: CPU: 20 PID: 1704 at mm/pagewalk.c:815 folio_walk_start+0x48f/0x6e0
[ 54.336455] Modules linked in: ...
[ 54.345009] CPU: 20 UID: 0 PID: 1704 Comm: move-pages-pmd- Not tainted 6.12.0-rc2+ #81
[ 54.346529] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-2.fc40 04/01/2014
[ 54.348191] RIP: 0010:folio_walk_start+0x48f/0x6e0
[ 54.349134] Code: b5 ad 48 8d 35 00 00 00 00 e8 6d 59 d7 ff e8 08 74 da ff e9 9c fe ff ff 4c 8b 7c 24 08 4c 89 ff e8 26 2b be 00 e9 8a fe ff ff <0f> 0b e9 ec fe ff ff f7 c2 ff 0f 00 00 0f 85 81 fe ff ff 48 8b 02
[ 54.352660] RSP: 0018:
ffffb7e4c430bc78 EFLAGS:
00010282
[ 54.353679] RAX:
80000002a3e008e7 RBX:
ffff9946039aa580 RCX:
ffff994380000000
[ 54.355056] RDX:
ffff994606aec000 RSI:
00007f004b000000 RDI:
0000000000000000
[ 54.356440] RBP:
00007f004b000000 R08:
0000000000000591 R09:
0000000000000001
[ 54.357820] R10:
0000000000000200 R11:
0000000000000001 R12:
ffffb7e4c430bd10
[ 54.359198] R13:
ffff994606aec2c0 R14:
0000000000000002 R15:
ffff994604a89b00
[ 54.360564] FS:
00007f004ae006c0(0000) GS:
ffff9947f7400000(0000) knlGS:
0000000000000000
[ 54.362111] CS: 0010 DS: 0000 ES: 0000 CR0:
0000000080050033
[ 54.363242] CR2:
00007f004adffe58 CR3:
0000000281e12005 CR4:
0000000000770ef0
[ 54.364615] PKRU:
55555554
[ 54.365153] Call Trace:
[ 54.365646] <TASK>
[ 54.366073] ? __warn.cold+0xb7/0x14d
[ 54.366796] ? folio_walk_start+0x48f/0x6e0
[ 54.367628] ? report_bug+0xff/0x140
[ 54.368324] ? handle_bug+0x58/0x90
[ 54.369019] ? exc_invalid_op+0x17/0x70
[ 54.369771] ? asm_exc_invalid_op+0x1a/0x20
[ 54.370606] ? folio_walk_start+0x48f/0x6e0
[ 54.371415] ? folio_walk_start+0x9e/0x6e0
[ 54.372227] do_pages_move+0x1c5/0x680
[ 54.372972] kernel_move_pages+0x1a1/0x2b0
[ 54.373804] __x64_sys_move_pages+0x25/0x30
Link: https://lkml.kernel.org/r/[email protected]
Fixes: aa39ca6940f1 ("mm/pagewalk: introduce folio_walk_start() + folio_walk_end()")
Signed-off-by: David Hildenbrand <[email protected]>
Reported-by: [email protected]
Closes: https://lkml.kernel.org/r/[email protected]
Acked-by: Kirill A. Shutemov <[email protected]>
Acked-by: Qi Zheng <[email protected]>
Cc: Jann Horn <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>