Re: [PATCH v4] mm/hugetlb: fix hugetlb vs. core-mm PT locking

Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx> · Fri, 2 Aug 2024 17:35:14 +0800

On 2024/8/2 16:43, David Hildenbrand wrote:
On 02.08.24 05:56, Baolin Wang wrote:

On 2024/8/2 04:47, David Hildenbrand wrote:
We recently made GUP's common page table walking code to also walk 
hugetlb
VMAs without most hugetlb special-casing, preparing for the future of
having less hugetlb-specific page table walking code in the codebase.
Turns out that we missed one page table locking detail: page table 
locking
for hugetlb folios that are not mapped using a single PMD/PUD.

Assume we have hugetlb folio that spans multiple PTEs (e.g., 64 KiB
hugetlb folios on arm64 with 4 KiB base page size). GUP, as it walks the
page tables, will perform a pte_offset_map_lock() to grab the PTE table
lock.

However, hugetlb that concurrently modifies these page tables would
actually grab the mm->page_table_lock: with USE_SPLIT_PTE_PTLOCKS, the
locks would differ. Something similar can happen right now with hugetlb
folios that span multiple PMDs when USE_SPLIT_PMD_PTLOCKS.

This issue can be reproduced [1], for example triggering:

[ 3105.936100] ------------[ cut here ]------------
[ 3105.939323] WARNING: CPU: 31 PID: 2732 at mm/gup.c:142 
try_grab_folio+0x11c/0x188
[ 3105.944634] Modules linked in: [...]
[ 3105.974841] CPU: 31 PID: 2732 Comm: reproducer Not tainted 
6.10.0-64.eln141.aarch64 #1
[ 3105.980406] Hardware name: QEMU KVM Virtual Machine, BIOS 
edk2-20240524-4.fc40 05/24/2024
[ 3105.986185] pstate: 60000005 (nZCv daif -PAN -UAO -TCO -DIT -SSBS 
BTYPE=--)
[ 3105.991108] pc : try_grab_folio+0x11c/0x188
[ 3105.994013] lr : follow_page_pte+0xd8/0x430
[ 3105.996986] sp : ffff80008eafb8f0
[ 3105.999346] x29: ffff80008eafb900 x28: ffffffe8d481f380 x27: 
00f80001207cff43
[ 3106.004414] x26: 0000000000000001 x25: 0000000000000000 x24: 
ffff80008eafba48
[ 3106.009520] x23: 0000ffff9372f000 x22: ffff7a54459e2000 x21: 
ffff7a546c1aa978
[ 3106.014529] x20: ffffffe8d481f3c0 x19: 0000000000610041 x18: 
0000000000000001
[ 3106.019506] x17: 0000000000000001 x16: ffffffffffffffff x15: 
0000000000000000
[ 3106.024494] x14: ffffb85477fdfe08 x13: 0000ffff9372ffff x12: 
0000000000000000
[ 3106.029469] x11: 1fffef4a88a96be1 x10: ffff7a54454b5f0c x9 : 
ffffb854771b12f0
[ 3106.034324] x8 : 0008000000000000 x7 : ffff7a546c1aa980 x6 : 
0008000000000080
[ 3106.038902] x5 : 00000000001207cf x4 : 0000ffff9372f000 x3 : 
ffffffe8d481f000
[ 3106.043420] x2 : 0000000000610041 x1 : 0000000000000001 x0 : 
0000000000000000
[ 3106.047957] Call trace:
[ 3106.049522]  try_grab_folio+0x11c/0x188
[ 3106.051996]  follow_pmd_mask.constprop.0.isra.0+0x150/0x2e0
[ 3106.055527]  follow_page_mask+0x1a0/0x2b8
[ 3106.058118]  __get_user_pages+0xf0/0x348
[ 3106.060647]  faultin_page_range+0xb0/0x360
[ 3106.063651]  do_madvise+0x340/0x598

Let's make huge_pte_lockptr() effectively use the same PT locks as any
core-mm page table walker would. Add ptep_lockptr() to obtain the PTE
page table lock using a pte pointer -- unfortunately we cannot convert
pte_lockptr() because virt_to_page() doesn't work with kmap'ed page
tables we can have with CONFIG_HIGHPTE.

Handle CONFIG_PGTABLE_LEVELS correctly by checking in reverse order,
such that when e.g., CONFIG_PGTABLE_LEVELS==2 with
PGDIR_SIZE==P4D_SIZE==PUD_SIZE==PMD_SIZE will work as expected.
Document why that works.

There is one ugly case: powerpc 8xx, whereby we have an 8 MiB hugetlb
folio being mapped using two PTE page tables.  While hugetlb wants to 
take
the PMD table lock, core-mm would grab the PTE table lock of one of both
PTE page tables.  In such corner cases, we have to make sure that both
locks match, which is (fortunately!) currently guaranteed for 8xx as it
does not support SMP and consequently doesn't use split PT locks.

[1] 
https://lore.kernel.org/all/1bbfcc7f-f222-45a5-ac44-c5a1381c596d@xxxxxxxxxx/

Fixes: 9cb28da54643 ("mm/gup: handle hugetlb in the generic 
follow_page_mask code")
Acked-by: Peter Xu <peterx@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Cc: Peter Xu <peterx@xxxxxxxxxx>
Cc: Oscar Salvador <osalvador@xxxxxxx>
Cc: Muchun Song <muchun.song@xxxxxxxxx>
Cc: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>

I tried your reproducer on my ARM64 machine, and this patch can fix the
problem.

Although I know nothing about HIGHPTE, the other parts look good to me.
So feel free to add:
Reviewed-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
Tested-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>

Thanks! Took longer than expected to get this (hopefully ;) ) right.

HIGHPTE means that we allocate PTE page tables from highmem on 32bit
architectures. I think it's only supported on x86 and arm.

If we allocate page tables from highmem, when we want to read/write
them, we have to kmap them into kernel virtual address space. This
is what the whole pte_offset_map_lock() / pte_unmap() does. On
!highmem configs, the "map/unmap" is a NOP.

Hugetlb doesn't use pte_offset_map_lock/pte_unmap() when accessing
page tables and assumes that they are never allocated from highmem.
So there is the implicit assumption that architectures that use
PTE page tables for hugetlb don't use HIGHPTE. For this reason, also
pte_alloc_huge() is guarded by CONFIG_HIGHPTE:

include/linux/hugetlb.h:
"
#ifndef CONFIG_HIGHPTE
/*
  * pte_offset_huge() and pte_alloc_huge() are helpers for those 
architectures
  * which may go down to the lowest PTE level in their huge_pte_offset() 
and
  * huge_pte_alloc(): to avoid reliance on pte_offset_map() without 
pte_unmap().
  */
"

Thanks for the explanation:)