Message-ID: <111927ac-752b-422e-b313-5a96bc736899@linux.alibaba.com>
Date: Fri, 2 Aug 2024 17:35:14 +0800
From: Baolin Wang <baolin.wang@...ux.alibaba.com>
To: David Hildenbrand <david@...hat.com>, linux-kernel@...r.kernel.org
Cc: linux-mm@...ck.org, Peter Xu <peterx@...hat.com>, stable@...r.kernel.org,
 Oscar Salvador <osalvador@...e.de>, Muchun Song <muchun.song@...ux.dev>
Subject: Re: [PATCH v4] mm/hugetlb: fix hugetlb vs. core-mm PT locking



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@redhat.com/
>>>
>>> Fixes: 9cb28da54643 ("mm/gup: handle hugetlb in the generic 
>>> follow_page_mask code")
>>> Acked-by: Peter Xu <peterx@...hat.com>
>>> Cc: <stable@...r.kernel.org>
>>> Cc: Peter Xu <peterx@...hat.com>
>>> Cc: Oscar Salvador <osalvador@...e.de>
>>> Cc: Muchun Song <muchun.song@...ux.dev>
>>> Cc: Baolin Wang <baolin.wang@...ux.alibaba.com>
>>> Signed-off-by: David Hildenbrand <david@...hat.com>
>>
>> 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@...ux.alibaba.com>
>> Tested-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
> 
> 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:)

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