linux-kernel - Re: [PATCH -next 1/1] mm: hugetlb_vmemmap: Fix WARN_ON in vmemmap_remap

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Date:   Wed, 26 Oct 2022 10:36:02 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     Muchun Song <muchun.song@...ux.dev>,
        Wupeng Ma <mawupeng1@...wei.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Muchun Song <songmuchun@...edance.com>,
        Michal Hocko <mhocko@...e.com>,
        Oscar Salvador <osalvador@...e.de>, catalin.marinas@....com,
        Linux Memory Management List <linux-mm@...ck.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH -next 1/1] mm: hugetlb_vmemmap: Fix WARN_ON in
 vmemmap_remap_pte



On 10/25/22 12:06, Muchun Song wrote:
> 
> 
>> On Oct 25, 2022, at 09:42, Wupeng Ma <mawupeng1@...wei.com> wrote:
>>
>> From: Ma Wupeng <mawupeng1@...wei.com>
>>
>> Commit f41f2ed43ca5 ("mm: hugetlb: free the vmemmap pages associated with
>> each HugeTLB page") add vmemmap_remap_pte to remap the tail pages as
>> read-only to catch illegal write operation to the tail page.
>>
>> However this will lead to WARN_ON in arm64 in __check_racy_pte_update()
> 
> Thanks for your finding this issue.
> 
>> since this may lead to dirty state cleaned. This check is introduced by
>> commit 2f4b829c625e ("arm64: Add support for hardware updates of the
>> access and dirty pte bits") and the initial check is as follow:
>>
>>  BUG_ON(pte_write(*ptep) && !pte_dirty(pte));
>>
>> Since we do need to mark this pte as read-only to catch illegal write
>> operation to the tail pages, use set_pte  to replace set_pte_at to bypass
>> this check.
> 
> In theory, the waring does not affect anything since the tail vmemmap
> pages are supposed to be read-only. So, skipping this check for vmemmap

Tails vmemmap pages are supposed to be read-only, in practice but their
backing pages do have pte_write() enabled. Otherwise the VM_WARN_ONCE()
warning would not have triggered.

        VM_WARN_ONCE(pte_write(old_pte) && !pte_dirty(pte),
                     "%s: racy dirty state clearing: 0x%016llx -> 0x%016llx",
                     __func__, pte_val(old_pte), pte_val(pte));

Also, is not it true that the pte being remapped into a different page
as read only, than what it had originally (which will be freed up) i.e 
the PFN in 'old_pte' and 'pte' will be different. Hence is there still
a possibility for a race condition even when the PFN changes ?

> pages seem feasible. But I am not sure whether it is general to use
> set_pte here instead of set_pte_at, I didn’t see any users of set_pte
> from the common code routines except the code from arch/xxx. And this
> issue is specific for arm64, so I suggest fixing it in __check_racy_pte_update()
> itself.

Right, should not change it to yet lower level platform helper set_pte()
just to work around this warning. Instead, __check_racy_pte_update() is
the right place where it should be fixed.

> 
> Something like (Just some thoughts from mine):
> 
> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> index b5df82aa99e6..df7716965a93 100644
> --- a/arch/arm64/include/asm/pgtable.h
> +++ b/arch/arm64/include/asm/pgtable.h
> @@ -292,7 +292,8 @@ extern void __sync_icache_dcache(pte_t pteval);
>   *   PTE_DIRTY || (PTE_WRITE && !PTE_RDONLY)
>   */
> 
> -static inline void __check_racy_pte_update(struct mm_struct *mm, pte_t *ptep,
> +static inline void __check_racy_pte_update(struct mm_struct *mm,
> +                                          unsigned long addr, pte_t *ptep,
>                                            pte_t pte)
>  {
>         pte_t old_pte;
> @@ -307,6 +308,10 @@ static inline void __check_racy_pte_update(struct mm_struct *mm, pte_t *ptep,
>         if (mm != current->active_mm && atomic_read(&mm->mm_users) <= 1)
>                 return;
> 
> +       if (IS_ENABLED(CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP) &&
> +           addr >= VMEMMAP_START && addr <= VMEMMAP_END)
> +               return;
> +
>         /*
>          * Check for potential race with hardware updates of the pte
>          * (ptep_set_access_flags safely changes valid ptes without going
> 
>>
>> The following shell command can be used to reproduce this WARN_ON in
>> 6.1.0-rc1:
>>
>>  echo 1 > /proc/sys/vm/hugetlb_optimize_vmemmap
>>  cat /proc/sys/vm/hugetlb_optimize_vmemmap
>>
>>  echo 1024 > /proc/sys/vm/nr_overcommit_hugepages
>>  mkdir -p /root/hugetlb
>>  mount none /root/hugetlb -t hugetlbfs
>>  fallocate -l 2g /root/hugetlb/xx &
>>
>> Here is the detail WARN_ON log:
>>
>> ------------[ cut here ]------------
>> __check_racy_pte_update: racy dirty state clearing: 0x0068000416899f03 -> 0x0060000416898f83
>> WARNING: CPU: 3 PID: 394 at arch/arm64/include/asm/pgtable.h:318 vmemmap_remap_pte+0x118/0x120
>> Modules linked in:
>> CPU: 3 PID: 394 Comm: fallocate Not tainted 6.1.0-rc1 #224
>> Hardware name: QEMU QEMU Virtual Machine, BIOS 0.0.0 02/06/2015
>> Call trace:
>> vmemmap_remap_pte+0x118/0x120
>> vmemmap_remap_range+0x30c/0x560
>> hugetlb_vmemmap_optimize+0x158/0x408
>> __prep_new_huge_page+0x24/0x150
>> prep_new_huge_page+0x30/0x60
>> alloc_fresh_huge_page+0x1c4/0x1e0
>> alloc_surplus_huge_page+0xa0/0x168
>> alloc_huge_page+0x264/0x5b8
>> hugetlbfs_fallocate+0x294/0x680
>> vfs_fallocate+0x12c/0x568
>> ksys_fallocate+0x50/0xa0
>> __arm64_sys_fallocate+0x28/0x38
>> invoke_syscall+0x4c/0x110
>> el0_svc_common.constprop.0+0x68/0x128
>> do_el0_svc+0x34/0xd0
>> el0_svc+0x48/0xb8
>> el0t_64_sync_handler+0xb8/0xc0
>> el0t_64_sync+0x18c/0x190
>> ---[ end trace 0000000000000000 ]---
>>
>> Fixes: f41f2ed43ca5 ("mm: hugetlb: free the vmemmap pages associated with each HugeTLB page")
> 
> Actually, this commit does not pose the issue for arm64. I think the correct commit
> which should be fixed is 1e63ac088f20f7a4425c430c31ecd3cf167fb3f2.
> 
> Thanks.
> 
>> Signed-off-by: Ma Wupeng <mawupeng1@...wei.com>
>> ---
>> mm/hugetlb_vmemmap.c | 2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
>> index ba2a2596fb4e..cb056265c31e 100644
>> --- a/mm/hugetlb_vmemmap.c
>> +++ b/mm/hugetlb_vmemmap.c
>> @@ -249,7 +249,7 @@ static void vmemmap_remap_pte(pte_t *pte, unsigned long addr,
>> struct page *page = pte_page(*pte);
>>
>> list_add_tail(&page->lru, walk->vmemmap_pages);
>> - set_pte_at(&init_mm, addr, pte, entry);
>> + set_pte(pte, entry);
>> }
>>
>> /*
>> -- 
>> 2.25.1
>>
>>
> 
>