Re: [PATCH -next 1/1] mm: hugetlb_vmemmap: Fix WARN_ON in vmemmap_remap_pte

[Muchun Song <muchun.song@linux.dev>]

> On Oct 26, 2022, at 13:06, Anshuman Khandual <anshuman.khandual@arm.com> wrote:
> 
> 
> 
> On 10/25/22 12:06, Muchun Song wrote:
>> 
>> 
>>> On Oct 25, 2022, at 09:42, Wupeng Ma <mawupeng1@huawei.com> wrote:
>>> 
>>> From: Ma Wupeng <mawupeng1@huawei.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.

Right.

> 
>        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

Right.

> a possibility for a race condition even when the PFN changes ?

Sorry, I didn't get this question. Did you mean the PTE is changed from
new (pte) to the old one (old_pte) by the hardware because of the update
of dirty bit when a concurrent write operation to the tail vmemmap page?

Muchun,
Thanks.

> 
>> 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@huawei.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