Re: [PATCH] arm64: Fix the ptep_set_wrprotect() to set PTE_DIRTY if (PTE_DBM && !PTE_RDONLY)

[Ganapatrao Kulkarni]

On Wed, Mar 9, 2016 at 11:13 PM, Ganapatrao Kulkarni
<gpkulkarni@xxxxxxxxx> wrote:
> On Wed, Mar 9, 2016 at 9:33 PM, Catalin Marinas <catalin.marinas@xxxxxxx> wrote:
>> On Wed, Mar 09, 2016 at 05:17:39PM +0530, Ganapatrao Kulkarni wrote:
>>> On Wed, Mar 9, 2016 at 3:36 PM, Catalin Marinas <catalin.marinas@xxxxxxx> wrote:
>>> > On Wed, Mar 09, 2016 at 10:32:48AM +0530, Ganapatrao Kulkarni wrote:
>>> >> Commit 2f4b829c625e ("arm64: Add support for hardware updates of the
>>> >> access and dirty pte bits") introduced support for handling hardware
>>> >> updates of the access flag and dirty status.
>>> >>
>>> >> ptep_set_wrprotect is setting PTR_DIRTY if !PTE_RDONLY,
>>> >> however by design it suppose to set PTE_DIRTY
>>> >> only if (PTE_DBM && !PTE_RDONLY). This patch addes code to
>>> >> test and set accordingly.
>>> >
>>> > The reasoning behind the original code is that if !PTE_RDONLY, you have
>>> > no way to tell whether the page was written or not since it is already
>>> > writable, independent of the DBM. So by clearing the DBM bit (making the
>>> > page read-only), we need to ensure that a potential dirty state is
>>> > transferred to the software PTE_DIRTY bit.
>>> >
>>> > By checking PTE_DBM && !PTE_RDONLY, you kind of imply that you can have
>>> > a page with !PTE_DBM && !PTE_RDONLY. Given that PTE_DBM is actually
>>> > PTE_WRITE, PTE_RDONLY must always be set when !PTE_DBM. The bug may be
>>> > elsewhere not setting these bits correctly.
>>>
>>> but i do see this macro,
>>> #define pte_hw_dirty(pte)       (pte_write(pte) && !(pte_val(pte) & PTE_RDONLY))
>>
>> This was added in commit b847415ce96e ("arm64: Fix the pte_hw_dirty()
>> check when AF/DBM is enabled") for the pte_modify() case which is not
>> called on the actual PTE but a local variable. A pte passed to this
>> function as !PTE_DBM && !PTE_RDONLY should not be assumed dirty since
>> PTE_RDONLY will be set later by set_pte_at() when the actual page table
>> write occurs.
>>
>> ptep_set_wrprotect() is run directly on the actual PTE, so here a
>> !PTE_RDONLY only means potentially dirty, independent of the PTE_DBM
>> bit. I consider the additional PTE_DBM check superfluous in this case
>> but we need to understand when we would actually get a pte with both
>> PTE_DBM and PTE_RDONLY cleared.
>>
>> The only way I see this happening is if the pte doesn't have PTE_VALID
>> set, IOW it probably has PTE_PROT_NONE set which is used by the NUMA
>> balancing. So calling set_pte_at() on a !PTE_VALID && !PTE_DBM pte does
>> not currently set PTE_RDONLY and ptep_set_wrprotect() wrongly assumes it
>> is dirty.
>>
>>> i dont see this issue, if i comment out arm64 implementation of
>>> ptep_set_wrprotect()
>>
>> Because the default implementation discards any existing hw dirty
>> information by clearing the PTE_DBM bit and setting PTE_RDONLY via the
>> set_pte_at (of course, apart from the atomicity issues).
>>
>>> >> This patch fixes BUG,
>>> >> kernel BUG at /build/linux-StrpB2/linux-4.4.0/fs/ext4/inode.c:2394!
>>> >> Internal error: Oops - BUG: 0 [#1] SMP
>>> >
>>> > Which bug is this? It's a PageWriteback() check in the for-next/core
>>> > branch. What kernel version are you using?
>>>
>>> i am using 4.4.0
>>
>> I guess with additional NUMA patches since it only fails when you enable
>> the NUMA_BALANCING configuration.
>>
>>> > BTW, in 4.5-rc2 we pushed commit ac15bd63bbb2 ("arm64: Honour !PTE_WRITE
>>> > in set_pte_at() for kernel mappings"), though not sure that's what you
>>> > are hitting.
>>>
>>> i have tried this patch, but issue still exist. crash log below
>>>
>>> root@ubuntu:/home/ganapat/test# [  733.853009] kernel BUG at
>>> fs/ext4/inode.c:2394!
>>
>> Is this the BUG_ON in page_buffers(!PagePrivate(page))? I can see in the
>> code above this that wrongly marking a page as dirty could have some
>> side effects.
>>
>> Can you give this patch a try, on top of commit ac15bd63bbb2?
>
> thanks, this fixes the issue, i have tried making pte_valid same as pte_present
> however, i have overlooked that set_pte_at is using pte_valid_user(in 4.4)
>
>
>>
>> -------------8<----------------------
>>
>> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
>> index 7c73b365fcfa..b409a983f870 100644
>> --- a/arch/arm64/include/asm/pgtable.h
>> +++ b/arch/arm64/include/asm/pgtable.h
>> @@ -201,7 +201,7 @@ extern void __sync_icache_dcache(pte_t pteval, unsigned long addr);
>>  static inline void set_pte_at(struct mm_struct *mm, unsigned long addr,
>>                               pte_t *ptep, pte_t pte)
>>  {
>> -       if (pte_valid(pte)) {
>> +       if (pte_present(pte)) {
>>                 if (pte_sw_dirty(pte) && pte_write(pte))
>>                         pte_val(pte) &= ~PTE_RDONLY;
>>                 else
>
this diff works for me.

Tested-by: Ganapatrao Kulkarni <gkulkarni@xxxxxxxxxx>

> Ganapat