Re: [PATCH v4 1/3] mm/mprotect: Fix soft-dirty check in can_change_pte_writable()

[Muhammad Usama Anjum <usama.anjum@collabora.com>]

Hi Peter,

Thank you so much for replying.

On 11/19/22 4:14 AM, Peter Xu wrote:
> On Sat, Nov 19, 2022 at 01:16:26AM +0500, Muhammad Usama Anjum wrote:
>> Hi Peter and David,
> 
> Hi, Muhammad,
> 
>>
>> On 7/25/22 7:20 PM, Peter Xu wrote:
>>> The check wanted to make sure when soft-dirty tracking is enabled we won't
>>> grant write bit by accident, as a page fault is needed for dirty tracking.
>>> The intention is correct but we didn't check it right because VM_SOFTDIRTY
>>> set actually means soft-dirty tracking disabled.  Fix it.
>> [...]
>>> +static inline bool vma_soft_dirty_enabled(struct vm_area_struct *vma)
>>> +{
>>> +	/*
>>> +	 * NOTE: we must check this before VM_SOFTDIRTY on soft-dirty
>>> +	 * enablements, because when without soft-dirty being compiled in,
>>> +	 * VM_SOFTDIRTY is defined as 0x0, then !(vm_flags & VM_SOFTDIRTY)
>>> +	 * will be constantly true.
>>> +	 */
>>> +	if (!IS_ENABLED(CONFIG_MEM_SOFT_DIRTY))
>>> +		return false;
>>> +
>>> +	/*
>>> +	 * Soft-dirty is kind of special: its tracking is enabled when the
>>> +	 * vma flags not set.
>>> +	 */
>>> +	return !(vma->vm_flags & VM_SOFTDIRTY);
>>> +}
>> I'm sorry. I'm unable to understand the inversion here.
>>> its tracking is enabled when the vma flags not set.
>> VM_SOFTDIRTY is set on the VMA when new VMA is allocated to mark is
>> soft-dirty. When we write to clear_refs to clear soft-dirty bit,
>> VM_SOFTDIRTY is cleared from the VMA as well. Then why do you say tracking
>> is enabled when the vma flags not set?
> 
> Because only when 4>clear_refs happens would VM_SOFTDIRTY be cleared, and
> only until then the real tracking starts (by removing write bits on ptes).
But even if the VM_SOFTDIRTY is set on the VMA, the individual pages are
still marked soft-dirty. Both are independent.

It means tracking is enabled all the time in individual pages. Only the
soft-dirty bit status in individual page isn't significant if VM_SOFTDIRTY
already is set. Right?

> 
>> I'm missing some obvious thing.  Maybe the meaning of tracking is to see
>> if VM_SOFTDIRTY needs to be set. If VM_SOFTDIRTY is already set, tracking
>> isn't needed. Can you give an example here?
> 
> If VM_SOFTDIRTY is set, pagemap will treat all pages as soft-dirty, please
> see pagemap_pmd_range():
> 
> 		if (vma->vm_flags & VM_SOFTDIRTY)
> 			flags |= PM_SOFT_DIRTY;
> 
> So fundamentally it reports nothing useful when VM_SOFTDIRTY set.  That's
> also why we need the clear_refs first before we can have anything useful.
> 
> Feel free to reference to the doc page (admin-guide/mm/soft-dirty.rst):
> 
> ---8<---
> The soft-dirty is a bit on a PTE which helps to track which pages a task
> writes to. In order to do this tracking one should
> 
>   1. Clear soft-dirty bits from the task's PTEs.
> 
>      This is done by writing "4" into the ``/proc/PID/clear_refs`` file of the
>      task in question.
> 
>   2. Wait some time.
> 
>   3. Read soft-dirty bits from the PTEs.
> 
>      This is done by reading from the ``/proc/PID/pagemap``. The bit 55 of the
>      64-bit qword is the soft-dirty one. If set, the respective PTE was
>      written to since step 1.
> ---8<---
> 
> The tracking starts at step 1, where is when the flag is cleared.
> 
> Thanks,
> 

-- 
BR,
Muhammad Usama Anjum