Re: [PATCH v6 0/3] Implement IOCTL to get and/or the clear info about PTEs

[Muhammad Usama Anjum]

Hello,

Thank you for replying.

On 11/14/22 8:46 PM, David Hildenbrand wrote:
>> The soft-dirtiness is stored in the PTE. VMA is marked dirty to store the
>> dirtiness for reused regions. Clearing the soft-dirty status of whole
>> process is straight forward. When we want to clear/monitor the
>> soft-dirtiness of a part of the virtual memory, there is a lot of internal
>> noise. We don't want the non-dirty pages to become dirty because of how the
>> soft-dirty feature has been working. Soft-dirty feature wasn't being used
>> the way we want to use now. While monitoring a part of memory, it is not
>> acceptable to get non-dirty pages as dirty. Non-dirty pages become dirty
>> when the two VMAs are merged without considering if they both are dirty or
>> not (34228d473efe). To monitor changes over the memory, sometimes VMAs are
>> split to clear the soft-dirty bit in the VMA flags. But sometimes kernel
>> decide to merge them backup. It is so waste of resources.
> 
> Maybe you'd want a per-process option to not merge if the VM_SOFTDIRTY
> property differs. But that might be just one alternative for handling this
> case.
> 
>>
>> To keep things consistent, the default behavior of the IOCTL is to output
>> even the extra non-dirty pages as dirty from the kernel noise. A optional
>> PAGEMAP_NO_REUSED_REGIONS flag is added for those use cases which aren't
>> tolerant of extra non-dirty pages. This flag can be considered as something
>> which is by-passing the already present buggy implementation in the kernel.
>> It is not buggy per say as the issue can be solved if we don't allow the
>> two VMA which have different soft-dirty bits to get merged. But we are
>> allowing that so that the total number of VMAs doesn't increase. This was
>> acceptable at the time, but now with the use case of monitoring a part of
>> memory for soft-dirty doesn't want this merging. So either we need to
>> revert 34228d473efe and PAGEMAP_NO_REUSED_REGIONS flag will not be needed
>> or we should allow PAGEMAP_NO_REUSED_REGIONS or similar mechanism to ignore
>> the extra dirty pages which aren't dirty in reality.
>>
>> When PAGEMAP_NO_REUSED_REGIONS flag is used, only the PTEs are checked to
>> find if the pages are dirty. So re-used regions cannot be detected. This
>> has the only side-effect of not checking the VMAs. So this is limitation of
>> using this flag which should be acceptable in the current state of code.
>> This limitation is okay for the users as they can clear the soft-dirty bit
>> of the VMA before starting to monitor a range of memory for soft-dirtiness.
>>
>>
>>> Please separate that part out from the other changes; I am still not
>>> convinced that we want this and what the semantical implications are.
>>>
>>> Let's take a look at an example: can_change_pte_writable()
>>>
>>>      /* Do we need write faults for softdirty tracking? */
>>>      if (vma_soft_dirty_enabled(vma) && !pte_soft_dirty(pte))
>>>          return false;
>>>
>>> We care about PTE softdirty tracking, if it is enabled for the VMA.
>>> Tracking is enabled if: vma_soft_dirty_enabled()
>>>
>>>      /*
>>>       * Soft-dirty is kind of special: its tracking is enabled when
>>>       * the vma flags not set.
>>>       */
>>>      return !(vma->vm_flags & VM_SOFTDIRTY);
>>>
>>> Consequently, if VM_SOFTDIRTY is set, we are not considering the soft_dirty
>>> PTE bits accordingly.
>> Sorry, I'm unable to completely grasp the meaning of the example. We have
>> followed clear_refs_write() to write the soft-dirty bit clearing code in
>> the current patch. Dirtiness of the VMA and the PTE may be set
>> independently. Newer allocated memory has dirty bit set in the VMA. When
>> something is written the memory, the soft dirty bit is set in the PTEs as
>> well regardless if the soft dirty bit is set in the VMA or not.
>>
> 
> Let me try to find a simple explanation:
> 
> After clearing a SOFTDIRTY PTE flag inside an area with VM_SOFTDIRTY set,
> there are ways that PTE could get written to and it could become dirty,
> without the PTE becoming softdirty.
> 
> Essentially, inside a VMA with VM_SOFTDIRTY set, the PTE softdirty values
> might be stale: there might be entries that are softdirty even though the
> PTE is *not* marked softdirty.
Can someone please share the example to reproduce this? In all of my
testing, even if I ignore VM_SOFTDIRTY and only base my decision of
soft-dirtiness on individual pages, it always passes.

> 
> These are, AFAIU, the current semantics, and I am not sure if we want user
> space to explicitly work around that.
> 
>>>
>>>
>>> I'd suggest moving forward without this controversial
>>> PAGEMAP_NO_REUSED_REGIONS functionality for now, and preparing it as a
>>> clear add-on we can discuss separately.Like I've described above, I've
>>> only added this flag to not get the
>> non-dirty pages as dirty. Can there be some alternative to adding this
>> flag? Please suggest.
> 
> Please split it out into a separate patch for now. We can discuss further
> what the semantics are and if there are better alternatives for that. In
> the meantime, you could move forward without PAGEMAP_NO_REUSED_REGIONS
> while we are discussing them further.
> 

-- 
BR,
Muhammad Usama Anjum