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

David Hildenbrand
2022-11-30T12:11:59Z

On 30.11.22 12:42, Muhammad Usama Anjum wrote:
> On 11/21/22 8:55 PM, David Hildenbrand wrote:
>> On 21.11.22 16:00, Muhammad Usama Anjum wrote:
>>> 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.
>>
>> Quick reproducer (the first and easiest one that triggered :) )
>> attached.
>>
>> With no kernel changes, it works as expected.
>>
>> # ./softdirty_mprotect
>>
>>
>> With the following kernel change to simulate what you propose it fails:
>>
>> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
>> index d22687d2e81e..f2c682bf7f64 100644
>> --- a/fs/proc/task_mmu.c
>> +++ b/fs/proc/task_mmu.c
>> @@ -1457,8 +1457,8 @@ static pagemap_entry_t pte_to_pagemap_entry(struct
>> pagemapread *pm,
>>                 flags |= PM_FILE;
>>         if (page && !migration && page_mapcount(page) == 1)
>>                 flags |= PM_MMAP_EXCLUSIVE;
>> -       if (vma->vm_flags & VM_SOFTDIRTY)
>> -               flags |= PM_SOFT_DIRTY;
>> +       //if (vma->vm_flags & VM_SOFTDIRTY)
>> +       //      flags |= PM_SOFT_DIRTY;
>>
>>         return make_pme(frame, flags);
>>  }
>>
>>
>> # ./softdirty_mprotect
>> Page #1 should be softdirty
>>
> Thank you so much for sharing the issue and reproducer.
>
> After remapping the second part of the memory and m-protecting +
> m-unprotecting the whole memory, the PTE of the first half of the memory
> doesn't get marked as soft dirty even after writing multiple times to it.
> Even if soft-dirtiness is cleared on the whole process, the PTE of the
> first half memory doesn't get dirty. This seems like more of a bug in
> mprotect. The mprotect should not mess up with the soft-dirty flag in the PTEs.
>
> I'm debugging this. I hope to find the issue soon. Soft-dirty tracking in
> PTEs should be working correctly irrespective of the VM_SOFTDIRTY is set or
> not on the VMA.

No, it's not a bug and these are not the VM_SOFTDIRTY semantics -- just
because you think they should be like this. As people explained,
VM_SOFTDIRTY implies *until now* that any PTE is consideres softdirty.
And there are other scenarios that can similarly trigger something like
that, besides mprotect().

Sorry if I sound annoyed, but please

1) factor out that from your patch set for now
2) find a way to handle this cleanly, for example, not merging VMAs that
differ in VM_SOFTDIRTY

--
Thanks,

David / dhildenb