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.
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.
--
Thanks,
David / dhildenb
