Re: Regression: x86/mm: new _PTE_SWP_SOFT_DIRTY bit conflicts with existing use

[David Vrabel <david.vrabel@citrix.com>]

On 22/08/13 00:04, Linus Torvalds wrote:
> On Wed, Aug 21, 2013 at 12:03 PM, Cyrill Gorcunov <gorcunov@gmail.com> wrote:
>>
>> I personally don't see bug here because
>>
>>  - this swapped page soft dirty bit is set for non-present entries only,
>>    never for present ones, just at moment we form swap pte entry
>>
>>  - i don't find any code which would test for this bit directly without
>>    is_swap_pte call
> 
> Ok, having gone through the places that use swp_*soft_dirty(), I have
> to agree. Afaik, it's only ever used on a swap-entry that has (by
> definition) the P bit clear. So with or without Xen, I don't see how
> it can make any difference.
> 
> David/Konrad - did you actually see any issues, or was this just from
> (mis)reading the code?

There are no Xen related bugs in the code, we were misreading it.

It was my call to raise this as a regression without a repro and clearly
this was the wrong decision.

However, having looked at the soft dirty implementation and specifically
the userspace ABI I think that it is far to closely coupled to the
current implementation.  I think this will constrain future development
of the feature should userspace require a more efficient ABI than
scanning all of /proc/<pid>/pagemaps.

Minimal downtime during 'live' checkpointing of a running task needs the
checkpointer to find and write out dirty pages faster than the task can
dirty them.  This seems less likely to be possible if every iteration
all PTEs have to be scanned by the checkpointer instead of (e.g.,)
accessing a separate list of dirtied pages.

David