Re: [Xen-devel] Regression: x86/mm: new _PTE_SWP_SOFT_DIRTY bit conflicts with existing use

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 22.08.13 at 01:04, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Wed, Aug 21, 2013 at 12:03 PM, Cyrill Gorcunov <gorcunov@xxxxxxxxx> 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?

It was actually me (mis)reading the code - as pointed out to Cyrill
already, setting _PAGE_PAT in a pte_t without even a comment
saying that this can only ever be done with a non-present entry
made me expect problems on Xen, because it's clear that to date
bare metal Linux doesn't care about the state of _PAGE_PAT in
present entries due to the way the PAT MSR gets set (and hence
quite likely no-one would have noticed the supposed problem
while testing).

So a comment either alongside the definition of _PAGE_SWP_SOFT_DIRTY
or directly in pte_swp_{mk,clear_}soft_dirty() would have been
the minimal thing I'd have expected for this sort of re-use of bits.
Ideally even a VM_BUG_ON(pte_present()) or similar. And perhaps
pte_swp_soft_dirty() should be either looking at the present bit
too or similarly asserting that it's clear...

In any event - I'm sorry for the red herring.

Jan


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