Re: [RFC v5 01/11] mm: Dont assume page-table invariance during faults

[Balbir Singh]

On Mon, 10 Jul 2017 19:48:43 +0200
Laurent Dufour <ldufour@xxxxxxxxxxxxxxxxxx> wrote:

> On 07/07/2017 09:07, Balbir Singh wrote:
> > On Fri, 2017-06-16 at 19:52 +0200, Laurent Dufour wrote:  
> >> From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> >>
> >> One of the side effects of speculating on faults (without holding
> >> mmap_sem) is that we can race with free_pgtables() and therefore we
> >> cannot assume the page-tables will stick around.
> >>
> >> Remove the relyance on the pte pointer.  
> >              ^^ reliance
> > 
> > Looking at the changelog and the code the impact is not clear.
> > It looks like after this patch we always assume the pte is not
> > the same. What is the impact of this patch?  
> 
> Hi Balbir,
> 
> In most of the case pte_unmap_same() was returning 1, which meaning that
> do_swap_page() should do its processing.
> 
> So in most of the case there will be no impact.
> 
> Now regarding the case where pte_unmap_safe() was returning 0, and thus
> do_swap_page return 0 too, this happens when the page has already been
> swapped back. This may happen before do_swap_page() get called or while in
> the call to do_swap_page(). In that later case, the check done when
> swapin_readahead() returns will detect that case.
> 
> The worst case would be that a page fault is occuring on 2 threads at the
> same time on the same swapped out page. In that case one thread will take
> much time looping in __read_swap_cache_async(). But in the regular page
> fault path, this is even worse since the thread would wait for semaphore to
> be released before starting anything.
> 
>

Sounds good!

Thanks,
Balbir Singh