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

From: Laurent Dufour
Date: Mon Jul 10 2017 - 13:48:56 EST

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.