On Mon, 10 Jul 2017 19:48:43 +0200 Laurent Dufour <lduf...@linux.vnet.ibm.com> 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 <pet...@infradead.org> > >> > >> 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
