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. Cheers, Laurent.
