On 07/10/2017 11:18 PM, Laurent Dufour 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. Can we move the detection of swap in of the same struct page back into the page table bit earlier, ideally where pte_unmap_same() present to speed up detection for the bail out case ? -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
