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

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Date:   Tue, 8 Aug 2017 15:34:41 +0530
From:   Anshuman Khandual <khandual@...ux.vnet.ibm.com>
To:     Laurent Dufour <ldufour@...ux.vnet.ibm.com>,
        Balbir Singh <bsingharora@...il.com>,
        paulmck@...ux.vnet.ibm.com, peterz@...radead.org,
        akpm@...ux-foundation.org, kirill@...temov.name,
        ak@...ux.intel.com, mhocko@...nel.org, dave@...olabs.net,
        jack@...e.cz, Matthew Wilcox <willy@...radead.org>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        haren@...ux.vnet.ibm.com, khandual@...ux.vnet.ibm.com,
        npiggin@...il.com, Tim Chen <tim.c.chen@...ux.intel.com>
Subject: Re: [RFC v5 01/11] mm: Dont assume page-table invariance during
 faults

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@...radead.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.

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 ?