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

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Message-Id: <d719a861-d712-1876-b46c-7f9c1360196c@linux.vnet.ibm.com>
Date:   Mon, 10 Jul 2017 19:48:43 +0200
From:   Laurent Dufour <ldufour@...ux.vnet.ibm.com>
To:     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 07/07/2017 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.

Cheers,
Laurent.