lkml.org 
[lkml]   [2017]   [Aug]   [8]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: [RFC v5 01/11] mm: Dont assume page-table invariance during faults
From
Date
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@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.

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 ?

\
 
 \ /
  Last update: 2017-08-08 12:06    [W:2.669 / U:0.004 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site