linux-kernel - Re: [PATCH v7 3/6] mm: Introduce VM

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Date:	Mon, 24 Aug 2015 15:30:31 +0200
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Konstantin Khlebnikov <koct9i@...il.com>,
	Eric B Munson <emunson@...mai.com>
Cc:	Michal Hocko <mhocko@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jonathan Corbet <corbet@....net>,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	dri-devel <dri-devel@...ts.freedesktop.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH v7 3/6] mm: Introduce VM_LOCKONFAULT

On 08/24/2015 12:17 PM, Konstantin Khlebnikov wrote:
>>
>> I am in the middle of implementing lock on fault this way, but I cannot
>> see how we will hanlde mremap of a lock on fault region.  Say we have
>> the following:
>>
>>      addr = mmap(len, MAP_ANONYMOUS, ...);
>>      mlock(addr, len, MLOCK_ONFAULT);
>>      ...
>>      mremap(addr, len, 2 * len, ...)
>>
>> There is no way for mremap to know that the area being remapped was lock
>> on fault so it will be locked and prefaulted by remap.  How can we avoid
>> this without tracking per vma if it was locked with lock or lock on
>> fault?
>
> remap can count filled ptes and prefault only completely populated areas.

Does (and should) mremap really prefault non-present pages? Shouldn't it 
just prepare the page tables and that's it?

> There might be a problem after failed populate: remap will handle them
> as lock on fault. In this case we can fill ptes with swap-like non-present
> entries to remember that fact and count them as should-be-locked pages.

I don't think we should strive to have mremap try to fix the inherent 
unreliability of mmap (MAP_POPULATE)?
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