linux-kernel - Re: [PATCH v8 3/6] mm: Introduce VM

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Message-ID: <55E07618.9090905@suse.cz>
Date:	Fri, 28 Aug 2015 16:54:16 +0200
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Eric B Munson <emunson@...mai.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Michal Hocko <mhocko@...e.cz>, Jonathan Corbet <corbet@....net>,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	linux-api@...r.kernel.org
Subject: Re: [PATCH v8 3/6] mm: Introduce VM_LOCKONFAULT

On 08/26/2015 08:24 PM, Eric B Munson wrote:
> The cost of faulting in all memory to be locked can be very high when
> working with large mappings.  If only portions of the mapping will be
> used this can incur a high penalty for locking.
>
> For the example of a large file, this is the usage pattern for a large
> statical language model (probably applies to other statical or graphical
> models as well).  For the security example, any application transacting
> in data that cannot be swapped out (credit card data, medical records,
> etc).
>
> This patch introduces the ability to request that pages are not
> pre-faulted, but are placed on the unevictable LRU when they are finally
> faulted in.  The VM_LOCKONFAULT flag will be used together with
> VM_LOCKED and has no effect when set without VM_LOCKED.  Setting the
> VM_LOCKONFAULT flag for a VMA will cause pages faulted into that VMA to
> be added to the unevictable LRU when they are faulted or if they are
> already present, but will not cause any missing pages to be faulted in.
>
> Exposing this new lock state means that we cannot overload the meaning
> of the FOLL_POPULATE flag any longer.  Prior to this patch it was used
> to mean that the VMA for a fault was locked.  This means we need the
> new FOLL_MLOCK flag to communicate the locked state of a VMA.
> FOLL_POPULATE will now only control if the VMA should be populated and
> in the case of VM_LOCKONFAULT, it will not be set.
>
> Signed-off-by: Eric B Munson <emunson@...mai.com>
> Cc: Michal Hocko <mhocko@...e.cz>
> Cc: Vlastimil Babka <vbabka@...e.cz>
> Cc: Jonathan Corbet <corbet@....net>
> Cc: "Kirill A. Shutemov" <kirill@...temov.name>
> Cc: linux-kernel@...r.kernel.org
> Cc: linux-mm@...ck.org
> Cc: linux-api@...r.kernel.org

Acked-by: Vlastimil Babka <vbabka@...e.cz>

I just wonder if the call to populate_vma_page_range from mprotect_fixup 
is just an potentially expensive no-op for VM_LOCKONFAULT vma's? It 
might find many cow candidates but faultin_page() won't do anything. And 
it shouldn't find any existing pages to put on the unevictable list from 
this context.

But it's a corner case and preventing it would mean putting in another 
VM_LOCKONFAULT check so maybe we can leave it like this.
-
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