# Re: \[PATCH v8 3/6\] mm: Introduce VM\_LOCKONFAULT

- From: Vlastimil Babka \<hidden\>
- Date: 2015-08-28 14:54:21 
- List: linux-api
- Page: https://ratatoskr.run/linux-api/2015/08/4566165
- Thread: https://ratatoskr.run/linux-mm/2015/08/4564806/t.md

```text
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 <redacted>
> Cc: Michal Hocko <redacted>
> Cc: Vlastimil Babka <redacted>
> Cc: Jonathan Corbet <redacted>
> Cc: "Kirill A. Shutemov" <redacted>
> Cc: <redacted>
> Cc: <redacted>
> Cc: <redacted>

Acked-by: Vlastimil Babka <redacted>

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.
-
```