Re: [PATCH 0/3] Guarantee faults for processes that call mmap(MAP_PRIVATE) on hugetlbfs v2

[Andrew Hastings <abh@cray.com>]

Andy Whitcroft wrote:
> Without patch 3 the parent is still vunerable during the period the
> child exists.  Even if that child does nothing with the pages not even
> referencing them, and then execs immediatly.  As soon as we fork any
> reference from the parent will trigger a COW, at which point there may
> be no pages available and the parent will have to be killed.  That is
> regardless of the fact the child is not going to reference the page and
> leave the address space shortly.  With patch 3 on COW if we find no memory
> available the page may be stolen for the parent saving it, and the _risk_
> of reference death moves to the child; the child is killed only should it
> then re-reference the page.
> 
> Without patch 3 a both the parent and child are immediatly vunerable on
> fork() until the child leaves the address space.  With patch 3 only the
> child is vunerable.  The main scenario where mapper protection is useful
> is for main payload applications which fork helpers.  The parent by
> definition is using the mapping heavily whereas we do not expect the
> children to even be aware of it.  As the child will not touch the
> mapping both parent and child should be safe even if we do have to steal
> to save the parent.

I agree, it's important to close this window for the parent.  This will 
be helpful for our customers as we move towards wider use of libhugetlbfs.

Thanks, Mel!

-Andrew Hastings
  Cray Inc.

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