On 05/30/2017 06:06 PM, Andrea Arcangeli wrote:

I'm not sure if it should be considered a bug, the prctl is intended
to use normally by wrappers so it looks optimal as implemented this
way: affecting future vmas only, which will all be created after
execve executed by the wrapper.

What's the point of messing with the prctl so it mangles over the
wrapper process own vmas before exec? Messing with those vmas is pure
wasted CPUs for the wrapper use case which is what the prctl was
created for.

Furthermore there would be the risk a program that uses the prctl not
as a wrapper and then calls the prctl to clear VM_NOHUGEPAGE from
def_flags assuming the current kABI. The program could assume those
vmas that were instantiated before disabling the prctl are still with
VM_NOHUGEPAGE set (they would not after the change you propose).

Adding a scan of all vmas to PR_SET_THP_DISABLE to clear VM_NOHUGEPAGE
on existing vmas looks more complex too and less finegrined so
probably more complex for userland to manage

I would expect the prctl wouldn't iterate all vma's, nor would it modify
def_flags anymore. It would just set a flag somewhere in mm struct that
would be considered in addition to the per-vma flags when deciding
whether to use THP. We could consider whether MADV_HUGEPAGE should be
able to override the prctl or not.

but ignoring all above
considerations it would be a functional alternative for CRIU's
needs. However if you didn't like the complexity of the new madvise
which is functionally a one-liner equivalent to MADV_NORMAL, I
wouldn't expect you to prefer to make the prctl even more complex with
a loop over all vmas that despite being fairly simple it'll still be
more than a trivial one liner.

Thanks,
Andrea

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