Re: [PATCH v3 0/7] prctl: introduce PR_SET/GET_THP_POLICY

["Liam R. Howlett" <Liam.Howlett@oracle.com>]

* Usama Arif <usamaarif642@gmail.com> [250521 05:31]:
> 
> 
> On 21/05/2025 03:33, Liam R. Howlett wrote:
> > * Usama Arif <usamaarif642@gmail.com> [250519 18:34]:
> >> This series allows to change the THP policy of a process, according to the
> >> value set in arg2, all of which will be inherited during fork+exec:
> >> - PR_DEFAULT_MADV_HUGEPAGE: This will set VM_HUGEPAGE and clear VM_NOHUGEPAGE
> >>   for the default VMA flags. It will also iterate through every VMA in the
> >>   process and call hugepage_madvise on it, with MADV_HUGEPAGE policy.
> >>   This effectively allows setting MADV_HUGEPAGE on the entire process.
> >>   In an environment where different types of workloads are run on the
> >>   same machine, this will allow workloads that benefit from always having
> >>   hugepages to do so, without regressing those that don't.
> >> - PR_DEFAULT_MADV_NOHUGEPAGE: This will set VM_NOHUGEPAGE and clear VM_HUGEPAGE
> >>   for the default VMA flags. It will also iterate through every VMA in the
> >>   process and call hugepage_madvise on it, with MADV_NOHUGEPAGE policy.
> >>   This effectively allows setting MADV_NOHUGEPAGE on the entire process.
> >>   In an environment where different types of workloads are run on the
> >>   same machine,this will allow workloads that benefit from having
> >>   hugepages on an madvise basis only to do so, without regressing those
> >>   that benefit from having hugepages always.
> >> - PR_THP_POLICY_SYSTEM: This will reset (clear) both VM_HUGEPAGE and
> >>   VM_NOHUGEPAGE process for the default flags.
> >>
> > 
> > Subject seems outdated now?  PR_DEFAULT_ vs PR_SET/GET_THP ?
> 
> No its not.
> 
> prctl takes 5 args, the first 2 are relevant here.
> 
> The first arg is to decide the op. This series introduces 2 ops. PR_SET_THP_POLICY
> and PR_GET_THP_POLICY to set and get the policy. This is the subject.
> 
> The 2nd arg describes the policies: PR_DEFAULT_MADV_HUGEPAGE, PR_DEFAULT_MADV_NOHUGEPAGE
> and PR_THP_POLICY_SYSTEM.
> 
> The subject is correct.

Thanks, that makes sense.  You are adding an entire new configuration
item to the prctl fun.

> 
> > 
> > On that note, doesn't it make sense to change the default mm flag under
> > PR_SET_MM?  PR_SET_MM_FLAG maybe?
> 
> I don't think thats the right approach. PR_SET_MM is used to modify kernel
> memory map descriptor fields. Thats not what we are doing here.

Fair enough, you are changing the memory map default flags for vmas.

So we are going to add another top level THP specific prctl that changes
flags, but now def_flags and that's communicated by the word POLICY.

I'm not sure this is the right approach either.

Thanks,
Liam