linux-kernel - Re: [PATCH 1/5] prctl: extend PR_SET_THP_DISABLE to optionally exclude VM

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Message-ID: <82f41b2f-fbf1-419c-9ce8-6875929de814@lucifer.local>
Date: Thu, 31 Jul 2025 10:09:08 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: David Hildenbrand <david@...hat.com>
Cc: Usama Arif <usamaarif642@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, corbet@....net, rppt@...nel.org,
        surenb@...gle.com, mhocko@...e.com, hannes@...xchg.org,
        baohua@...nel.org, shakeel.butt@...ux.dev, riel@...riel.com,
        ziy@...dia.com, laoar.shao@...il.com, dev.jain@....com,
        baolin.wang@...ux.alibaba.com, npache@...hat.com,
        Liam.Howlett@...cle.com, ryan.roberts@....com, vbabka@...e.cz,
        jannh@...gle.com, Arnd Bergmann <arnd@...db.de>, sj@...nel.org,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        kernel-team@...a.com, Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH 1/5] prctl: extend PR_SET_THP_DISABLE to optionally
 exclude VM_HUGEPAGE

On Thu, Jul 31, 2025 at 10:38:49AM +0200, David Hildenbrand wrote:
> Thanks Lorenzo for the review, I'll leave handling all that to Usama from
> this point :)
>
> On 31.07.25 10:29, Lorenzo Stoakes wrote:
> > Just a ping on the man page stuff - you will do that right? :>)
> >
>
> I'm hoping that Usama can take over that part. If not, I'll handle it (had
> planned it for once it's in mm-stable / going upstream).
>
> [ ... ]

Thanks

>
> > > > > +/*
> > > > > + * Don't disable THPs when explicitly advised (e.g., MADV_HUGEPAGE /
> > > > > + * VM_HUGEPAGE).
> > > > > + */
> > > > > +# define PR_THP_DISABLE_EXCEPT_ADVISED	(1 << 1)
> > > >
> > > > NO space after # please.
> > > >
> > >
> > > I think this is following the file convention, the space is there in other flags
> > > all over this file. I dont like the space as well.
> >
> > Yeah yuck. It's not a big deal, but ideally I'd prefer us to be sane even
> > if the rest of the header is less so here.
>
> I'm afraid us doing something different here will not make prctl() any
> better as a whole, so let's keep it consistent in this questionable file.

Sure not a big deal.

>
> >
> > >
> > > > >   #define PR_GET_THP_DISABLE	42
> > > > >
> > > > >   /*
> > > > > diff --git a/kernel/sys.c b/kernel/sys.c
> > > > > index b153fb345ada..b87d0acaab0b 100644
> > > > > --- a/kernel/sys.c
> > > > > +++ b/kernel/sys.c
> > > > > @@ -2423,6 +2423,50 @@ static int prctl_get_auxv(void __user *addr, unsigned long len)
> > > > >   	return sizeof(mm->saved_auxv);
> > > > >   }
> > > > >
> > > > > +static int prctl_get_thp_disable(unsigned long arg2, unsigned long arg3,
> > > > > +				 unsigned long arg4, unsigned long arg5)
> > > > > +{
> > > > > +	unsigned long *mm_flags = &current->mm->flags;
> > > > > +
> > > > > +	if (arg2 || arg3 || arg4 || arg5)
> > > > > +		return -EINVAL;
> > > > > +
> > > >
> > > > Can we have a comment here about what we're doing below re: the return
> > > > value?
> > > >
> > >
> > > Do you mean add returning 1 for MMF_DISABLE_THP_COMPLETELY and 3 for MMF_DISABLE_THP_EXCEPT_ADVISED?
> >
> > Well more so something about we return essentially flags indicating what is
> > enabled or not, if bit 0 is set then it's disabled, if bit 1 is set then
> > it's that with the exception of VM_HUGEPAGE VMAs.
>
> We have that documented above the defines for flags etc. Maybe simply here:
>
> /* If disabled, we return "1 | flags", otherwise 0. */

Yup I know it was documented, but nice to have here as I'd definitely read this
code and think 'huh?'.

What you suggest is fine!

>
> --
> Cheers,
>
> David / dhildenb
>

Thanks, Lorenzo