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

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Message-ID: <36cae7e8-97d0-4d53-968b-7f39b34fa5c0@redhat.com>
Date: Thu, 31 Jul 2025 10:38:49 +0200
From: David Hildenbrand <david@...hat.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Usama Arif <usamaarif642@...il.com>
Cc: 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

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).

[ ... ]

>>>> +/*
>>>> + * 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.

> 
>>
>>>>   #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. */

-- 
Cheers,

David / dhildenb