Message-ID: <8c5d607d-498e-4a34-a781-faafb3a5fdef@lucifer.local>
Date: Wed, 30 Jul 2025 20:31:14 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Usama Arif <usamaarif642@...il.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, david@...hat.com,
        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

An aside - we should update the man page for this - see
https://man7.org/linux/man-pages/man2/PR_SET_THP_DISABLE.2const.html

This has to be done separately from the series I think.

On Fri, Jul 25, 2025 at 05:22:40PM +0100, Usama Arif wrote:
> From: David Hildenbrand <david@...hat.com>
>
> People want to make use of more THPs, for example, moving from
> the "never" system policy to "madvise", or from "madvise" to "always".
>
> While this is great news for every THP desperately waiting to get
> allocated out there, apparently there are some workloads that require a
> bit of care during that transition: individual processes may need to
> opt-out from this behavior for various reasons, and this should be
> permitted without needing to make all other workloads on the system
> similarly opt-out.
>
> The following scenarios are imaginable:
>
> (1) Switch from "none" system policy to "madvise"/"always", but keep THPs
>     disabled for selected workloads.
>
> (2) Stay at "none" system policy, but enable THPs for selected
>     workloads, making only these workloads use the "madvise" or "always"
>     policy.
>
> (3) Switch from "madvise" system policy to "always", but keep the
>     "madvise" policy for selected workloads: allocate THPs only when
>     advised.
>
> (4) Stay at "madvise" system policy, but enable THPs even when not advised
>     for selected workloads -- "always" policy.
>
> Once can emulate (2) through (1), by setting the system policy to
> "madvise"/"always" while disabling THPs for all processes that don't want
> THPs. It requires configuring all workloads, but that is a user-space
> problem to sort out.
>
> (4) can be emulated through (3) in a similar way.
>
> Back when (1) was relevant in the past, as people started enabling THPs,
> we added PR_SET_THP_DISABLE, so relevant workloads that were not ready
> yet (i.e., used by Redis) were able to just disable THPs completely. Redis
> still implements the option to use this interface to disable THPs
> completely.
>
> With PR_SET_THP_DISABLE, we added a way to force-disable THPs for a
> workload -- a process, including fork+exec'ed process hierarchy.
> That essentially made us support (1): simply disable THPs for all workloads
> that are not ready for THPs yet, while still enabling THPs system-wide.
>
> The quest for handling (3) and (4) started, but current approaches
> (completely new prctl, options to set other policies per process,
> alternatives to prctl -- mctrl, cgroup handling) don't look particularly
> promising. Likely, the future will use bpf or something similar to
> implement better policies, in particular to also make better decisions
> about THP sizes to use, but this will certainly take a while as that work
> just started.
>
> Long story short: a simple enable/disable is not really suitable for the
> future, so we're not willing to add completely new toggles.
>
> While we could emulate (3)+(4) through (1)+(2) by simply disabling THPs
> completely for these processes, this is a step backwards, because these
> processes can no longer allocate THPs in regions where THPs were
> explicitly advised: regions flagged as VM_HUGEPAGE. Apparently, that
> imposes a problem for relevant workloads, because "not THPs" is certainly
> worse than "THPs only when advised".
>
> Could we simply relax PR_SET_THP_DISABLE, to "disable THPs unless not
> explicitly advised by the app through MAD_HUGEPAGE"? *maybe*, but this
> would change the documented semantics quite a bit, and the versatility
> to use it for debugging purposes, so I am not 100% sure that is what we
> want -- although it would certainly be much easier.
>
> So instead, as an easy way forward for (3) and (4), add an option to
> make PR_SET_THP_DISABLE disable *less* THPs for a process.
>
> In essence, this patch:
>
> (A) Adds PR_THP_DISABLE_EXCEPT_ADVISED, to be used as a flag in arg3
>     of prctl(PR_SET_THP_DISABLE) when disabling THPs (arg2 != 0).
>
>     prctl(PR_SET_THP_DISABLE, 1, PR_THP_DISABLE_EXCEPT_ADVISED).
>
> (B) Makes prctl(PR_GET_THP_DISABLE) return 3 if
>     PR_THP_DISABLE_EXCEPT_ADVISED was set while disabling.
>
>     Previously, it would return 1 if THPs were disabled completely. Now
>     it returns the set flags as well: 3 if PR_THP_DISABLE_EXCEPT_ADVISED
>     was set.
>
> (C) Renames MMF_DISABLE_THP to MMF_DISABLE_THP_COMPLETELY, to express
>     the semantics clearly.
>
>     Fortunately, there are only two instances outside of prctl() code.
>
> (D) Adds MMF_DISABLE_THP_EXCEPT_ADVISED to express "no THP except for VMAs
>     with VM_HUGEPAGE" -- essentially "thp=madvise" behavior
>
>     Fortunately, we only have to extend vma_thp_disabled().
>
> (E) Indicates "THP_enabled: 0" in /proc/pid/status only if THPs are
>     disabled completely
>
>     Only indicating that THPs are disabled when they are really disabled
>     completely, not only partially.
>
>     For now, we don't add another interface to obtained whether THPs
>     are disabled partially (PR_THP_DISABLE_EXCEPT_ADVISED was set). If
>     ever required, we could add a new entry.
>
> The documented semantics in the man page for PR_SET_THP_DISABLE
> "is inherited by a child created via fork(2) and is preserved across
> execve(2)" is maintained. This behavior, for example, allows for
> disabling THPs for a workload through the launching process (e.g.,
> systemd where we fork() a helper process to then exec()).
>
> For now, MADV_COLLAPSE will *fail* in regions without VM_HUGEPAGE and
> VM_NOHUGEPAGE. As MADV_COLLAPSE is a clear advise that user space
> thinks a THP is a good idea, we'll enable that separately next
> (requiring a bit of cleanup first).
>
> There is currently not way to prevent that a process will not issue
> PR_SET_THP_DISABLE itself to re-enable THP. There are not really known
> users for re-enabling it, and it's against the purpose of the original
> interface. So if ever required, we could investigate just forbidding to
> re-enable them, or make this somehow configurable.
>
> Acked-by: Usama Arif <usamaarif642@...il.com>
> Tested-by: Usama Arif <usamaarif642@...il.com>
> Cc: Jonathan Corbet <corbet@....net>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> Cc: Zi Yan <ziy@...dia.com>
> Cc: Baolin Wang <baolin.wang@...ux.alibaba.com>
> Cc: "Liam R. Howlett" <Liam.Howlett@...cle.com>
> Cc: Nico Pache <npache@...hat.com>
> Cc: Ryan Roberts <ryan.roberts@....com>
> Cc: Dev Jain <dev.jain@....com>
> Cc: Barry Song <baohua@...nel.org>
> Cc: Vlastimil Babka <vbabka@...e.cz>
> Cc: Mike Rapoport <rppt@...nel.org>
> Cc: Suren Baghdasaryan <surenb@...gle.com>
> Cc: Michal Hocko <mhocko@...e.com>
> Cc: Usama Arif <usamaarif642@...il.com>
> Cc: SeongJae Park <sj@...nel.org>
> Cc: Jann Horn <jannh@...gle.com>
> Cc: Liam R. Howlett <Liam.Howlett@...cle.com>
> Cc: Yafang Shao <laoar.shao@...il.com>
> Cc: Matthew Wilcox <willy@...radead.org>
> Signed-off-by: David Hildenbrand <david@...hat.com>
>
> ---
>
> At first, I thought of "why not simply relax PR_SET_THP_DISABLE", but I
> think there might be real use cases where we want to disable any THPs --
> in particular also around debugging THP-related problems, and
> "never" not meaning ... "never" anymore ever since we add MADV_COLLAPSE.
> PR_SET_THP_DISABLE will also block MADV_COLLAPSE, which can be very
> helpful for debugging purposes. Of course, I thought of having a
> system-wide config option to modify PR_SET_THP_DISABLE behavior, but
> I just don't like the semantics.
>
> "prctl: allow overriding system THP policy to always"[1] proposed
> "overriding policies to always", which is just the wrong way around: we
> should not add mechanisms to "enable more" when we already have an
> interface/mechanism to "disable" them (PR_SET_THP_DISABLE). It all gets
> weird otherwise.
>
> "[PATCH 0/6] prctl: introduce PR_SET/GET_THP_POLICY"[2] proposed
> setting the default of the VM_HUGEPAGE, which is similarly the wrong way
> around I think now.
>
> The ideas explored by Lorenzo to extend process_madvise()[3] and mctrl()[4]
> similarly were around the "default for VM_HUGEPAGE" idea, but after the
> discussion, I think we should better leave VM_HUGEPAGE untouched.
>
> Happy to hear naming suggestions for "PR_THP_DISABLE_EXCEPT_ADVISED" where
> we essentially want to say "leave advised regions alone" -- "keep THP
> enabled for advised regions",
>
> The only thing I really dislike about this is using another MMF_* flag,
> but well, no way around it -- and seems like we could easily support
> more than 32 if we want to (most users already treat it like a proper
> bitmap).
>
> I think this here (modifying an existing toggle) is the only prctl()
> extension that we might be willing to accept. In general, I agree like
> most others, that prctl() is a very bad interface for that -- but
> PR_SET_THP_DISABLE is already there and is getting used.
>
> Long-term, I think the answer will be something based on bpf[5]. Maybe
> in that context, I there could still be value in easily disabling THPs for
> selected workloads (esp. debugging purposes).
>
> Jann raised valid concerns[6] about new flags that are persistent across
> exec[6]. As this here is a relaxation to existing PR_SET_THP_DISABLE I
> consider it having a similar security risk as our existing
> PR_SET_THP_DISABLE, but devil is in the detail.
>
> [1] https://lore.kernel.org/r/20250507141132.2773275-1-usamaarif642@gmail.com
> [2] https://lkml.kernel.org/r/20250515133519.2779639-2-usamaarif642@gmail.com
> [3] https://lore.kernel.org/r/cover.1747686021.git.lorenzo.stoakes@oracle.com
> [4] https://lkml.kernel.org/r/85778a76-7dc8-4ea8-8827-acb45f74ee05@lucifer.local
> [5] https://lkml.kernel.org/r/20250608073516.22415-1-laoar.shao@gmail.com
> [6] https://lore.kernel.org/r/CAG48ez3-7EnBVEjpdoW7z5K0hX41nLQN5Wb65Vg-1p8DdXRnjg@mail.gmail.com
>
> Signed-off-by: David Hildenbrand <david@...hat.com>
> ---
>  Documentation/filesystems/proc.rst |  5 +--
>  fs/proc/array.c                    |  2 +-
>  include/linux/huge_mm.h            | 20 ++++++++---
>  include/linux/mm_types.h           | 13 +++----
>  include/uapi/linux/prctl.h         | 10 ++++++
>  kernel/sys.c                       | 58 +++++++++++++++++++++++-------
>  mm/khugepaged.c                    |  2 +-
>  7 files changed, 81 insertions(+), 29 deletions(-)
>
> diff --git a/Documentation/filesystems/proc.rst b/Documentation/filesystems/proc.rst
> index 2971551b7235..915a3e44bc12 100644
> --- a/Documentation/filesystems/proc.rst
> +++ b/Documentation/filesystems/proc.rst
> @@ -291,8 +291,9 @@ It's slow but very precise.
>   HugetlbPages                size of hugetlb memory portions
>   CoreDumping                 process's memory is currently being dumped
>                               (killing the process may lead to a corrupted core)
> - THP_enabled		     process is allowed to use THP (returns 0 when
> -			     PR_SET_THP_DISABLE is set on the process
> + THP_enabled                 process is allowed to use THP (returns 0 when
> +                             PR_SET_THP_DISABLE is set on the process to disable
> +                             THP completely, not just partially)
>   Threads                     number of threads
>   SigQ                        number of signals queued/max. number for queue
>   SigPnd                      bitmap of pending signals for the thread
> diff --git a/fs/proc/array.c b/fs/proc/array.c
> index d6a0369caa93..c4f91a784104 100644
> --- a/fs/proc/array.c
> +++ b/fs/proc/array.c
> @@ -422,7 +422,7 @@ static inline void task_thp_status(struct seq_file *m, struct mm_struct *mm)
>  	bool thp_enabled = IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE);
>
>  	if (thp_enabled)
> -		thp_enabled = !test_bit(MMF_DISABLE_THP, &mm->flags);
> +		thp_enabled = !test_bit(MMF_DISABLE_THP_COMPLETELY, &mm->flags);
>  	seq_printf(m, "THP_enabled:\t%d\n", thp_enabled);
>  }
>
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index 7748489fde1b..71db243a002e 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -318,16 +318,26 @@ struct thpsize {
>  	(transparent_hugepage_flags &					\
>  	 (1<<TRANSPARENT_HUGEPAGE_USE_ZERO_PAGE_FLAG))
>
> +/*
> + * Check whether THPs are explicitly disabled for this VMA, for example,
> + * through madvise or prctl.
> + */
>  static inline bool vma_thp_disabled(struct vm_area_struct *vma,
>  		vm_flags_t vm_flags)
>  {
> +	/* Are THPs disabled for this VMA? */
> +	if (vm_flags & VM_NOHUGEPAGE)
> +		return true;
> +	/* Are THPs disabled for all VMAs in the whole process? */
> +	if (test_bit(MMF_DISABLE_THP_COMPLETELY, &vma->vm_mm->flags))
> +		return true;
>  	/*
> -	 * Explicitly disabled through madvise or prctl, or some
> -	 * architectures may disable THP for some mappings, for
> -	 * example, s390 kvm.
> +	 * Are THPs disabled only for VMAs where we didn't get an explicit
> +	 * advise to use them?
>  	 */
> -	return (vm_flags & VM_NOHUGEPAGE) ||
> -	       test_bit(MMF_DISABLE_THP, &vma->vm_mm->flags);
> +	if (vm_flags & VM_HUGEPAGE)
> +		return false;

Hm is this correct? This means that VM_NOHUGEPAGE no longer results in THP being
disabled here no?

> +	return test_bit(MMF_DISABLE_THP_EXCEPT_ADVISED, &vma->vm_mm->flags);
>  }
>
>  static inline bool thp_disabled_by_hw(void)
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 1ec273b06691..123fefaa4b98 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -1743,19 +1743,16 @@ enum {
>  #define MMF_VM_MERGEABLE	16	/* KSM may merge identical pages */
>  #define MMF_VM_HUGEPAGE		17	/* set when mm is available for khugepaged */
>
> -/*
> - * This one-shot flag is dropped due to necessity of changing exe once again
> - * on NFS restore
> - */
> -//#define MMF_EXE_FILE_CHANGED	18	/* see prctl_set_mm_exe_file() */
> +#define MMF_HUGE_ZERO_PAGE	18      /* mm has ever used the global huge zero page */
>
>  #define MMF_HAS_UPROBES		19	/* has uprobes */
>  #define MMF_RECALC_UPROBES	20	/* MMF_HAS_UPROBES can be wrong */
>  #define MMF_OOM_SKIP		21	/* mm is of no interest for the OOM killer */
>  #define MMF_UNSTABLE		22	/* mm is unstable for copy_from_user */
> -#define MMF_HUGE_ZERO_PAGE	23      /* mm has ever used the global huge zero page */
> -#define MMF_DISABLE_THP		24	/* disable THP for all VMAs */
> -#define MMF_DISABLE_THP_MASK	(1 << MMF_DISABLE_THP)
> +#define MMF_DISABLE_THP_EXCEPT_ADVISED	23	/* no THP except when advised (e.g., VM_HUGEPAGE) */
> +#define MMF_DISABLE_THP_COMPLETELY	24	/* no THP for all VMAs */

It's almost a bit weird to have these as separate flags, since they're distinct
(of course, I don't think there's necessarily another way).

Though this makes me think maybe in future we can have a new mode where both
enabled == something else :P

But perhaps I've been infected with 'bit packing' disease.

Anyway as discussed in the THP meeting, I'm going to be (hopefully!) making
the mm flags a bitmap soon so we'll get more flags available.

> +#define MMF_DISABLE_THP_MASK	((1 << MMF_DISABLE_THP_COMPLETELY) |\
> +				 (1 << MMF_DISABLE_THP_EXCEPT_ADVISED))
>  #define MMF_OOM_REAP_QUEUED	25	/* mm was queued for oom_reaper */
>  #define MMF_MULTIPROCESS	26	/* mm is shared between processes */
>  /*
> diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
> index 43dec6eed559..9c1d6e49b8a9 100644
> --- a/include/uapi/linux/prctl.h
> +++ b/include/uapi/linux/prctl.h
> @@ -177,7 +177,17 @@ struct prctl_mm_map {
>
>  #define PR_GET_TID_ADDRESS	40
>
> +/*
> + * Flags for PR_SET_THP_DISABLE are only applicable when disabling. Bit 0
> + * is reserved, so PR_GET_THP_DISABLE can return "1 | flags", to effectively
> + * return "1" when no flags were specified for PR_SET_THP_DISABLE.
> + */
>  #define PR_SET_THP_DISABLE	41
> +/*
> + * 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.

>  #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?

> +	if (test_bit(MMF_DISABLE_THP_COMPLETELY, mm_flags))
> +		return 1;
> +	else if (test_bit(MMF_DISABLE_THP_EXCEPT_ADVISED, mm_flags))
> +		return 1 | PR_THP_DISABLE_EXCEPT_ADVISED;
> +	return 0;
> +}
> +
> +static int prctl_set_thp_disable(bool thp_disable, unsigned long flags,
> +				 unsigned long arg4, unsigned long arg5)
> +{
> +	unsigned long *mm_flags = &current->mm->flags;
> +
> +	if (arg4 || arg5)
> +		return -EINVAL;
> +
> +	/* Flags are only allowed when disabling. */
> +	if ((!thp_disable && flags) || (flags & ~PR_THP_DISABLE_EXCEPT_ADVISED))
> +		return -EINVAL;
> +	if (mmap_write_lock_killable(current->mm))
> +		return -EINTR;
> +	if (thp_disable) {
> +		if (flags & PR_THP_DISABLE_EXCEPT_ADVISED) {
> +			clear_bit(MMF_DISABLE_THP_COMPLETELY, mm_flags);
> +			set_bit(MMF_DISABLE_THP_EXCEPT_ADVISED, mm_flags);
> +		} else {
> +			set_bit(MMF_DISABLE_THP_COMPLETELY, mm_flags);
> +			clear_bit(MMF_DISABLE_THP_EXCEPT_ADVISED, mm_flags);
> +		}
> +	} else {
> +		clear_bit(MMF_DISABLE_THP_COMPLETELY, mm_flags);
> +		clear_bit(MMF_DISABLE_THP_EXCEPT_ADVISED, mm_flags);
> +	}
> +	mmap_write_unlock(current->mm);
> +	return 0;
> +}
> +
>  SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
>  		unsigned long, arg4, unsigned long, arg5)
>  {
> @@ -2596,20 +2640,10 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
>  			return -EINVAL;
>  		return task_no_new_privs(current) ? 1 : 0;
>  	case PR_GET_THP_DISABLE:
> -		if (arg2 || arg3 || arg4 || arg5)
> -			return -EINVAL;
> -		error = !!test_bit(MMF_DISABLE_THP, &me->mm->flags);
> +		error = prctl_get_thp_disable(arg2, arg3, arg4, arg5);
>  		break;
>  	case PR_SET_THP_DISABLE:
> -		if (arg3 || arg4 || arg5)
> -			return -EINVAL;
> -		if (mmap_write_lock_killable(me->mm))
> -			return -EINTR;
> -		if (arg2)
> -			set_bit(MMF_DISABLE_THP, &me->mm->flags);
> -		else
> -			clear_bit(MMF_DISABLE_THP, &me->mm->flags);
> -		mmap_write_unlock(me->mm);
> +		error = prctl_set_thp_disable(arg2, arg3, arg4, arg5);
>  		break;
>  	case PR_MPX_ENABLE_MANAGEMENT:
>  	case PR_MPX_DISABLE_MANAGEMENT:
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 1ff0c7dd2be4..2c9008246785 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -410,7 +410,7 @@ static inline int hpage_collapse_test_exit(struct mm_struct *mm)
>  static inline int hpage_collapse_test_exit_or_disable(struct mm_struct *mm)
>  {
>  	return hpage_collapse_test_exit(mm) ||
> -	       test_bit(MMF_DISABLE_THP, &mm->flags);
> +	       test_bit(MMF_DISABLE_THP_COMPLETELY, &mm->flags);
>  }
>
>  static bool hugepage_pmd_enabled(void)
> --
> 2.47.3
>

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