linux-kernel - Re: [PATCH POC] prctl: extend PR_SET_THP_DISABLE to optionally exclude VM

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Message-ID: <383a957e-badb-49fc-9913-30bea3a1c5a8@redhat.com>
Date: Wed, 23 Jul 2025 20:02:52 +0200
From: David Hildenbrand <david@...hat.com>
To: Usama Arif <usamaarif642@...il.com>, linux-kernel@...r.kernel.org
Cc: linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
 linux-doc@...r.kernel.org, Jonathan Corbet <corbet@....net>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, Zi Yan <ziy@...dia.com>,
 Baolin Wang <baolin.wang@...ux.alibaba.com>,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>, Nico Pache <npache@...hat.com>,
 Ryan Roberts <ryan.roberts@....com>, Dev Jain <dev.jain@....com>,
 Barry Song <baohua@...nel.org>, Vlastimil Babka <vbabka@...e.cz>,
 Mike Rapoport <rppt@...nel.org>, Suren Baghdasaryan <surenb@...gle.com>,
 Michal Hocko <mhocko@...e.com>, SeongJae Park <sj@...nel.org>,
 Jann Horn <jannh@...gle.com>, Yafang Shao <laoar.shao@...il.com>,
 Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH POC] prctl: extend PR_SET_THP_DISABLE to optionally
 exclude VM_HUGEPAGE

On 23.07.25 19:07, Usama Arif wrote:
>>> Thanks for the patch David!
>>>
>>> As discussed in the other thread, with the below diff
>>>
>>> diff --git a/kernel/sys.c b/kernel/sys.c
>>> index 2a34b2f70890..3912f5b6a02d 100644
>>> --- a/kernel/sys.c
>>> +++ b/kernel/sys.c
>>> @@ -2447,7 +2447,7 @@ static int prctl_set_thp_disable(unsigned long thp_disable, unsigned long flags,
>>>                   return -EINVAL;
>>>             /* Flags are only allowed when disabling. */
>>> -       if (!thp_disable || (flags & ~PR_THP_DISABLE_EXCEPT_ADVISED))
>>> +       if ((!thp_disable && flags) || (flags & ~PR_THP_DISABLE_EXCEPT_ADVISED))
>>>                   return -EINVAL;
>>>           if (mmap_write_lock_killable(current->mm))
>>>                   return -EINTR;
>>>
>>>
>>> I tested with the below selftest, and it works. It hopefully covers
>>> majority of the cases including fork and re-enabling THPs.
>>> Let me know if it looks ok and please feel free to add this in the
>>> next revision you send.
>>>
>>>
>>> Once the above diff is included, please feel free to add
>>>
>>> Acked-by: Usama Arif <usamaarif642@...il.com>
>>> Tested-by: Usama Arif <usamaarif642@...il.com>
>>
>> Thanks!
>>
>> The latest version lives at
>>
>>    https://github.com/davidhildenbrand/linux/tree/PR_SET_THP_DISABLE
>>
>> With all current review feedback addressed (primarily around description+comments) + that one fix.
>>
>>
> 
> Hi David,
> 
> Just wanted to check if the above branch is up to date?
> 

No, I forgot to push. Now pushed.

This is the diff (excluding description changes):

diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
index c4127104d9bc3..527aa4c9645fd 100644
--- a/include/linux/huge_mm.h
+++ b/include/linux/huge_mm.h
@@ -324,8 +324,8 @@ struct thpsize {
          (1<<TRANSPARENT_HUGEPAGE_USE_ZERO_PAGE_FLAG))
  
  /*
- * Check whether THPs are explicitly disabled through madvise or prctl, or some
- * architectures may disable THP for some mappings, for example, s390 kvm.
+ * 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)
diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
index 1949bb9270d48..60e496ecabe04 100644
--- a/include/uapi/linux/prctl.h
+++ b/include/uapi/linux/prctl.h
@@ -179,8 +179,8 @@ struct prctl_mm_map {
  
  /*
   * Flags for PR_SET_THP_DISABLE are only applicable when disabling. Bit 0
- * is reserved, so PR_GET_THP_DISABLE can return 1 when no other flags were
- * specified for PR_SET_THP_DISABLE.
+ * 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 (MADV_HUGEPAGE / VM_HUGEPAGE). */
diff --git a/kernel/sys.c b/kernel/sys.c
index 2a34b2f708900..b87d0acaab0be 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -2438,7 +2438,7 @@ static int prctl_get_thp_disable(unsigned long arg2, unsigned long arg3,
         return 0;
  }
  
-static int prctl_set_thp_disable(unsigned long thp_disable, unsigned long flags,
+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;
@@ -2447,7 +2447,7 @@ static int prctl_set_thp_disable(unsigned long thp_disable, unsigned long flags,
                 return -EINVAL;
  
         /* Flags are only allowed when disabling. */
-       if (!thp_disable || (flags & ~PR_THP_DISABLE_EXCEPT_ADVISED))
+       if ((!thp_disable && flags) || (flags & ~PR_THP_DISABLE_EXCEPT_ADVISED))
                 return -EINVAL;
         if (mmap_write_lock_killable(current->mm))
                 return -EINTR;


-- 
Cheers,

David / dhildenb