Re: [PATCH POC] prctl: extend PR_SET_THP_DISABLE to optionally exclude VM_HUGEPAGE

[David Hildenbrand]

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@xxxxxxxxx>
> > > Tested-by: Usama Arif <usamaarif642@xxxxxxxxx>
> >
> > 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