On Wed, May 31, 2017 at 10:24:14AM +0200, Michal Hocko wrote:
> On Wed 31-05-17 08:30:08, Vlastimil Babka wrote:
> > On 05/30/2017 06:06 PM, Andrea Arcangeli wrote:
> > >
> > > I'm not sure if it should be considered a bug, the prctl is intended
> > > to use normally by wrappers so it looks optimal as implemented this
> > > way: affecting future vmas only, which will all be created after
> > > execve executed by the wrapper.
> > >
> > > What's the point of messing with the prctl so it mangles over the
> > > wrapper process own vmas before exec? Messing with those vmas is pure
> > > wasted CPUs for the wrapper use case which is what the prctl was
> > > created for.
> > >
> > > Furthermore there would be the risk a program that uses the prctl not
> > > as a wrapper and then calls the prctl to clear VM_NOHUGEPAGE from
> > > def_flags assuming the current kABI. The program could assume those
> > > vmas that were instantiated before disabling the prctl are still with
> > > VM_NOHUGEPAGE set (they would not after the change you propose).
> > >
> > > Adding a scan of all vmas to PR_SET_THP_DISABLE to clear VM_NOHUGEPAGE
> > > on existing vmas looks more complex too and less finegrined so
> > > probably more complex for userland to manage
> >
> > I would expect the prctl wouldn't iterate all vma's, nor would it modify
> > def_flags anymore. It would just set a flag somewhere in mm struct that
> > would be considered in addition to the per-vma flags when deciding
> > whether to use THP.
>
> Exactly. Something like the below (not even compile tested).
I did a quick go with the patch, compiles just fine :)
It worked for my simple examples, the THP is enabled/disabled as expected
and the vma->vm_flags are indeed unaffected.
> > We could consider whether MADV_HUGEPAGE should be
> > able to override the prctl or not.
>
> This should be a master override to any per vma setting.
Here you've introduced a change to the current behaviour. Consider the
following sequence:
{
prctl(PR_SET_THP_DISABLE);
address = mmap(...);
madvise(address, len, MADV_HUGEPAGE);
}
Currently, for the vma that backs the address
transparent_hugepage_enabled(vma) will return true, and after your patch it
will return false.
The new behaviour may be more correct, I just wanted to bring the change to
attention.
> ---
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index a3762d49ba39..9da053ced864 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -92,6 +92,7 @@ extern bool is_vma_temporary_stack(struct vm_area_struct *vma);
> (1<<TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG) && \
> ((__vma)->vm_flags & VM_HUGEPAGE))) && \
> !((__vma)->vm_flags & VM_NOHUGEPAGE) && \
> + !test_bit(MMF_DISABLE_THP, &(__vma)->vm_mm->flags) && \
> !is_vma_temporary_stack(__vma))
> #define transparent_hugepage_use_zero_page() \
> (transparent_hugepage_flags & \
> diff --git a/include/linux/khugepaged.h b/include/linux/khugepaged.h
> index 5d9a400af509..f0d7335336cd 100644
> --- a/include/linux/khugepaged.h
> +++ b/include/linux/khugepaged.h
> @@ -48,7 +48,8 @@ static inline int khugepaged_enter(struct vm_area_struct *vma,
> if (!test_bit(MMF_VM_HUGEPAGE, &vma->vm_mm->flags))
> if ((khugepaged_always() ||
> (khugepaged_req_madv() && (vm_flags & VM_HUGEPAGE))) &&
> - !(vm_flags & VM_NOHUGEPAGE))
> + !(vm_flags & VM_NOHUGEPAGE) &&
> + !test_bit(MMF_DISABLE_THP, &vma->vm_mm->flags))
> if (__khugepaged_enter(vma->vm_mm))
> return -ENOMEM;
> return 0;
> diff --git a/include/linux/sched/coredump.h b/include/linux/sched/coredump.h
> index 69eedcef8f03..2c07b244090a 100644
> --- a/include/linux/sched/coredump.h
> +++ b/include/linux/sched/coredump.h
> @@ -68,6 +68,7 @@ static inline int get_dumpable(struct mm_struct *mm)
> #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_INIT_MASK (MMF_DUMPABLE_MASK | MMF_DUMP_FILTER_MASK)
>
> diff --git a/kernel/sys.c b/kernel/sys.c
> index 8a94b4eabcaa..e48f0636c7fd 100644
> --- a/kernel/sys.c
> +++ b/kernel/sys.c
> @@ -2266,7 +2266,7 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
> case PR_GET_THP_DISABLE:
> if (arg2 || arg3 || arg4 || arg5)
> return -EINVAL;
> - error = !!(me->mm->def_flags & VM_NOHUGEPAGE);
> + error = !!test_bit(MMF_DISABLE_THP, &me->mm->flags);
> break;
> case PR_SET_THP_DISABLE:
> if (arg3 || arg4 || arg5)
> @@ -2274,9 +2274,9 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
> if (down_write_killable(&me->mm->mmap_sem))
> return -EINTR;
> if (arg2)
> - me->mm->def_flags |= VM_NOHUGEPAGE;
> + set_bit(MMF_DISABLE_THP, &me->mm->flags);
> else
> - me->mm->def_flags &= ~VM_NOHUGEPAGE;
> + clear_bit(MMF_DISABLE_THP, &me->mm->flags);
> up_write(&me->mm->mmap_sem);
> break;
> case PR_MPX_ENABLE_MANAGEMENT:
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index ce29e5cc7809..57e31f4752b3 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -818,7 +818,8 @@ khugepaged_alloc_page(struct page **hpage, gfp_t gfp, int node)
> static bool hugepage_vma_check(struct vm_area_struct *vma)
> {
> if ((!(vma->vm_flags & VM_HUGEPAGE) && !khugepaged_always()) ||
> - (vma->vm_flags & VM_NOHUGEPAGE))
> + (vma->vm_flags & VM_NOHUGEPAGE) ||
> + test_bit(MMF_DISABLE_THP, &vma->vm_mm->flags))
> return false;
> if (shmem_file(vma->vm_file)) {
> if (!IS_ENABLED(CONFIG_TRANSPARENT_HUGE_PAGECACHE))
> diff --git a/mm/shmem.c b/mm/shmem.c
> index e67d6ba4e98e..27fe1bbf813b 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -1977,10 +1977,11 @@ static int shmem_fault(struct vm_fault *vmf)
> }
>
> sgp = SGP_CACHE;
> - if (vma->vm_flags & VM_HUGEPAGE)
> - sgp = SGP_HUGE;
> - else if (vma->vm_flags & VM_NOHUGEPAGE)
> +
> + if ((vma->vm_flags & VM_NOHUGEPAGE) || test_bit(MMF_DISABLE_THP, &vma->vm_mm->flags))
> sgp = SGP_NOHUGE;
> + else if (vma->vm_flags & VM_HUGEPAGE)
> + sgp = SGP_HUGE;
>
> error = shmem_getpage_gfp(inode, vmf->pgoff, &vmf->page, sgp,
> gfp, vma, vmf, &ret);
> --
> Michal Hocko
> SUSE Labs
>
On Thu 01-06-17 14:00:48, Mike Rapoport wrote:
> On Wed, May 31, 2017 at 10:24:14AM +0200, Michal Hocko wrote:
> > On Wed 31-05-17 08:30:08, Vlastimil Babka wrote:
> > > On 05/30/2017 06:06 PM, Andrea Arcangeli wrote:
> > > >
> > > > I'm not sure if it should be considered a bug, the prctl is intended
> > > > to use normally by wrappers so it looks optimal as implemented this
> > > > way: affecting future vmas only, which will all be created after
> > > > execve executed by the wrapper.
> > > >
> > > > What's the point of messing with the prctl so it mangles over the
> > > > wrapper process own vmas before exec? Messing with those vmas is pure
> > > > wasted CPUs for the wrapper use case which is what the prctl was
> > > > created for.
> > > >
> > > > Furthermore there would be the risk a program that uses the prctl not
> > > > as a wrapper and then calls the prctl to clear VM_NOHUGEPAGE from
> > > > def_flags assuming the current kABI. The program could assume those
> > > > vmas that were instantiated before disabling the prctl are still with
> > > > VM_NOHUGEPAGE set (they would not after the change you propose).
> > > >
> > > > Adding a scan of all vmas to PR_SET_THP_DISABLE to clear VM_NOHUGEPAGE
> > > > on existing vmas looks more complex too and less finegrined so
> > > > probably more complex for userland to manage
> > >
> > > I would expect the prctl wouldn't iterate all vma's, nor would it modify
> > > def_flags anymore. It would just set a flag somewhere in mm struct that
> > > would be considered in addition to the per-vma flags when deciding
> > > whether to use THP.
> >
> > Exactly. Something like the below (not even compile tested).
>
> I did a quick go with the patch, compiles just fine :)
> It worked for my simple examples, the THP is enabled/disabled as expected
> and the vma->vm_flags are indeed unaffected.
>
> > > We could consider whether MADV_HUGEPAGE should be
> > > able to override the prctl or not.
> >
> > This should be a master override to any per vma setting.
>
> Here you've introduced a change to the current behaviour. Consider the
> following sequence:
>
> {
> prctl(PR_SET_THP_DISABLE);
> address = mmap(...);
> madvise(address, len, MADV_HUGEPAGE);
> }
>
> Currently, for the vma that backs the address
> transparent_hugepage_enabled(vma) will return true, and after your patch it
> will return false.
> The new behaviour may be more correct, I just wanted to bring the change to
> attention.
The system wide disable should override any VMA specific setting
IMHO. Why would we disable the THP for the whole process otherwise?
Anyway this needs to be discussed at linux-api mailing list. I will try
to make my change into a proper patch and post it there.
--
Michal Hocko
SUSE Labs