Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE

Michal Hocko
2017-05-31T08:24:24Z

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).

> 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.

---
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