Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE

[Mike Rapoport]

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).

If we set aside the argument for keeping the kABI, this seems, hmm, a bit
more complex than new madvise() :)

It seems that for CRIU usecase such behaviour of prctl will work and it
probably will be even more convenient than madvise(). Nonetheless, I think
madvise() is the more elegant and correct solution.

> > 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.

Currently, MADV_HUGEPAGE overrides the prctl(PR_SET_THP_DISABLE)...
AFAIU, the prctl was intended to work with applications unaware of THP and
for the cases where addition of MADV_*HUGEPAGE to the application was not
an option.
 
> ---
> 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
>