Re: [RFC PATCH 2/3] mm, thp, proc: report THP eligibility for each vma

[Michal Hocko]

Damn, David somehow didn't make it to the CC list. Sorry about that.

On Tue 20-11-18 11:35:14, Michal Hocko wrote:
> From: Michal Hocko <mhocko@xxxxxxxx>
> 
> Userspace falls short when trying to find out whether a specific memory
> range is eligible for THP. There are usecases that would like to know
> that
> http://lkml.kernel.org/r/alpine.DEB.2.21.1809251248450.50347@xxxxxxxxxxxxxxxxxxxxxxxxx
> : This is used to identify heap mappings that should be able to fault thp
> : but do not, and they normally point to a low-on-memory or fragmentation
> : issue.
> 
> The only way to deduce this now is to query for hg resp. nh flags and
> confronting the state with the global setting. Except that there is
> also PR_SET_THP_DISABLE that might change the picture. So the final
> logic is not trivial. Moreover the eligibility of the vma depends on
> the type of VMA as well. In the past we have supported only anononymous
> memory VMAs but things have changed and shmem based vmas are supported
> as well these days and the query logic gets even more complicated
> because the eligibility depends on the mount option and another global
> configuration knob.
> 
> Simplify the current state and report the THP eligibility in
> /proc/<pid>/smaps for each existing vma. Reuse transparent_hugepage_enabled
> for this purpose. The original implementation of this function assumes
> that the caller knows that the vma itself is supported for THP so make
> the core checks into __transparent_hugepage_enabled and use it for
> existing callers. __show_smap just use the new transparent_hugepage_enabled
> which also checks the vma support status (please note that this one has
> to be out of line due to include dependency issues).
> 
> Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
> ---
>  Documentation/filesystems/proc.txt |  3 +++
>  fs/proc/task_mmu.c                 |  2 ++
>  include/linux/huge_mm.h            | 13 ++++++++++++-
>  mm/huge_memory.c                   | 12 +++++++++++-
>  mm/memory.c                        |  4 ++--
>  5 files changed, 30 insertions(+), 4 deletions(-)
> 
> diff --git a/Documentation/filesystems/proc.txt b/Documentation/filesystems/proc.txt
> index b1fda309f067..06562bab509a 100644
> --- a/Documentation/filesystems/proc.txt
> +++ b/Documentation/filesystems/proc.txt
> @@ -425,6 +425,7 @@ SwapPss:               0 kB
>  KernelPageSize:        4 kB
>  MMUPageSize:           4 kB
>  Locked:                0 kB
> +THPeligible:           0
>  VmFlags: rd ex mr mw me dw
>  
>  the first of these lines shows the same information as is displayed for the
> @@ -462,6 +463,8 @@ replaced by copy-on-write) part of the underlying shmem object out on swap.
>  "SwapPss" shows proportional swap share of this mapping. Unlike "Swap", this
>  does not take into account swapped out page of underlying shmem objects.
>  "Locked" indicates whether the mapping is locked in memory or not.
> +"THPeligible" indicates whether the mapping is eligible for THP pages - 1 if
> +true, 0 otherwise.
>  
>  "VmFlags" field deserves a separate description. This member represents the kernel
>  flags associated with the particular virtual memory area in two letter encoded
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index 47c3764c469b..c9f160eb9fbc 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -790,6 +790,8 @@ static int show_smap(struct seq_file *m, void *v)
>  
>  	__show_smap(m, &mss);
>  
> +	seq_printf(m, "THPeligible:    %d\n", transparent_hugepage_enabled(vma));
> +
>  	if (arch_pkeys_enabled())
>  		seq_printf(m, "ProtectionKey:  %8u\n", vma_pkey(vma));
>  	show_smap_vma_flags(m, vma);
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index 4663ee96cf59..381e872bfde0 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -93,7 +93,11 @@ extern bool is_vma_temporary_stack(struct vm_area_struct *vma);
>  
>  extern unsigned long transparent_hugepage_flags;
>  
> -static inline bool transparent_hugepage_enabled(struct vm_area_struct *vma)
> +/*
> + * to be used on vmas which are known to support THP.
> + * Use transparent_hugepage_enabled otherwise
> + */
> +static inline bool __transparent_hugepage_enabled(struct vm_area_struct *vma)
>  {
>  	if (vma->vm_flags & VM_NOHUGEPAGE)
>  		return false;
> @@ -117,6 +121,8 @@ static inline bool transparent_hugepage_enabled(struct vm_area_struct *vma)
>  	return false;
>  }
>  
> +bool transparent_hugepage_enabled(struct vm_area_struct *vma);
> +
>  #define transparent_hugepage_use_zero_page()				\
>  	(transparent_hugepage_flags &					\
>  	 (1<<TRANSPARENT_HUGEPAGE_USE_ZERO_PAGE_FLAG))
> @@ -257,6 +263,11 @@ static inline bool thp_migration_supported(void)
>  
>  #define hpage_nr_pages(x) 1
>  
> +static inline bool __transparent_hugepage_enabled(struct vm_area_struct *vma)
> +{
> +	return false;
> +}
> +
>  static inline bool transparent_hugepage_enabled(struct vm_area_struct *vma)
>  {
>  	return false;
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 55478ab3c83b..f64733c23067 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -62,6 +62,16 @@ static struct shrinker deferred_split_shrinker;
>  static atomic_t huge_zero_refcount;
>  struct page *huge_zero_page __read_mostly;
>  
> +bool transparent_hugepage_enabled(struct vm_area_struct *vma)
> +{
> +	if (vma_is_anonymous(vma))
> +		return __transparent_hugepage_enabled(vma);
> +	if (shmem_mapping(vma->vm_file->f_mapping) && shmem_huge_enabled(vma))
> +		return __transparent_hugepage_enabled(vma);
> +
> +	return false;
> +}
> +
>  static struct page *get_huge_zero_page(void)
>  {
>  	struct page *zero_page;
> @@ -1303,7 +1313,7 @@ vm_fault_t do_huge_pmd_wp_page(struct vm_fault *vmf, pmd_t orig_pmd)
>  	get_page(page);
>  	spin_unlock(vmf->ptl);
>  alloc:
> -	if (transparent_hugepage_enabled(vma) &&
> +	if (__transparent_hugepage_enabled(vma) &&
>  	    !transparent_hugepage_debug_cow()) {
>  		huge_gfp = alloc_hugepage_direct_gfpmask(vma, haddr);
>  		new_page = alloc_pages_vma(huge_gfp, HPAGE_PMD_ORDER, vma,
> diff --git a/mm/memory.c b/mm/memory.c
> index 4ad2d293ddc2..3c2716ec7fbd 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -3830,7 +3830,7 @@ static vm_fault_t __handle_mm_fault(struct vm_area_struct *vma,
>  	vmf.pud = pud_alloc(mm, p4d, address);
>  	if (!vmf.pud)
>  		return VM_FAULT_OOM;
> -	if (pud_none(*vmf.pud) && transparent_hugepage_enabled(vma)) {
> +	if (pud_none(*vmf.pud) && __transparent_hugepage_enabled(vma)) {
>  		ret = create_huge_pud(&vmf);
>  		if (!(ret & VM_FAULT_FALLBACK))
>  			return ret;
> @@ -3856,7 +3856,7 @@ static vm_fault_t __handle_mm_fault(struct vm_area_struct *vma,
>  	vmf.pmd = pmd_alloc(mm, vmf.pud, address);
>  	if (!vmf.pmd)
>  		return VM_FAULT_OOM;
> -	if (pmd_none(*vmf.pmd) && transparent_hugepage_enabled(vma)) {
> +	if (pmd_none(*vmf.pmd) && __transparent_hugepage_enabled(vma)) {
>  		ret = create_huge_pmd(&vmf);
>  		if (!(ret & VM_FAULT_FALLBACK))
>  			return ret;
> -- 
> 2.19.1

-- 
Michal Hocko
SUSE Labs