Re: [PATCH 14/16] mm/thp: add THP allocation helper

[Ralph Campbell]

On 6/21/20 5:15 PM, Zi Yan wrote:
> On 19 Jun 2020, at 17:56, Ralph Campbell wrote:
>
> > Transparent huge page allocation policy is controlled by several sysfs
> > variables. Rather than expose these to each device driver that needs to
> > allocate THPs, provide a helper function.
> >
> > Signed-off-by: Ralph Campbell <rcampbell@xxxxxxxxxx>
> > ---
> >   include/linux/gfp.h | 10 ++++++++++
> >   mm/huge_memory.c    | 16 ++++++++++++++++
> >   2 files changed, 26 insertions(+)
> >
> > diff --git a/include/linux/gfp.h b/include/linux/gfp.h
> > index 67a0774e080b..1c7d968a27d3 100644
> > --- a/include/linux/gfp.h
> > +++ b/include/linux/gfp.h
> > @@ -562,6 +562,16 @@ extern struct page *alloc_pages_vma(gfp_t gfp_mask, int order,
> >   	alloc_pages_vma(gfp_mask, 0, vma, addr, numa_node_id(), false)
> >   #define alloc_page_vma_node(gfp_mask, vma, addr, node)		\
> >   	alloc_pages_vma(gfp_mask, 0, vma, addr, node, false)
> > +#ifdef CONFIG_ARCH_ENABLE_THP_MIGRATION
> > +extern struct page *alloc_transhugepage(struct vm_area_struct *vma,
> > +					unsigned long addr);
> > +#else
> > +static inline struct page *alloc_transhugepage(struct vm_area_struct *vma,
> > +						unsigned long addr)
> > +{
> > +	return NULL;
> > +}
> > +#endif
> >
> >   extern unsigned long __get_free_pages(gfp_t gfp_mask, unsigned int order);
> >   extern unsigned long get_zeroed_page(gfp_t gfp_mask);
> > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > index 25d95f7b1e98..f749633ed350 100644
> > --- a/mm/huge_memory.c
> > +++ b/mm/huge_memory.c
> > @@ -775,6 +775,22 @@ vm_fault_t do_huge_pmd_anonymous_page(struct vm_fault *vmf)
> >   	return __do_huge_pmd_anonymous_page(vmf, page, gfp);
> >   }
> >
> > +#ifdef CONFIG_ARCH_ENABLE_THP_MIGRATION
> > +struct page *alloc_transhugepage(struct vm_area_struct *vma,
> > +				 unsigned long haddr)
> > +{
> > +	gfp_t gfp;
> > +	struct page *page;
> > +
> > +	gfp = alloc_hugepage_direct_gfpmask(vma);
> > +	page = alloc_hugepage_vma(gfp, vma, haddr, HPAGE_PMD_ORDER);
> > +	if (page)
> > +		prep_transhuge_page(page);
> > +	return page;
> > +}
> > +EXPORT_SYMBOL_GPL(alloc_transhugepage);
> > +#endif
> > +
> >   static void insert_pfn_pmd(struct vm_area_struct *vma, unsigned long addr,
> >   		pmd_t *pmd, pfn_t pfn, pgprot_t prot, bool write,
> >   		pgtable_t pgtable)
> > --
> > 2.20.1
>
> Why use CONFIG_ARCH_ENABLE_THP_MIGRATION to guard THP allocator helper?
> Shouldnât CONFIG_TRANSPARENT_HUGEPAGE be used? Also the helper still allocates
> a THP even if transparent_hugepage_enabled(vma) is false, which is wrong, right?
>
>
> --
> Best Regards,
> Yan Zi

Oops, I'm not sure why I thought that was needed. The whole file is only compiled
if CONFIG_TRANSPARENT_HUGEPAGE is defined and the calls to alloc_hugepage_vma()
and alloc_hugepage_direct_gfpmask() are unprotected just above this in
do_huge_pmd_anonymous_page(). I'll fix that in v2.

The helper is intended to be called by a device driver to allocate a THP when
migrating device private memory back to system memory. The THP should never be
migrated to device private memory in the first place if
transparent_hugepage_enabled(vma) is false.
I suppose I could add a if (WARN_ON_ONCE()) return NULL as a sanity check.
The real checks are in migrate_vma_setup() and migrate_vma_pages().