Re: [PATCH 1/1] mm: Add folio_map_local()

[Uladzislau Rezki <urezki@gmail.com>]

On Tue, Nov 01, 2022 at 07:12:21PM +0900, Hyeonggon Yoo wrote:
> On Fri, Oct 28, 2022 at 04:15:26PM +0100, Matthew Wilcox (Oracle) wrote:
> > Some filesystems benefit from being able to map the entire folio.
> > On 32-bit platforms with HIGHMEM, we fall back to using vmap, which
> > will be slow.  If it proves to be a performance problem, we can look at
> > optimising it in a number of ways.
> > 
> > Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> > ---
> >  include/linux/highmem.h | 40 ++++++++++++++++++++++++++++++++-
> >  include/linux/vmalloc.h |  6 +++--
> >  mm/vmalloc.c            | 50 +++++++++++++++++++++++++++++++++++++++++
> >  3 files changed, 93 insertions(+), 3 deletions(-)
> > 
> > diff --git a/include/linux/highmem.h b/include/linux/highmem.h
> > index e9912da5441b..e8159243d88d 100644
> > --- a/include/linux/highmem.h
> > +++ b/include/linux/highmem.h
> > @@ -10,6 +10,7 @@
> >  #include <linux/mm.h>
> >  #include <linux/uaccess.h>
> >  #include <linux/hardirq.h>
> > +#include <linux/vmalloc.h>
> >  
> >  #include "highmem-internal.h"
> >  
> > @@ -132,6 +133,44 @@ static inline void *kmap_local_page(struct page *page);
> >   */
> >  static inline void *kmap_local_folio(struct folio *folio, size_t offset);
> >  
> > +/**
> > + * folio_map_local - Map an entire folio.
> > + * @folio: The folio to map.
> > + *
> > + * Unlike kmap_local_folio(), map an entire folio.  This should be undone
> > + * with folio_unmap_local().  The address returned should be treated as
> > + * stack-based, and local to this CPU, like kmap_local_folio().
> > + *
> > + * Context: May allocate memory using GFP_KERNEL if it takes the vmap path.
> > + * Return: A kernel virtual address which can be used to access the folio,
> > + * or NULL if the mapping fails.
> > + */
> > +static inline __must_check void *folio_map_local(struct folio *folio)
> > +{
> > +	might_alloc(GFP_KERNEL);
> > +
> > +	if (!IS_ENABLED(CONFIG_HIGHMEM))
> > +		return folio_address(folio);
> > +	if (folio_test_large(folio))
> > +		return vm_map_folio(folio);
> > +	return kmap_local_page(&folio->page);
> > +}
> > +
> > +/**
> > + * folio_unmap_local - Unmap an entire folio.
> > + * @addr: Address returned from folio_map_local()
> > + *
> > + * Undo the result of a previous call to folio_map_local().
> > + */
> > +static inline void folio_unmap_local(const void *addr)
> > +{
> > +	if (!IS_ENABLED(CONFIG_HIGHMEM))
> > +		return;
> > +	if (is_vmalloc_addr(addr))
> > +		vunmap(addr);
> 
> I think it should be vm_unmap_ram(); (and pass number of pages to
> folio_unmap_local()) as the vmap area might be allocated using
> vb_alloc().
> 
> > +	kunmap_local(addr);
> > +}
> 
> missing else statement?
> 
> > +
> >  /**
> >   * kmap_atomic - Atomically map a page for temporary usage - Deprecated!
> >   * @page:	Pointer to the page to be mapped
> > @@ -426,5 +465,4 @@ static inline void folio_zero_range(struct folio *folio,
> >  {
> >  	zero_user_segments(&folio->page, start, start + length, 0, 0);
> >  }
> > -
> >  #endif /* _LINUX_HIGHMEM_H */
> > diff --git a/include/linux/vmalloc.h b/include/linux/vmalloc.h
> > index 096d48aa3437..4bb34c939c01 100644
> > --- a/include/linux/vmalloc.h
> > +++ b/include/linux/vmalloc.h
> > @@ -13,6 +13,7 @@
> >  #include <asm/vmalloc.h>
> >  
> >  struct vm_area_struct;		/* vma defining user mapping in mm_types.h */
> > +struct folio;			/* also mm_types.h */
> >  struct notifier_block;		/* in notifier.h */
> >  
> >  /* bits in flags of vmalloc's vm_struct below */
> > @@ -163,8 +164,9 @@ extern void *vcalloc(size_t n, size_t size) __alloc_size(1, 2);
> >  extern void vfree(const void *addr);
> >  extern void vfree_atomic(const void *addr);
> >  
> > -extern void *vmap(struct page **pages, unsigned int count,
> > -			unsigned long flags, pgprot_t prot);
> > +void *vmap(struct page **pages, unsigned int count, unsigned long flags,
> > +		pgprot_t prot);
> > +void *vm_map_folio(struct folio *folio);
> >  void *vmap_pfn(unsigned long *pfns, unsigned int count, pgprot_t prot);
> >  extern void vunmap(const void *addr);
> >  
> > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > index ccaa461998f3..265b860c9550 100644
> > --- a/mm/vmalloc.c
> > +++ b/mm/vmalloc.c
> > @@ -2283,6 +2283,56 @@ void *vm_map_ram(struct page **pages, unsigned int count, int node)
> >  }
> >  EXPORT_SYMBOL(vm_map_ram);
> >  
> > +#ifdef CONFIG_HIGHMEM
> > +/**
> > + * vm_map_folio() - Map an entire folio into virtually contiguous space.
> > + * @folio: The folio to map.
> > + *
> > + * Maps all pages in @folio into contiguous kernel virtual space.  This
> > + * function is only available in HIGHMEM builds; for !HIGHMEM, use
> > + * folio_address().  The pages are mapped with PAGE_KERNEL permissions.
> > + *
> > + * Return: The address of the area or %NULL on failure
> > + */
> > +void *vm_map_folio(struct folio *folio)
> > +{
> > +	size_t size = folio_size(folio);
> > +	unsigned long addr;
> > +	void *mem;
> > +
> > +	might_sleep();
> > +
> > +	if (likely(folio_nr_pages(folio) <= VMAP_MAX_ALLOC)) {
> > +		mem = vb_alloc(size, GFP_KERNEL);
> > +		if (IS_ERR(mem))
> > +			return NULL;
> > +		addr = (unsigned long)mem;
> > +	} else {
> > +		struct vmap_area *va;
> > +		va = alloc_vmap_area(size, PAGE_SIZE, VMALLOC_START,
> > +				VMALLOC_END, NUMA_NO_NODE, GFP_KERNEL);
> > +		if (IS_ERR(va))
> > +			return NULL;
> > +
> > +		addr = va->va_start;
> > +		mem = (void *)addr;
> > +	}
> > +
> > +	if (vmap_range_noflush(addr, addr + size,
> > +				folio_pfn(folio) << PAGE_SHIFT,
> > +				PAGE_KERNEL, folio_shift(folio))) {
> > +		vm_unmap_ram(mem, folio_nr_pages(folio));
> > +		return NULL;
> > +	}
> > +	flush_cache_vmap(addr, addr + size);
> > +
> > +	mem = kasan_unpoison_vmalloc(mem, size, KASAN_VMALLOC_PROT_NORMAL);
> > +
> > +	return mem;
> > +}
> > +EXPORT_SYMBOL(vm_map_folio);
> > +#endif
> 
> it's a bit of copy & paste but yeah, it seems unavoidable at this point.
> 
Agree.

It is worth to rework the void *vm_map_folio(struct folio *folio) function.
All the rest looks reasonable.

--
Uladzislau Rezki