Re: [PATCH v9 01/10] memblock: Introduce MEMBLOCK_LLMAP

[Vincent Donnefort]

On Sun, Sep 06, 2026 at 10:33:11PM +0300, Mike Rapoport wrote:
> On Wed, Sep 02, 2026 at 11:47:03AM +0100, Vincent Donnefort wrote:
> > Keeping last-level mappings is interesting on some architectures as it
> > allows mapping/unmapping pages from the kernel direct map without the
> > risk of splitting blocks which, under the break-before-make rule, may
> > trigger page-faults the kernel can't handle.
> > 
> > However, mapping the entire direct map at PTE-level is costly. So
> > instead, create a new memblock flag MEMBLOCK_LLMAP to enable the system
> 
> I believe MEMBLOCK_PTE_MAP sounds more descriptive.

The idea was to have something close from NOMAP, to emphasis it is one or the
other. But PTE_MAP sounds good too.

> 
> > to decide which region must be covered by mappings up to the last-level.
> > 
> > Signed-off-by: Vincent Donnefort <vdonnefort@xxxxxxxxxx>
> > 
> > diff --git a/include/linux/memblock.h b/include/linux/memblock.h
> > index d62db9e776cf..d40a5ded188d 100644
> > --- a/include/linux/memblock.h
> > +++ b/include/linux/memblock.h
> > @@ -52,6 +52,7 @@ extern unsigned long long max_possible_pfn;
> >   * kernel that we know is good to use. It is the only memory that
> >   * allocations may happen from in this phase.
> >   * @MEMBLOCK_RSRV_HUGETLB: memory is reserved for hugetlb pages
> > + * @MEMBLOCK_LLMAP: memory region to be mapped using last-level mapping
> >   */
> >  enum memblock_flags {
> >  	MEMBLOCK_NONE		= 0x0,	/* No special request */
> > @@ -63,6 +64,7 @@ enum memblock_flags {
> >  	MEMBLOCK_RSRV_KERN	= 0x20,	/* memory reserved for kernel use */
> >  	MEMBLOCK_KHO_SCRATCH	= 0x40,	/* scratch memory for kexec handover */
> >  	MEMBLOCK_RSRV_HUGETLB	= 0x80, /* memory reserved for hugetlb pages */
> > +	MEMBLOCK_LLMAP		= 0x100,/* last-level mapping */
> >  };
> >  
> >  /**
> > @@ -160,6 +162,8 @@ int memblock_reserved_mark_noinit(phys_addr_t base, phys_addr_t size);
> >  int memblock_reserved_mark_kern(phys_addr_t base, phys_addr_t size);
> >  int memblock_mark_kho_scratch(phys_addr_t base, phys_addr_t size);
> >  int memblock_clear_kho_scratch(phys_addr_t base, phys_addr_t size);
> > +int memblock_mark_llmap(phys_addr_t base, phys_addr_t size);
> > +int memblock_clear_llmap(phys_addr_t base, phys_addr_t size);
> >  
> >  void memblock_free(void *ptr, size_t size);
> >  void reset_all_zones_managed_pages(void);
> > @@ -306,6 +310,11 @@ static inline bool memblock_is_kho_scratch(struct memblock_region *m)
> >  	return m->flags & MEMBLOCK_KHO_SCRATCH;
> >  }
> >  
> > +static inline bool memblock_is_llmap(struct memblock_region *m)
> > +{
> > +	return m->flags & MEMBLOCK_LLMAP;
> > +}
> > +
> >  int memblock_search_pfn_nid(unsigned long pfn, unsigned long *start_pfn,
> >  			    unsigned long  *end_pfn);
> >  void __next_mem_pfn_range(int *idx, int nid, unsigned long *out_start_pfn,
> > diff --git a/mm/memblock.c b/mm/memblock.c
> > index 9ce86349a29f..1591b50503ed 100644
> > --- a/mm/memblock.c
> > +++ b/mm/memblock.c
> > @@ -1119,6 +1119,16 @@ int __init_memblock memblock_mark_mirror(phys_addr_t base, phys_addr_t size)
> >   */
> >  int __init_memblock memblock_mark_nomap(phys_addr_t base, phys_addr_t size)
> >  {
> > +	struct memblock_region *r;
> > +
> > +	memblock_cap_size(base, &size);
> > +
> > +	for_each_mem_region(r) {
> > +		if (memblock_is_llmap(r) &&
> > +		    memblock_addrs_overlap(base, size, r->base, r->size))
> > +			return -EINVAL;
> > +	}
> 
> I'm not very fond of implicit skips here. memblock has no idea what's the
> caller intention, maybe it actually wants to change the memory from nomap
> to pte-mapped.
> 
> I'd rather warn in memblock_is_nomap() and memblock_is_llmap() if they both
> are set and let the caller deal with making sure they are not.

ack.

> 
> > +
> >  	return memblock_setclr_flag(&memblock.memory, base, size, 1, MEMBLOCK_NOMAP);
> >  }
> >  
> > @@ -1204,6 +1214,45 @@ __init int memblock_clear_kho_scratch(phys_addr_t base, phys_addr_t size)
> >  				    MEMBLOCK_KHO_SCRATCH);
> >  }
> >  
> > +/**
> > + * memblock_mark_llmap - Mark a memory region with flag MEMBLOCK_LLMAP.
> > + * @base: the base phys addr of the region
> > + * @size: the size of the region
> > + *
> > + * If supported by the architecture, such region is mapped at the last-level in
> > + * the kernel direct map.
> > + *
> > + * Return: 0 on success, -errno on failure.
> > + */
> > +int __init_memblock memblock_mark_llmap(phys_addr_t base, phys_addr_t size)
> > +{
> > +	struct memblock_region *r;
> > +
> > +	memblock_cap_size(base, &size);
> > +
> > +	for_each_mem_region(r) {
> > +		if (memblock_is_nomap(r) &&
> > +		    memblock_addrs_overlap(base, size, r->base, r->size))
> > +			return -EINVAL;
> > +	}
> 
> same here

ack

> 
> > +
> > +	return memblock_setclr_flag(&memblock.memory, base, size, 1,
> > +				    MEMBLOCK_LLMAP);
> > +}
> > +
> > +/**
> > + * memblock_clear_llmap - Clear flag MEMBLOCK_LLMAP for a specified region.
> > + * @base: the base phys addr of the region
> > + * @size: the size of the region
> > + *
> > + * Return: 0 on success, -errno on failure.
> > + */
> > +int __init_memblock memblock_clear_llmap(phys_addr_t base, phys_addr_t size)
> > +{
> > +	return memblock_setclr_flag(&memblock.memory, base, size, 0,
> > +				    MEMBLOCK_LLMAP);
> > +}
> > +
> >  static bool should_skip_region(struct memblock_type *type,
> >  			       struct memblock_region *m,
> >  			       int nid, int flags)
> > @@ -2886,6 +2935,7 @@ static const char * const flagname[] = {
> >  	[ilog2(MEMBLOCK_RSRV_NOINIT)] = "RSV_NIT",
> >  	[ilog2(MEMBLOCK_RSRV_KERN)] = "RSV_KERN",
> >  	[ilog2(MEMBLOCK_KHO_SCRATCH)] = "KHO_SCRATCH",
> > +	[ilog2(MEMBLOCK_LLMAP)] = "LLMAP",
> >  };
> >  
> >  static int memblock_debug_show(struct seq_file *m, void *private)
> > -- 
> > 2.55.0.970.g62bdec98f9-goog
> > 
> 
> -- 
> Sincerely yours,
> Mike.

-- 
Vincent