Re: [RFC 14/43] mm: memblock: PKRAM: prevent memblock resize from clobbering preserved pages

[Anthony Yznaga <anthony.yznaga@oracle.com>]

On 5/11/20 6:57 AM, Mike Rapoport wrote:
> On Wed, May 06, 2020 at 05:41:40PM -0700, Anthony Yznaga wrote:
>> The size of the memblock reserved array may be increased while preserved
>> pages are being reserved. When this happens, preserved pages that have
>> not yet been reserved are at risk for being clobbered when space for a
>> larger array is allocated.
>> When called from memblock_double_array(), a wrapper around
>> memblock_find_in_range() walks the preserved pages pagetable to find
>> sufficiently sized ranges without preserved pages and passes them to
>> memblock_find_in_range().
> I'd suggest to create an array of memblock_region's that will contain
> the PKRAM ranges before kexec and pass this array to the new kernel.
> Then, somewhere in start_kerenel() replace replace
> memblock.reserved->regions with that array. 

I'll look into doing this.  Thanks!

Anthony

>
>> Signed-off-by: Anthony Yznaga <anthony.yznaga@oracle.com>
>> ---
>>  include/linux/pkram.h |  3 +++
>>  mm/memblock.c         | 15 +++++++++++++--
>>  mm/pkram.c            | 51 +++++++++++++++++++++++++++++++++++++++++++++++++++
>>  3 files changed, 67 insertions(+), 2 deletions(-)
>>
>> diff --git a/include/linux/pkram.h b/include/linux/pkram.h
>> index edc5d8bef9d3..409022e1472f 100644
>> --- a/include/linux/pkram.h
>> +++ b/include/linux/pkram.h
>> @@ -62,6 +62,9 @@ struct page *pkram_load_page(struct pkram_stream *ps, unsigned long *index,
>>  ssize_t pkram_write(struct pkram_stream *ps, const void *buf, size_t count);
>>  size_t pkram_read(struct pkram_stream *ps, void *buf, size_t count);
>>  
>> +phys_addr_t pkram_memblock_find_in_range(phys_addr_t start, phys_addr_t end,
>> +					 phys_addr_t size, phys_addr_t align);
>> +
>>  #ifdef CONFIG_PKRAM
>>  extern unsigned long pkram_reserved_pages;
>>  void pkram_reserve(void);
>> diff --git a/mm/memblock.c b/mm/memblock.c
>> index c79ba6f9920c..69ae883b8d21 100644
>> --- a/mm/memblock.c
>> +++ b/mm/memblock.c
>> @@ -16,6 +16,7 @@
>>  #include <linux/kmemleak.h>
>>  #include <linux/seq_file.h>
>>  #include <linux/memblock.h>
>> +#include <linux/pkram.h>
>>  
>>  #include <asm/sections.h>
>>  #include <linux/io.h>
>> @@ -349,6 +350,16 @@ phys_addr_t __init_memblock memblock_find_in_range(phys_addr_t start,
>>  	return ret;
>>  }
>>  
>> +phys_addr_t __init_memblock __memblock_find_in_range(phys_addr_t start,
>> +					phys_addr_t end, phys_addr_t size,
>> +					phys_addr_t align)
>> +{
>> +	if (IS_ENABLED(CONFIG_PKRAM))
>> +		return pkram_memblock_find_in_range(start, end, size, align);
>> +	else
>> +		return memblock_find_in_range(start, end, size, align);
>> +}
>> +
>>  static void __init_memblock memblock_remove_region(struct memblock_type *type, unsigned long r)
>>  {
>>  	type->total_size -= type->regions[r].size;
>> @@ -447,11 +458,11 @@ static int __init_memblock memblock_double_array(struct memblock_type *type,
>>  		if (type != &memblock.reserved)
>>  			new_area_start = new_area_size = 0;
>>  
>> -		addr = memblock_find_in_range(new_area_start + new_area_size,
>> +		addr = __memblock_find_in_range(new_area_start + new_area_size,
>>  						memblock.current_limit,
>>  						new_alloc_size, PAGE_SIZE);
>>  		if (!addr && new_area_size)
>> -			addr = memblock_find_in_range(0,
>> +			addr = __memblock_find_in_range(0,
>>  				min(new_area_start, memblock.current_limit),
>>  				new_alloc_size, PAGE_SIZE);
>>  
>> diff --git a/mm/pkram.c b/mm/pkram.c
>> index dd3c89614010..e49c9bcd3854 100644
>> --- a/mm/pkram.c
>> +++ b/mm/pkram.c
>> @@ -1238,3 +1238,54 @@ void pkram_free_pgt(void)
>>  	__free_pages_core(virt_to_page(pkram_pgd), 0);
>>  	pkram_pgd = NULL;
>>  }
>> +
>> +static int __init_memblock pkram_memblock_find_cb(struct pkram_pg_state *st, unsigned long base, unsigned long size)
>> +{
>> +	unsigned long end = base + size;
>> +	unsigned long addr;
>> +
>> +	if (size < st->min_size)
>> +		return 0;
>> +
>> +	addr =  memblock_find_in_range(base, end, st->min_size, PAGE_SIZE);
>> +	if (!addr)
>> +		return 0;
>> +
>> +	st->retval = addr;
>> +	return 1;
>> +}
>> +
>> +/*
>> + * It may be necessary to allocate a larger reserved memblock array
>> + * while populating it with ranges of preserved pages.  To avoid
>> + * trampling preserved pages that have not yet been added to the
>> + * memblock reserved list this function implements a wrapper around
>> + * memblock_find_in_range() that restricts searches to subranges
>> + * that do not contain preserved pages.
>> + */
>> +phys_addr_t __init_memblock pkram_memblock_find_in_range(phys_addr_t start,
>> +					phys_addr_t end, phys_addr_t size,
>> +					phys_addr_t align)
>> +{
>> +	struct pkram_pg_state st = {
>> +		.range_cb = pkram_memblock_find_cb,
>> +		.min_addr = start,
>> +		.max_addr = end,
>> +		.min_size = PAGE_ALIGN(size),
>> +		.find_holes = true,
>> +	};
>> +
>> +	if (!pkram_reservation_in_progress)
>> +		return memblock_find_in_range(start, end, size, align);
>> +
>> +	if (!pkram_pgd) {
>> +		WARN_ONCE(1, "No preserved pages pagetable\n");
>> +		return memblock_find_in_range(start, end, size, align);
>> +	}
>> +
>> +	WARN_ONCE(memblock_bottom_up(), "PKRAM: bottom up memblock allocation not yet supported\n");
>> +
>> +	pkram_walk_pgt_rev(&st, pkram_pgd);
>> +
>> +	return st.retval;
>> +}
>> -- 
>> 2.13.3
>>