Re: [PATCH v4 02/14] memblock: add MEMBLOCK_RSRV_KERN flag

[Wei Yang]

On Wed, Feb 26, 2025 at 02:09:15AM +0000, Wei Yang wrote:
>On Tue, Feb 25, 2025 at 09:46:28AM +0200, Mike Rapoport wrote:
>>On Mon, Feb 24, 2025 at 01:31:31AM +0000, Wei Yang wrote:
>>> On Wed, Feb 19, 2025 at 09:24:31AM +0200, Mike Rapoport wrote:
>>> >Hi,
>>> >
>>> >On Tue, Feb 18, 2025 at 03:50:04PM +0000, Wei Yang wrote:
>>> >> On Thu, Feb 06, 2025 at 03:27:42PM +0200, Mike Rapoport wrote:
>>> >> >From: "Mike Rapoport (Microsoft)" <rppt@xxxxxxxxxx>
>>> >> >
>>> >> >to denote areas that were reserved for kernel use either directly with
>>> >> >memblock_reserve_kern() or via memblock allocations.
>>> >> >
>>> >> >Signed-off-by: Mike Rapoport (Microsoft) <rppt@xxxxxxxxxx>
>>> >> >---
>>> >> > include/linux/memblock.h | 16 +++++++++++++++-
>>> >> > mm/memblock.c            | 32 ++++++++++++++++++++++++--------
>>> >> > 2 files changed, 39 insertions(+), 9 deletions(-)
>>> >> >
>>> >> >diff --git a/include/linux/memblock.h b/include/linux/memblock.h
>>> >> >index e79eb6ac516f..65e274550f5d 100644
>>> >> >--- a/include/linux/memblock.h
>>> >> >+++ b/include/linux/memblock.h
>>> >> >@@ -50,6 +50,7 @@ enum memblock_flags {
>>> >> > 	MEMBLOCK_NOMAP		= 0x4,	/* don't add to kernel direct mapping */
>>> >> > 	MEMBLOCK_DRIVER_MANAGED = 0x8,	/* always detected via a driver */
>>> >> > 	MEMBLOCK_RSRV_NOINIT	= 0x10,	/* don't initialize struct pages */
>>> >> >+	MEMBLOCK_RSRV_KERN	= 0x20,	/* memory reserved for kernel use */
>>> >> 
>>> >> Above memblock_flags, there are comments on explaining those flags.
>>> >> 
>>> >> Seems we miss it for MEMBLOCK_RSRV_KERN.
>>> >
>>> >Right, thanks!
>>> > 
>>> >> > 
>>> >> > #ifdef CONFIG_HAVE_MEMBLOCK_PHYS_MAP
>>> >> >@@ -1459,14 +1460,14 @@ phys_addr_t __init memblock_alloc_range_nid(phys_addr_t size,
>>> >> > again:
>>> >> > 	found = memblock_find_in_range_node(size, align, start, end, nid,
>>> >> > 					    flags);
>>> >> >-	if (found && !memblock_reserve(found, size))
>>> >> >+	if (found && !__memblock_reserve(found, size, nid, MEMBLOCK_RSRV_KERN))
>>> >> 
>>> >> Maybe we could use memblock_reserve_kern() directly. If my understanding is
>>> >> correct, the reserved region's nid is not used.
>>> >
>>> >We use nid of reserved regions in reserve_bootmem_region() (commit
>>> >61167ad5fecd ("mm: pass nid to reserve_bootmem_region()")) but KHO needs to
>>> >know the distribution of reserved memory among the nodes before
>>> >memmap_init_reserved_pages().
>>> > 
>>> >> BTW, one question here. How we handle concurrent memblock allocation? If two
>>> >> threads find the same available range and do the reservation, it seems to be a
>>> >> problem to me. Or I missed something?
>>> >
>>> >memblock allocations end before smp_init(), there is no possible concurrency.
>>> > 
>>> 
>>> Thanks, I still have one question here.
>>> 
>>> Below is a simplified call flow.
>>> 
>>>     mm_core_init()
>>>         mem_init()
>>>             memblock_free_all()
>>>                 free_low_memory_core_early()
>>>                     memmap_init_reserved_pages()
>>>                         memblock_set_node(..., memblock.reserved, )   --- (1)
>>>                     __free_memory_core()
>>>         kmem_cache_init()
>>>             slab_state = UP;                                          --- (2)
>>> 
>>> And memblock_allloc_range_nid() is not supposed to be called after
>>> slab_is_available(). Even someone do dose it, it will get memory from slab
>>> instead of reserve region in memblock.
>>> 
>>> From the above call flow and background, there are three cases when
>>> memblock_alloc_range_nid() would be called:
>>> 
>>>   * If it is called before (1), memblock.reserved's nid would be adjusted correctly.
>>>   * If it is called after (2), we don't touch memblock.reserved.
>>>   * If it happens between (1) and (2), it looks would break the consistency of
>>>     nid information in memblock.reserved. Because when we use
>>>     memblock_reserve_kern(), NUMA_NO_NODE would be stored in region.
>>> 
>>> So my question is if the third case happens, would it introduce a bug? If it
>>> won't happen, seems we don't need to specify the nid here?
>>
>>We don't really care about proper assignment of nodes between (1) and (2)
>>from one side and the third case does not happen on the other side. Nothing
>>should call membloc_alloc() after memblock_free_all(). 
>>
>
>My point is if no one would call memblock_alloc() after memblock_free_all(),
>which set nid in memblock.reserved properly, it seems not necessary to do
>__memblock_reserve() with exact nid during memblock_alloc()? 
>
>As you did __memblock_reserve(found, size, nid, MEMBLOCK_RSRV_KERN) in this
>patch.
>

Hi, Mike

Do you think my understanding is reasonable?

-- 
Wei Yang
Help you, Help me