linux-kernel - Re: [PATCH v4 02/14] memblock: add MEMBLOCK_RSRV

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Message-ID: <Z8_Ju0p63GBCdSG9@kernel.org>
Date: Tue, 11 Mar 2025 07:27:23 +0200
From: Mike Rapoport <rppt@...nel.org>
To: Wei Yang <richard.weiyang@...il.com>
Cc: linux-kernel@...r.kernel.org, Alexander Graf <graf@...zon.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andy Lutomirski <luto@...nel.org>,
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Subject: Re: [PATCH v4 02/14] memblock: add MEMBLOCK_RSRV_KERN flag

Hi Wei,

On Mon, Mar 10, 2025 at 09:51:24AM +0000, Wei Yang wrote:
> On Sun, Feb 23, 2025 at 12:22:29AM +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@...nel.org>
> >>> >
> >>> >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@...nel.org>
> >>> >---
> >>> > 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().
> >> 
> >
> >I took another look into this commit. There maybe a very corner case in which
> >will leave a reserved region with no nid set.
> >
> >memmap_init_reserved_pages()
> >    for_each_mem_region() {
> >        ...
> >	memblock_set_node(start, end, &memblock.reserved, nid);
> >    }
> >
> >We leverage the iteration here to set nid to all regions in memblock.reserved.
> >But memblock_set_node() may call memblock_double_array() to expand the array,
> >which may get a range before current start. So we would miss to set the
> >correct nid to the new reserved region.
> >
> >I have tried to create a case in memblock test. This would happen when there
> >are 126 memblock.reserved regions. And the last region is across the last two
> >node.
> >
> >One way to fix this is compare type->max in memblock_set_node(). Then check
> >this return value in memmap_init_reserved_pages(). If we found the size
> >changes, repeat the iteration.
> >
> >But this is a very trivial one, not sure it worth fix.
> >
> 
> Hi, Mike
> 
> I have done a user space test which shows we may have a chance to leave a
> region with non-nid set.
> 
> Not sure you are ok with my approach of fixing.

Wouldn't it be better to check for a change in reserved.max in
memmap_init_reserved_pages()?
 
> -- 
> Wei Yang
> Help you, Help me

-- 
Sincerely yours,
Mike.