linux-kernel - [PATCH 1/1] powerpc/kernel: Enables memory hot-remove after reboot on pseries guests

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Date:   Fri, 28 Feb 2020 03:04:39 -0300
From:   Leonardo Bras <leonardo@...ux.ibm.com>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Hari Bathini <hbathini@...ux.ibm.com>,
        Leonardo Bras <leonardo@...ux.ibm.com>,
        Christophe Leroy <christophe.leroy@....fr>,
        Thomas Gleixner <tglx@...utronix.de>,
        Claudio Carvalho <cclaudio@...ux.ibm.com>,
        mdroth@...ux.vnet.ibm.com
Cc:     linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: [PATCH 1/1] powerpc/kernel: Enables memory hot-remove after reboot on pseries guests

While providing guests, it's desirable to resize it's memory on demand.

By now, it's possible to do so by creating a guest with a small base
memory, hot-plugging all the rest, and using 'movable_node' kernel
command-line parameter, which puts all hot-plugged memory in
ZONE_MOVABLE, allowing it to be removed whenever needed.

But there is an issue regarding guest reboot:
If memory is hot-plugged, and then the guest is rebooted, all hot-plugged
memory goes to ZONE_NORMAL, which offers no guaranteed hot-removal.
It usually prevents this memory to be hot-removed from the guest.

It's possible to use device-tree information to fix that behavior, as
it stores flags for LMB ranges on ibm,dynamic-memory-vN.
It involves marking each memblock with the correct flags as hotpluggable
memory, which mm/memblock.c puts in ZONE_MOVABLE during boot if
'movable_node' is passed.

For base memory, qemu assigns these flags for it's LMBs:
(DRCONF_MEM_AI_INVALID | DRCONF_MEM_RESERVED)
For hot-plugged memory, it assigns (DRCONF_MEM_ASSIGNED).

While guest kernel reads the device-tree, early_init_drmem_lmb() is
called for every added LMBs, doing nothing for base memory, and adding
memblocks for hot-plugged memory. Skipping base memory happens here:

if ((lmb->flags & DRCONF_MEM_RESERVED) ||
    !(lmb->flags & DRCONF_MEM_ASSIGNED))
	return;

Marking memblocks added by this function as hotplugable memory
is enough to get the desirable behavior, and should cause no change
if 'movable_node' parameter is not passed to kernel.

Signed-off-by: Leonardo Bras <leonardo@...ux.ibm.com>
---
 arch/powerpc/kernel/prom.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/arch/powerpc/kernel/prom.c b/arch/powerpc/kernel/prom.c
index 6620f37abe73..f4d14c67bf53 100644
--- a/arch/powerpc/kernel/prom.c
+++ b/arch/powerpc/kernel/prom.c
@@ -518,6 +518,8 @@ static void __init early_init_drmem_lmb(struct drmem_lmb *lmb,
 		DBG("Adding: %llx -> %llx\n", base, size);
 		if (validate_mem_limit(base, &size))
 			memblock_add(base, size);
+
+		early_init_dt_mark_hotplug_memory_arch(base, size);
 	} while (--rngs);
 }
 #endif /* CONFIG_PPC_PSERIES */
-- 
2.24.1