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Subject[PATCH AUTOSEL 5.2 74/85] xen: let alloc_xenballooned_pages() fail if not enough memory free
Date
From: Juergen Gross <jgross@suse.com>

[ Upstream commit a1078e821b605813b63bf6bca414a85f804d5c66 ]

Instead of trying to allocate pages with GFP_USER in
add_ballooned_pages() check the available free memory via
si_mem_available(). GFP_USER is far less limiting memory exhaustion
than the test via si_mem_available().

This will avoid dom0 running out of memory due to excessive foreign
page mappings especially on ARM and on x86 in PVH mode, as those don't
have a pre-ballooned area which can be used for foreign mappings.

As the normal ballooning suffers from the same problem don't balloon
down more than si_mem_available() pages in one iteration. At the same
time limit the default maximum number of retries.

This is part of XSA-300.

Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
drivers/xen/balloon.c | 16 +++++++++++++---
1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/drivers/xen/balloon.c b/drivers/xen/balloon.c
index d37dd5bb7a8f..559768dc2567 100644
--- a/drivers/xen/balloon.c
+++ b/drivers/xen/balloon.c
@@ -538,8 +538,15 @@ static void balloon_process(struct work_struct *work)
state = reserve_additional_memory();
}

- if (credit < 0)
- state = decrease_reservation(-credit, GFP_BALLOON);
+ if (credit < 0) {
+ long n_pages;
+
+ n_pages = min(-credit, si_mem_available());
+ state = decrease_reservation(n_pages, GFP_BALLOON);
+ if (state == BP_DONE && n_pages != -credit &&
+ n_pages < totalreserve_pages)
+ state = BP_EAGAIN;
+ }

state = update_schedule(state);

@@ -578,6 +585,9 @@ static int add_ballooned_pages(int nr_pages)
}
}

+ if (si_mem_available() < nr_pages)
+ return -ENOMEM;
+
st = decrease_reservation(nr_pages, GFP_USER);
if (st != BP_DONE)
return -ENOMEM;
@@ -710,7 +720,7 @@ static int __init balloon_init(void)
balloon_stats.schedule_delay = 1;
balloon_stats.max_schedule_delay = 32;
balloon_stats.retry_count = 1;
- balloon_stats.max_retry_count = RETRY_UNLIMITED;
+ balloon_stats.max_retry_count = 4;

#ifdef CONFIG_XEN_BALLOON_MEMORY_HOTPLUG
set_online_page_callback(&xen_online_page);
--
2.20.1
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