[Xen-devel] [PATCH v2] mm/page_alloc: fix MEMF_no_dma allocations for single NUMA

[Sergey Dyasli <sergey.dyasli@xxxxxxxxxx>]

Currently dma_bitsize is zero by default on single NUMA node machines.
This makes all alloc_domheap_pages() calls with MEMF_no_dma return NULL.

There is only 1 user of MEMF_no_dma: dom0_memflags, which are used
during memory allocation for Dom0. Failing allocation with default
dom0_memflags is especially severe for the PV Dom0 case: it makes
alloc_chunk() to use suboptimal 2MB allocation algorithm with a search
for higher memory addresses.

This can lead to the NMI watchdog timeout during PV Dom0 construction
on some machines, which can be worked around by specifying "dma_bits"
in Xen's cmdline manually.

Fix the issue by ignoring MEMF_no_dma in cases when dma_bitsize is zero,
which means there is no DMA zone. This shouldn't cause any issues for
Dom0 because alloc_heap_pages() will first use higher memory addresses
for satisfying memory allocation requests.

Signed-off-by: Sergey Dyasli <sergey.dyasli@xxxxxxxxxx>
---
v2:
- A different approach as suggested by Jan, with wording changes

---
CC: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CC: Jan Beulich <jbeulich@xxxxxxxx>
CC: Julien Grall <julien.grall@xxxxxxx>
CC: Wei Liu <wei.liu2@xxxxxxxxxx>
CC: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
CC: George Dunlap <George.Dunlap@xxxxxxxxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
 xen/common/page_alloc.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/xen/common/page_alloc.c b/xen/common/page_alloc.c
index 37a52aaa0d..f71d3bb7a1 100644
--- a/xen/common/page_alloc.c
+++ b/xen/common/page_alloc.c
@@ -2314,7 +2314,9 @@ struct page_info *alloc_domheap_pages(
         return NULL;
     }
 
-    if ( dma_bitsize && ((dma_zone = bits_to_zone(dma_bitsize)) < zone_hi) )
+    if ( !dma_bitsize )
+        memflags &= ~MEMF_no_dma;
+    else if ( (dma_zone = bits_to_zone(dma_bitsize)) < zone_hi )
         pg = alloc_heap_pages(dma_zone + 1, zone_hi, order, memflags, d);
 
     if ( (pg == NULL) &&
-- 
2.17.1


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