On 2024/12/13 15:37, yangge1116@xxxxxxx wrote:
From: yangge <yangge1116@xxxxxxx>
Since commit 984fdba6a32e ("mm, compaction: use proper alloc_flags
in __compaction_suitable()") allow compaction to proceed when free
pages required for compaction reside in the CMA pageblocks, it's
possible that __compaction_suitable() always returns true, and in
some cases, it's not acceptable.
There are 4 NUMA nodes on my machine, and each NUMA node has 32GB
of memory. I have configured 16GB of CMA memory on each NUMA node,
and starting a 32GB virtual machine with device passthrough is
extremely slow, taking almost an hour.
During the start-up of the virtual machine, it will call
pin_user_pages_remote(..., FOLL_LONGTERM, ...) to allocate memory.
Long term GUP cannot allocate memory from CMA area, so a maximum
of 16 GB of no-CMA memory on a NUMA node can be used as virtual
machine memory. Since there is 16G of free CMA memory on the NUMA
node, watermark for order-0 always be met for compaction, so
__compaction_suitable() always returns true, even if the node is
unable to allocate non-CMA memory for the virtual machine.
For costly allocations, because __compaction_suitable() always
returns true, __alloc_pages_slowpath() can't exit at the appropriate
place, resulting in excessively long virtual machine startup times.
Call trace:
__alloc_pages_slowpath
if (compact_result == COMPACT_SKIPPED ||
compact_result == COMPACT_DEFERRED)
goto nopage; // should exit __alloc_pages_slowpath() from here
To sum up, during long term GUP flow, we should remove ALLOC_CMA
both in __compaction_suitable() and __isolate_free_page().
Fixes: 984fdba6a32e ("mm, compaction: use proper alloc_flags in __compaction_suitable()")
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: yangge <yangge1116@xxxxxxx>
---
mm/compaction.c | 8 +++++---
mm/page_alloc.c | 4 +++-
2 files changed, 8 insertions(+), 4 deletions(-)
diff --git a/mm/compaction.c b/mm/compaction.c
index 07bd227..044c2247 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -2384,6 +2384,7 @@ static bool __compaction_suitable(struct zone *zone, int order,
unsigned long wmark_target)
{
unsigned long watermark;
+ bool pin;
/*
* Watermarks for order-0 must be met for compaction to be able to
* isolate free pages for migration targets. This means that the
@@ -2395,14 +2396,15 @@ static bool __compaction_suitable(struct zone *zone, int order,
* even if compaction succeeds.
* For costly orders, we require low watermark instead of min for
* compaction to proceed to increase its chances.
- * ALLOC_CMA is used, as pages in CMA pageblocks are considered
- * suitable migration targets
+ * In addition to long term GUP flow, ALLOC_CMA is used, as pages in
+ * CMA pageblocks are considered suitable migration targets
*/
watermark = (order > PAGE_ALLOC_COSTLY_ORDER) ?
low_wmark_pages(zone) : min_wmark_pages(zone);
watermark += compact_gap(order);
+ pin = !!(current->flags & PF_MEMALLOC_PIN);
return __zone_watermark_ok(zone, 0, watermark, highest_zoneidx,
- ALLOC_CMA, wmark_target);
+ pin ? 0 : ALLOC_CMA, wmark_target);
}
Seems a little hack for me. Using the 'cc->alloc_flags' passed from the
caller to determin if ‘ALLOC_CMA’ is needed looks more reasonable to me.
/*
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index dde19db..9a5dfda 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2813,6 +2813,7 @@ int __isolate_free_page(struct page *page, unsigned int order)
{
struct zone *zone = page_zone(page);
int mt = get_pageblock_migratetype(page);
+ bool pin;
if (!is_migrate_isolate(mt)) {
unsigned long watermark;
@@ -2823,7 +2824,8 @@ int __isolate_free_page(struct page *page, unsigned int order)
* exists.
*/
watermark = zone->_watermark[WMARK_MIN] + (1UL << order);
- if (!zone_watermark_ok(zone, 0, watermark, 0, ALLOC_CMA))
+ pin = !!(current->flags & PF_MEMALLOC_PIN);
+ if (!zone_watermark_ok(zone, 0, watermark, 0, pin ? 0 : ALLOC_CMA))
return 0;
}
