在 2024/6/17 下午6:26, Barry Song 写道:
On Tue, Jun 4, 2024 at 9:15 PM <yangge1116@xxxxxxx> wrote:
From: yangge <yangge1116@xxxxxxx>
Since commit 5d0a661d808f ("mm/page_alloc: use only one PCP list for
THP-sized allocations") no longer differentiates the migration type
of pages in THP-sized PCP list, it's possible to get a CMA page from
the list, in some cases, it's not acceptable, for example, allocating
a non-CMA page with PF_MEMALLOC_PIN flag returns a CMA page.
The patch forbids allocating non-CMA THP-sized page from THP-sized
PCP list to avoid the issue above.
Could you please describe the impact on users in the commit log?
If a large number of CMA memory are configured in the system (for
example, the CMA memory accounts for 50% of the system memory), starting
virtual machine with device passthrough will get stuck.
During starting virtual machine, it will call pin_user_pages_remote(...,
FOLL_LONGTERM, ...) to pin memory. If a page is in CMA area,
pin_user_pages_remote() will migrate the page from CMA area to non-CMA
area because of FOLL_LONGTERM flag. If non-movable allocation requests
return CMA memory, pin_user_pages_remote() will enter endless loops.
backtrace:
pin_user_pages_remote
----__gup_longterm_locked //cause endless loops in this function
--------__get_user_pages_locked
--------check_and_migrate_movable_pages //always check fail and continue
to migrate
------------migrate_longterm_unpinnable_pages
----------------alloc_migration_target // non-movable allocation
Is it possible that some CMA memory might be used by non-movable
allocation requests?
Yes.
If so, will CMA somehow become unable to migrate, causing cma_alloc() to fail?
No, it will cause endless loops in __gup_longterm_locked(). If
non-movable allocation requests return CMA memory,
migrate_longterm_unpinnable_pages() will migrate a CMA page to another
CMA page, which is useless and cause endless loops in
__gup_longterm_locked().
backtrace:
pin_user_pages_remote
----__gup_longterm_locked //cause endless loops in this function
--------__get_user_pages_locked
--------check_and_migrate_movable_pages //always check fail and continue
to migrate
------------migrate_longterm_unpinnable_pages
Fixes: 5d0a661d808f ("mm/page_alloc: use only one PCP list for THP-sized allocations")
Signed-off-by: yangge <yangge1116@xxxxxxx>
---
mm/page_alloc.c | 10 ++++++++++
1 file changed, 10 insertions(+)
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 2e22ce5..0bdf471 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2987,10 +2987,20 @@ struct page *rmqueue(struct zone *preferred_zone,
WARN_ON_ONCE((gfp_flags & __GFP_NOFAIL) && (order > 1));
if (likely(pcp_allowed_order(order))) {
+#ifdef CONFIG_TRANSPARENT_HUGEPAGE
+ if (!IS_ENABLED(CONFIG_CMA) || alloc_flags & ALLOC_CMA ||
+ order != HPAGE_PMD_ORDER) {
+ page = rmqueue_pcplist(preferred_zone, zone, order,
+ migratetype, alloc_flags);
+ if (likely(page))
+ goto out;
+ }
This seems not ideal, because non-CMA THP gets no chance to use PCP. But it
still seems better than causing the failure of CMA allocation.
Is there a possible approach to avoiding adding CMA THP into pcp from the first
beginning? Otherwise, we might need a separate PCP for CMA.
+#else
page = rmqueue_pcplist(preferred_zone, zone, order,
migratetype, alloc_flags);
if (likely(page))
goto out;
+#endif
}
page = rmqueue_buddy(preferred_zone, zone, order, alloc_flags,
--
2.7.4
Thanks
Barry
