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mm/page_alloc: fix race condition in unaccepted memory handling

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commit fefc075182275057ce607effaa3daa9e6e3bdc73 upstream.

The page allocator tracks the number of zones that have unaccepted memory
using static_branch_enc/dec() and uses that static branch in hot paths to
determine if it needs to deal with unaccepted memory.

Borislav and Thomas pointed out that the tracking is racy: operations on
static_branch are not serialized against adding/removing unaccepted pages
to/from the zone.

Sanity checks inside static_branch machinery detects it:

WARNING: CPU: 0 PID: 10 at kernel/jump_label.c:276 __static_key_slow_dec_cpuslocked+0x8e/0xa0

The comment around the WARN() explains the problem:

	/*
	 * Warn about the '-1' case though; since that means a
	 * decrement is concurrent with a first (0->1) increment. IOW
	 * people are trying to disable something that wasn't yet fully
	 * enabled. This suggests an ordering problem on the user side.
	 */

The effect of this static_branch optimization is only visible on
microbenchmark.

Instead of adding more complexity around it, remove it altogether.

Link: https://lkml.kernel.org/r/20250506133207.1009676-1-kirill.shutemov@linux.intel.com
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Fixes: dcdfdd40fa ("mm: Add support for unaccepted memory")
Link: https://lore.kernel.org/all/20250506092445.GBaBnVXXyvnazly6iF@fat_crate.local
Reported-by: Borislav Petkov <bp@alien8.de>
Tested-by: Borislav Petkov (AMD) <bp@alien8.de>
Reported-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Brendan Jackman <jackmanb@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: <stable@vger.kernel.org>	[6.5+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Kirill A. Shutemov 2025-05-06 16:32:07 +03:00 committed by Greg Kroah-Hartman
parent 260ca2e961
commit 74953f93f4
1 changed files with 0 additions and 23 deletions
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23
mm/page_alloc.c
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@ -7041,9 +7041,6 @@ bool has_managed_dma(void)
#ifdef CONFIG_UNACCEPTED_MEMORY
/* Counts number of zones with unaccepted pages. */
static DEFINE_STATIC_KEY_FALSE(zones_with_unaccepted_pages);
static bool lazy_accept = true;
static int __init accept_memory_parse(char *p)
@ -7070,11 +7067,7 @@ static bool page_contains_unaccepted(struct page *page, unsigned int order)
static void __accept_page(struct zone *zone, unsigned long *flags,
struct page *page)
{
bool last;
list_del(&page->lru);
last = list_empty(&zone->unaccepted_pages);
account_freepages(zone, -MAX_ORDER_NR_PAGES, MIGRATE_MOVABLE);
__mod_zone_page_state(zone, NR_UNACCEPTED, -MAX_ORDER_NR_PAGES);
__ClearPageUnaccepted(page);
@ -7083,9 +7076,6 @@ static void __accept_page(struct zone *zone, unsigned long *flags,
accept_memory(page_to_phys(page), PAGE_SIZE << MAX_PAGE_ORDER);
__free_pages_ok(page, MAX_PAGE_ORDER, FPI_TO_TAIL);
if (last)
static_branch_dec(&zones_with_unaccepted_pages);
}
void accept_page(struct page *page)
@ -7122,19 +7112,11 @@ static bool try_to_accept_memory_one(struct zone *zone)
return true;
}
static inline bool has_unaccepted_memory(void)
{
return static_branch_unlikely(&zones_with_unaccepted_pages);
}
static bool cond_accept_memory(struct zone *zone, unsigned int order)
{
long to_accept, wmark;
bool ret = false;
if (!has_unaccepted_memory())
return false;
if (list_empty(&zone->unaccepted_pages))
return false;
@ -7168,22 +7150,17 @@ static bool __free_unaccepted(struct page *page)
{
struct zone *zone = page_zone(page);
unsigned long flags;
bool first = false;
if (!lazy_accept)
return false;
spin_lock_irqsave(&zone->lock, flags);
first = list_empty(&zone->unaccepted_pages);
list_add_tail(&page->lru, &zone->unaccepted_pages);
account_freepages(zone, MAX_ORDER_NR_PAGES, MIGRATE_MOVABLE);
__mod_zone_page_state(zone, NR_UNACCEPTED, MAX_ORDER_NR_PAGES);
__SetPageUnaccepted(page);
spin_unlock_irqrestore(&zone->lock, flags);
if (first)
static_branch_inc(&zones_with_unaccepted_pages);
return true;
}