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mm: kmemleak: move the struct page scan into a helper

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Patch series "mm: kmemleak: batch the struct page scan".

kmemleak walks the struct page array as a scan root and hands it to
scan_block() one page at a time.

scan_block() takes kmemleak_lock with interrupts disabled for the duration
of the call, so the scanner acquires the lock once per online PFN in order
to look at a single 64-byte struct page.

Batching adjacent pages up to that size keeps exactly the same pages under
the same maximum lock hold time, with MAX_SCAN_SIZE / pagesize fewer
acquisitions.

Patch 1 lifts the loop out of __kmemleak_scan() into scan_zone_pages()
with no functional change.  Patch 2 does the batching, which is then
contained in that one function.

This improves the performance due to less atomic operations, which is not
a big deal on a regular machine, but, given kmemleak usually comes with
extra debug options, such as PROVE_LOCKING, DEBUG_SPINLOCK, etc.

For instance, measuring Meta's "debug kernel flavor" on an arm64 hosts,
this improve the scan time by 20%.

It is safe to get more work into scan_block(), given it has the
protections, added by commit eb11f56eec ("mm/kmemleak: stop the task
stack scan early when interrupted")

MAX_SCAN_SIZE is also not a new maximum for a single scan_block() call. 
kmemleak_scan_task_stacks() already hands it a whole task stack in one go.
On arm64 and x86_64 THREAD_SIZE is never below 16 KiB, four times
MAX_SCAN_SIZE, and it is 64 KiB on arm64 with 64K pages.


This patch (of 2):

The struct page scanning loop sits inline in __kmemleak_scan(), nested
three levels deep and sharing the caller's "stop" variable with the
zone walk around it. Move it into scan_zone_pages(), which scans one
zone and returns 1 if scanning should stop.

No functional change; this only makes room for changing how the pages
are handed to scan_block().

Link: https://lore.kernel.org/20260921-b4-kmemleak-page-scan-v1-1-fb97d4801b3a@debian.org
Signed-off-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
This commit is contained in:
Breno Leitao 2026-09-21 05:30:26 -07:00 committed by Andrew Morton
parent 6fbcf40619
commit 1b4bbae111
1 changed files with 34 additions and 23 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

57
mm/kmemleak.c
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@ -1855,6 +1855,39 @@ static void dedup_flush(struct xarray *dedup)
}
}
/*
* Scan the struct pages of a zone, skipping memory holes, pages that belong to
* another zone and pages that are not in use. Returns 1 if the scan should be
* stopped.
*/
static int scan_zone_pages(struct zone *zone)
{
unsigned long start_pfn = zone->zone_start_pfn;
unsigned long end_pfn = zone_end_pfn(zone);
unsigned long pfn;
for (pfn = start_pfn; pfn < end_pfn; pfn++) {
struct page *page = pfn_to_online_page(pfn);
if (!(pfn & 63))
cond_resched_tasks_rcu_qs();
if (!page)
continue;
/* only scan pages belonging to this zone */
if (page_zone(page) != zone)
continue;
/* only scan if page is in use */
if (page_count(page) == 0)
continue;
if (scan_block(page, page + 1, NULL))
return 1;
}
return 0;
}
/*
* Scan data sections and all the referenced memory blocks allocated via the
* kernel's standard allocators. This function must be called with the
@ -1928,29 +1961,7 @@ static int __kmemleak_scan(bool full)
*/
get_online_mems();
for_each_populated_zone(zone) {
unsigned long start_pfn = zone->zone_start_pfn;
unsigned long end_pfn = zone_end_pfn(zone);
unsigned long pfn;
for (pfn = start_pfn; pfn < end_pfn; pfn++) {
struct page *page = pfn_to_online_page(pfn);
if (!(pfn & 63))
cond_resched_tasks_rcu_qs();
if (!page)
continue;
/* only scan pages belonging to this zone */
if (page_zone(page) != zone)
continue;
/* only scan if page is in use */
if (page_count(page) == 0)
continue;
stop = scan_block(page, page + 1, NULL);
if (stop)
break;
}
stop = scan_zone_pages(zone);
if (stop)
break;
}