linux-kernel - Re: [PATCH] mm:fix duplicate accounting of free pages in should_reclaim

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Message-Id: <20250825070505.407367-1-liuqiqi@kylinos.cn>
Date: Mon, 25 Aug 2025 15:05:05 +0800
From: liuqiqi@...inos.cn
To: akpm@...ux-foundation.org
Cc: linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm:fix duplicate accounting of free pages in should_reclaim_retry()

Duplicate accounting of free pages in should_reclaim_retry() effects:
The number of retry in the __alloc_pages_slowpath() function has increased.
The execution time of the kswapd process has increased.

static inline struct page * 
__alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
                                                struct alloc_context *ac)
{
	......
retry:
	/*
	 * Deal with possible cpuset update races or zonelist updates to avoid
	 * infinite retries.
	 */
	......
        if (should_reclaim_retry(gfp_mask, order, ac, alloc_flags,
                                 did_some_progress > 0, &no_progress_loops))
                goto retry;

The test program: continuously allocates 1k-sized memory through kmalloc(); 
counts the number of retry and the execution time of the kswapd process;
the test results also confirm this.

> Thanks. Does this have any significant runtime effects?

> In the zone_reclaimable_pages() function, if the page counts for
> NR_ZONE_INACTIVE_FILE, NR_ZONE_ACTIVE_FILE, NR_ZONE_INACTIVE_ANON,
> and NR_ZONE_ACTIVE_ANON are all zero,
> the function returns the number of free pages as the result.
>
> In this case, when should_reclaim_retry() calculates reclaimable pages,
> it will inadvertently double-count the free pages in its accounting.
>
> static inline bool
> should_reclaim_retry(gfp_t gfp_mask, unsigned order,
>                      struct alloc_context *ac, int alloc_flags,
>                      bool did_some_progress, int *no_progress_loops)
> {
>         ...
>                 available = reclaimable = zone_reclaimable_pages(zone);
>                 available += zone_page_state_snapshot(zone, NR_FREE_PAGES);

https://lore.kernel.org/all/20250817120016.8dcc091c5b7114d6993a29ae@linux-foundation.org/

---
Best Regards