[LTP] [PATCH v1] min_free_kbytes: Handle transient memory drops in check_monitor

[Wei Gao]

On Tue, May 26, 2026 at 04:10:54PM +0200, Petr Vorel wrote:
> Hi Wei,
> 
> > High memory pressure can cause MemFree to temporarily drop below the
> > min_free_kbytes threshold before the kernel reclaimer can catch up.
> > This results in intermittent test failures, particularly observed on
> > openQA aarch64 machines.
> 
> > Implement a 1-second grace period with exponential backoff polling
> > (from 1ms up to 512ms) in check_monitor() to allow the kernel time to
> > reclaim memory.
> 
> > Also the global 'end' flag is reset for multi-iteration
> > support (-i).
> IMHO this is not needed. Is it just wrong AI hint or you really spot it's
> needed?
Yes, reset 'end' flag is not needed, AI indeed complain wrong hint
firstly and later take this as defence programing.
> 
> > Signed-off-by: Wei Gao <wegao@suse.com>
> > ---
> >  .../kernel/mem/tunable/min_free_kbytes.c      | 36 +++++++++++++------
> >  1 file changed, 26 insertions(+), 10 deletions(-)
> 
> > diff --git a/testcases/kernel/mem/tunable/min_free_kbytes.c b/testcases/kernel/mem/tunable/min_free_kbytes.c
> > index a62e4ae9d..59ba6a9e1 100644
> > --- a/testcases/kernel/mem/tunable/min_free_kbytes.c
> > +++ b/testcases/kernel/mem/tunable/min_free_kbytes.c
> > @@ -1,6 +1,6 @@
> >  // SPDX-License-Identifier: GPL-2.0-or-later
> >  /*
> > - * Copyright (c) Linux Test Project, 2012-2025
> > + * Copyright (c) Linux Test Project, 2012-2026
> >   * Copyright (C) 2012-2017  Red Hat, Inc.
> >   */
> 
> > @@ -53,6 +53,8 @@ static void min_free_kbytes_test(void)
> >  	int pid, status;
> >  	struct sigaction sa;
> 
> > +	end = 0;
> Why I think it's not needed? Here you're in the parent. But the value is changed
> only in child (forked process), they run in a separate memory spaces (change in
> child is not visible for the parent).
> 
> In patch without it + updated commit message you may add:
> Reviewed-by: Petr Vorel <pvorel@suse.cz>
> 
> > +
> >  	sa.sa_handler = sighandler;
> >  	if (sigemptyset(&sa.sa_mask) < 0)
> >  		tst_brk(TBROK | TERRNO, "sigemptyset");
> > @@ -140,14 +142,13 @@ static void test_tune(unsigned long overcommit_policy)
> >  		} else {
> >  			if (WIFEXITED(status)) {
> >  				if (WEXITSTATUS(status) != 0) {
> > -					tst_res(TFAIL, "child unexpectedly "
> > -						 "failed: %d", status);
> > +					tst_res(TFAIL, "child unexpectedly failed: %d",
> > +						status);
> >  				}
> >  			} else if (!WIFSIGNALED(status) ||
> >  				   WTERMSIG(status) != SIGKILL) {
> > -				tst_res(TFAIL,
> > -					 "child unexpectedly failed: %d",
> > -					 status);
> > +				tst_res(TFAIL, "child unexpectedly failed: %d",
> > +					status);
> >  			}
> Unrelated cleanup, but thanks.
> 
> >  		}
> >  	}
> > @@ -183,18 +184,33 @@ static void check_monitor(void)
> >  {
> >  	unsigned long tune;
> >  	unsigned long memfree;
> > +	int i;
> 
> >  	while (!end) {
> >  		memfree = SAFE_READ_MEMINFO("MemFree:");
> >  		tune = TST_SYS_CONF_LONG_GET(MIN_FREE_KBYTES);
> 
> >  		if (memfree < tune) {
> > -			tst_res(TINFO, "MemFree is %lu kB, "
> > -				 "min_free_kbytes is %lu kB", memfree, tune);
> > -			tst_res(TFAIL, "MemFree < min_free_kbytes");
> > +			/*
> > +			 * Give it some time to reclaim. The kernel should keep
> > +			 * MemFree above min_free_kbytes, but transient drops
> > +			 * are possible under high pressure.
> > +			 */
> > +			for (i = 1; i < 1024; i *= 2) {
> > +				usleep(i * 1000);
> > +				memfree = SAFE_READ_MEMINFO("MemFree:");
> > +				if (memfree >= tune)
> > +					break;
> > +			}
> > +
> > +			if (memfree < tune) {
> > +				tst_res(TINFO, "MemFree is %lu kB, min_free_kbytes is %lu kB",
> > +					memfree, tune);
> > +				tst_res(TFAIL, "MemFree < min_free_kbytes");
> > +			}
> >  		}
> 
> > -		sleep(2);
> > +		usleep(100000);
> >  	}
> >  }