linux-kernel - Re: [PATCH 4/4] tools/testing/selftests: add VMA merge tests for KSM merge

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Message-ID: <48746a0c-b2c3-4295-81ac-79bef6bf177a@lucifer.local>
Date: Wed, 21 May 2025 09:10:41 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Chengming Zhou <chengming.zhou@...ux.dev>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>,
        Pedro Falcato <pfalcato@...e.de>, David Hildenbrand <david@...hat.com>,
        Xu Xin <xu.xin16@....com.cn>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 4/4] tools/testing/selftests: add VMA merge tests for KSM
 merge

On Wed, May 21, 2025 at 04:07:29PM +0800, Chengming Zhou wrote:
> On 2025/5/19 16:51, Lorenzo Stoakes wrote:
> > Add test to assert that we have now allowed merging of VMAs when KSM
> > merging-by-default has been set by prctl(PR_SET_MEMORY_MERGE, ...).
> >
> > We simply perform a trivial mapping of adjacent VMAs expecting a merge,
> > however prior to recent changes implementing this mode earlier than before,
> > these merges would not have succeeded.
> >
> > Assert that we have fixed this!
> >
> > Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
>
> Reviewed-by: Chengming Zhou <chengming.zhou@...ux.dev>
> Tested-by: Chengming Zhou <chengming.zhou@...ux.dev>

Thanks very much! :)

I hope to get a v2 out today, so will carry forward your tags there.

Cheers, Loreno

>
> Thanks!
>
> > ---
> >   tools/testing/selftests/mm/merge.c | 78 ++++++++++++++++++++++++++++++
> >   1 file changed, 78 insertions(+)
> >
> > diff --git a/tools/testing/selftests/mm/merge.c b/tools/testing/selftests/mm/merge.c
> > index c76646cdf6e6..2380a5a6a529 100644
> > --- a/tools/testing/selftests/mm/merge.c
> > +++ b/tools/testing/selftests/mm/merge.c
> > @@ -2,10 +2,12 @@
> >   #define _GNU_SOURCE
> >   #include "../kselftest_harness.h"
> > +#include <linux/prctl.h>
> >   #include <stdio.h>
> >   #include <stdlib.h>
> >   #include <unistd.h>
> >   #include <sys/mman.h>
> > +#include <sys/prctl.h>
> >   #include <sys/wait.h>
> >   #include "vm_util.h"
> > @@ -31,6 +33,11 @@ FIXTURE_TEARDOWN(merge)
> >   {
> >   	ASSERT_EQ(munmap(self->carveout, 12 * self->page_size), 0);
> >   	ASSERT_EQ(close_procmap(&self->procmap), 0);
> > +	/*
> > +	 * Clear unconditionally, as some tests set this. It is no issue if this
> > +	 * fails (KSM may be disabled for instance).
> > +	 */
> > +	prctl(PR_SET_MEMORY_MERGE, 0, 0, 0, 0);
> >   }
> >   TEST_F(merge, mprotect_unfaulted_left)
> > @@ -452,4 +459,75 @@ TEST_F(merge, forked_source_vma)
> >   	ASSERT_EQ(procmap->query.vma_end, (unsigned long)ptr2 + 5 * page_size);
> >   }
> > +TEST_F(merge, ksm_merge)
> > +{
> > +	unsigned int page_size = self->page_size;
> > +	char *carveout = self->carveout;
> > +	struct procmap_fd *procmap = &self->procmap;
> > +	char *ptr, *ptr2;
> > +	int err;
> > +
> > +	/*
> > +	 * Map two R/W immediately adjacent to one another, they should
> > +	 * trivially merge:
> > +	 *
> > +	 * |-----------|-----------|
> > +	 * |    R/W    |    R/W    |
> > +	 * |-----------|-----------|
> > +	 *      ptr         ptr2
> > +	 */
> > +
> > +	ptr = mmap(&carveout[page_size], page_size, PROT_READ | PROT_WRITE,
> > +		   MAP_ANON | MAP_PRIVATE | MAP_FIXED, -1, 0);
> > +	ASSERT_NE(ptr, MAP_FAILED);
> > +	ptr2 = mmap(&carveout[2 * page_size], page_size,
> > +		    PROT_READ | PROT_WRITE,
> > +		    MAP_ANON | MAP_PRIVATE | MAP_FIXED, -1, 0);
> > +	ASSERT_NE(ptr2, MAP_FAILED);
> > +	ASSERT_TRUE(find_vma_procmap(procmap, ptr));
> > +	ASSERT_EQ(procmap->query.vma_start, (unsigned long)ptr);
> > +	ASSERT_EQ(procmap->query.vma_end, (unsigned long)ptr + 2 * page_size);
> > +
> > +	/* Unmap the second half of this merged VMA. */
> > +	ASSERT_EQ(munmap(ptr2, page_size), 0);
> > +
> > +	/* OK, now enable global KSM merge. We clear this on test teardown. */
> > +	err = prctl(PR_SET_MEMORY_MERGE, 1, 0, 0, 0);
> > +	if (err == -1) {
> > +		int errnum = errno;
> > +
> > +		/* Only non-failure case... */
> > +		ASSERT_EQ(errnum, EINVAL);
> > +		/* ...but indicates we should skip. */
> > +		SKIP(return, "KSM memory merging not supported, skipping.");
> > +	}
> > +
> > +	/*
> > +	 * Now map a VMA adjacent to the existing that was just made
> > +	 * VM_MERGEABLE, this should merge as well.
> > +	 */
> > +	ptr2 = mmap(&carveout[2 * page_size], page_size,
> > +		    PROT_READ | PROT_WRITE,
> > +		    MAP_ANON | MAP_PRIVATE | MAP_FIXED, -1, 0);
> > +	ASSERT_NE(ptr2, MAP_FAILED);
> > +	ASSERT_TRUE(find_vma_procmap(procmap, ptr));
> > +	ASSERT_EQ(procmap->query.vma_start, (unsigned long)ptr);
> > +	ASSERT_EQ(procmap->query.vma_end, (unsigned long)ptr + 2 * page_size);
> > +
> > +	/* Now this VMA altogether. */
> > +	ASSERT_EQ(munmap(ptr, 2 * page_size), 0);
> > +
> > +	/* Try the same operation as before, asserting this also merges fine. */
> > +	ptr = mmap(&carveout[page_size], page_size, PROT_READ | PROT_WRITE,
> > +		   MAP_ANON | MAP_PRIVATE | MAP_FIXED, -1, 0);
> > +	ASSERT_NE(ptr, MAP_FAILED);
> > +	ptr2 = mmap(&carveout[2 * page_size], page_size,
> > +		    PROT_READ | PROT_WRITE,
> > +		    MAP_ANON | MAP_PRIVATE | MAP_FIXED, -1, 0);
> > +	ASSERT_NE(ptr2, MAP_FAILED);
> > +	ASSERT_TRUE(find_vma_procmap(procmap, ptr));
> > +	ASSERT_EQ(procmap->query.vma_start, (unsigned long)ptr);
> > +	ASSERT_EQ(procmap->query.vma_end, (unsigned long)ptr + 2 * page_size);
> > +}
> > +
> >   TEST_HARNESS_MAIN