linux-kernel - Re: [PATCH 1/2] selftests: mincore: fix tmpfs mincore test failure

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Message-ID: <662ad650-8c68-40ef-a109-2e489658880d@redhat.com>
Date: Mon, 7 Apr 2025 09:49:27 +0200
From: David Hildenbrand <david@...hat.com>
To: Baolin Wang <baolin.wang@...ux.alibaba.com>, akpm@...ux-foundation.org,
 hughd@...gle.com
Cc: willy@...radead.org, 21cnbao@...il.com, ryan.roberts@....com,
 ziy@...dia.com, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] selftests: mincore: fix tmpfs mincore test failure

On 07.04.25 05:49, Baolin Wang wrote:
> 
> 
> On 2025/4/1 20:54, David Hildenbrand wrote:
>> On 26.03.25 04:38, Baolin Wang wrote:
>>> When running mincore test cases, I encountered the following failures:
>>>
>>> "
>>> mincore_selftest.c:359:check_tmpfs_mmap:Expected ra_pages (511) == 0 (0)
>>> mincore_selftest.c:360:check_tmpfs_mmap:Read-ahead pages found in memory
>>> check_tmpfs_mmap: Test terminated by assertion
>>>             FAIL  global.check_tmpfs_mmap
>>> not ok 5 global.check_tmpfs_mmap
>>> FAILED: 4 / 5 tests passed
>>> "
>>>
>>> The reason for the test case failure is that my system automatically
>>> enabled
>>> tmpfs large folio allocation by adding the
>>> 'transparent_hugepage_tmpfs=always'
>>> cmdline. However, the test case still expects the tmpfs mounted on
>>> /dev/shm to
>>> allocate small folios, which leads to assertion failures when
>>> verifying readahead
>>> pages.
>>>
>>> To fix this issue, remount tmpfs to a new test directory and set the
>>> 'huge=never'
>>> parameter to avoid allocating large folios, which can pass the test.
>>>
>>> Signed-off-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
>>> ---
>>>    .../selftests/mincore/mincore_selftest.c      | 25 +++++++++++++++++--
>>>    1 file changed, 23 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/tools/testing/selftests/mincore/mincore_selftest.c
>>> b/tools/testing/selftests/mincore/mincore_selftest.c
>>> index e949a43a6145..e8d7a3a4739f 100644
>>> --- a/tools/testing/selftests/mincore/mincore_selftest.c
>>> +++ b/tools/testing/selftests/mincore/mincore_selftest.c
>>> @@ -12,6 +12,7 @@
>>>    #include <unistd.h>
>>>    #include <stdlib.h>
>>>    #include <sys/mman.h>
>>> +#include <sys/mount.h>
>>>    #include <string.h>
>>>    #include <fcntl.h>
>>> @@ -283,7 +284,7 @@ TEST(check_file_mmap)
>>>        free(vec);
>>>    }
>>> -
>>> +#define INPUT_MAX 80
>>>    /*
>>>     * Test mincore() behavior on a page backed by a tmpfs file.  This test
>>>     * performs the same steps as the previous one. However, we don't
>>> expect
>>> @@ -291,6 +292,9 @@ TEST(check_file_mmap)
>>>     */
>>>    TEST(check_tmpfs_mmap)
>>>    {
>>> +    char tmpfs_template[] = "/tmp/check_tmpfs_XXXXXX";
>>> +    const char *tmpfs_loc = mkdtemp(tmpfs_template);
>>> +    char testfile[INPUT_MAX];
>>>        unsigned char *vec;
>>>        int vec_size;
>>>        char *addr;
>>> @@ -300,6 +304,10 @@ TEST(check_tmpfs_mmap)
>>>        int i;
>>>        int ra_pages = 0;
>>> +    ASSERT_NE(NULL, tmpfs_loc) {
>>> +        TH_LOG("Can't mkdir tmpfs dentry\n");
>>> +    }
>>> +
>>>        page_size = sysconf(_SC_PAGESIZE);
>>>        vec_size = FILE_SIZE / page_size;
>>>        if (FILE_SIZE % page_size)
>>> @@ -311,7 +319,18 @@ TEST(check_tmpfs_mmap)
>>>        }
>>>        errno = 0;
>>> -    fd = open("/dev/shm", O_TMPFILE | O_RDWR, 0600);
>>> +    /* Do not use large folios for tmpfs mincore testing */
>>> +    retval = mount("tmpfs", tmpfs_loc, "tmpfs", 0,
>>> "huge=never,size=4M");
>>> +    ASSERT_EQ(0, retval) {
>>> +        TH_LOG("Unable to mount tmpfs for testing\n");
>>> +    }
>>> +
>>> +    retval = snprintf(testfile, INPUT_MAX, "%s/test_file", tmpfs_loc);
>>> +    ASSERT_GE(INPUT_MAX, retval) {
>>> +        TH_LOG("Unable to create a tmpfs for testing\n");
>>> +    }
>>> +
>>> +    fd = open(testfile, O_CREAT|O_RDWR, 0664);
>>>        ASSERT_NE(-1, fd) {
>>>            TH_LOG("Can't create temporary file: %s",
>>>                strerror(errno));
>>> @@ -363,6 +382,8 @@ TEST(check_tmpfs_mmap)
>>>        munmap(addr, FILE_SIZE);
>>>        close(fd);
>>>        free(vec);
>>> +    umount(tmpfs_loc);
>>> +    rmdir(tmpfs_loc);
>>>    }
>>>    TEST_HARNESS_MAIN
>>
>> Is there anything that cleans up the mount in case something goes wrong
>> (we run into an assertion), or will the directory+mount stick around
>> forever?
> 
> Good point, will cleanup the mount in the next version.
> 
>>
>> But I also wonder whether check_tmpfs_mmap() should be changed to live
>> with the fact that readahead ("faultaround") could now happen. What's
>> the reason for not doing that?
> 
>   From this test case's description, it doesn't expect any readahead.

Yes, but why are we testing for that *at all*? We don't make such 
assumptions/tests for anon memmory ("no faultaround happened").

Why not simply remove the "We expect only that page to be fetched into 
memory." documentation + checking?

-- 
Cheers,

David / dhildenb