linux-kernel - Re: [PATCH 2/2] mm/selftests: Don't prefault in gup

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Date: Mon, 29 Apr 2024 09:28:15 +0200
From: David Hildenbrand <david@...hat.com>
To: Peter Xu <peterx@...hat.com>, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org
Cc: Christophe Leroy <christophe.leroy@...roup.eu>,
 Andrew Morton <akpm@...ux-foundation.org>,
 "Aneesh Kumar K . V" <aneesh.kumar@...nel.org>,
 Lorenzo Stoakes <lstoakes@...il.com>, John Hubbard <jhubbard@...dia.com>,
 linuxppc-dev@...ts.ozlabs.org, Muchun Song <muchun.song@...ux.dev>,
 Jason Gunthorpe <jgg@...dia.com>
Subject: Re: [PATCH 2/2] mm/selftests: Don't prefault in gup_longterm tests

On 28.04.24 21:01, Peter Xu wrote:
> Prefault, especially with RW, makes the GUP test too easy, and may not yet
> reach the core of the test.
> 
> For example, R/O longterm pins will just hit, pte_write()==true for
> whatever cases, the unsharing logic won't be ever tested.
> 
> This patch remove the prefault.  This tortures more code paths at least to
> cover the unshare care for R/O longterm pins, in which case the first R/O
> GUP attempt will fault in the page R/O first, then the 2nd will go through
> the unshare path, checking whether an unshare is needed.
> 
> Cc: David Hildenbrand <david@...hat.com>
> Signed-off-by: Peter Xu <peterx@...hat.com>
> ---
>   tools/testing/selftests/mm/gup_longterm.c | 12 +++++++++---
>   1 file changed, 9 insertions(+), 3 deletions(-)
> 
> diff --git a/tools/testing/selftests/mm/gup_longterm.c b/tools/testing/selftests/mm/gup_longterm.c
> index ad168d35b23b..488e32186246 100644
> --- a/tools/testing/selftests/mm/gup_longterm.c
> +++ b/tools/testing/selftests/mm/gup_longterm.c
> @@ -119,10 +119,16 @@ static void do_test(int fd, size_t size, enum test_type type, bool shared)
>   	}
>   
>   	/*
> -	 * Fault in the page writable such that GUP-fast can eventually pin
> -	 * it immediately.
> +	 * Explicitly avoid pre-faulting in the page, this can help testing
> +	 * more code paths.
> +	 *
> +	 * Take example of an upcoming R/O pin test, if we RW prefault the
> +	 * page, such pin will directly skip R/O unsharing and the longterm
> +	 * pin will success mostly always.  When not prefaulted, R/O
> +	 * longterm pin will first fault in a RO page, then the 2nd round
> +	 * it'll go via the unshare check.  Otherwise those paths aren't
> +	 * covered.
>   	 */
This will mean that GUP-fast never succeeds, which removes quite some testing
coverage for most other tests here.

Note that the main motivation of this test was to test gup_fast_folio_allowed(),
where we had issues with GUP-fast during development.

Would the following also get the job done?

diff --git a/tools/testing/selftests/mm/gup_longterm.c b/tools/testing/selftests/mm/gup_longterm.c
index ad168d35b23b7..e917a7c58d571 100644
--- a/tools/testing/selftests/mm/gup_longterm.c
+++ b/tools/testing/selftests/mm/gup_longterm.c
@@ -92,7 +92,7 @@ static void do_test(int fd, size_t size, enum test_type type, bool shared)
  {
  	__fsword_t fs_type = get_fs_type(fd);
  	bool should_work;
-	char *mem;
+	char tmp, *mem;
  	int ret;
  
  	if (ftruncate(fd, size)) {
@@ -119,10 +119,19 @@ static void do_test(int fd, size_t size, enum test_type type, bool shared)
  	}
  
  	/*
-	 * Fault in the page writable such that GUP-fast can eventually pin
-	 * it immediately.
+	 * Fault in the page such that GUP-fast might be able to pin it
+	 * immediately. To cover more cases, don't fault in pages writable when
+	 * R/O pinning.
  	 */
-	memset(mem, 0, size);
+	switch (type) {
+	case TEST_TYPE_RO:
+	case TEST_TYPE_RO_FAST:
+		tmp = *mem;
+		asm volatile("" : "+r" (tmp));
+		break;
+	default:
+		memset(mem, 0, size);
+	};
  
  	switch (type) {
  	case TEST_TYPE_RO:
-- 
2.44.0


-- 
Cheers,

David / dhildenb