selftests/vm: retry on EAGAIN for MADV_COLLAPSE selftest

MADV_COLLAPSE is a best-effort request that will set errno to an
actionable value if the request cannot be performed.

For example, if pages are not found on the LRU, or if they are currently
locked by something else, MADV_COLLAPSE will fail and set errno to EAGAIN
to inform callers that they may try again.

Since the khugepaged selftest is the first public use of MADV_COLLAPSE,
set a best practice of checking errno and retrying on EAGAIN.

Link: https://lkml.kernel.org/r/20220922184651.1016461-2-zokeefe@google.com
Fixes: 9330694d ("selftests/vm: add MADV_COLLAPSE collapse context to selftests")
Signed-off-by: Zach O'Keefe <zokeefe@google.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Chris Kennelly <ckennelly@google.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: James Houghton <jthoughton@google.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Pasha Tatashin <pasha.tatashin@soleen.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Rongwei Wang <rongwei.wang@linux.alibaba.com>
Cc: SeongJae Park <sj@kernel.org>
Cc: Song Liu <songliubraving@fb.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Yang Shi <shy828301@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

tools/testing/selftests/vm/khugepaged.c

 #define _GNU_SOURCE
+#include <errno.h>
 #include <fcntl.h>
 #include <limits.h>
 #include <signal.h>
@@ -477,6 +478,26 @@ static void fill_memory(int *p, unsigned long start, unsigned long end)
 		p[i * page_size / sizeof(*p)] = i + 0xdead0000;
 }
 
+/*
+ * MADV_COLLAPSE is a best-effort request and may fail if an internal
+ * resource is temporarily unavailable, in which case it will set errno to
+ * EAGAIN.  In such a case, immediately reattempt the operation one more
+ * time.
+ */
+static int madvise_collapse_retry(void *p, unsigned long size)
+{
+	bool retry = true;
+	int ret;
+
+retry:
+	ret = madvise(p, size, MADV_COLLAPSE);
+	if (ret && errno == EAGAIN && retry) {
+		retry = false;
+		goto retry;
+	}
+	return ret;
+}
+
 /*
  * Returns pmd-mapped hugepage in VMA marked VM_HUGEPAGE, filled with
  * validate_memory()'able contents.
@@ -531,7 +552,7 @@ static void madvise_collapse(const char *msg, char *p, int nr_hpages,
 
 	/* Clear VM_NOHUGEPAGE */
 	madvise(p, nr_hpages * hpage_pmd_size, MADV_HUGEPAGE);
-	ret = madvise(p, nr_hpages * hpage_pmd_size, MADV_COLLAPSE);
+	ret = madvise_collapse_retry(p, nr_hpages * hpage_pmd_size);
 	if (((bool)ret) == expect)
 		fail("Fail: Bad return value");
 	else if (check_huge(p, nr_hpages) != expect)