selftest: mm: Test if hugepage does not get leaked during __bio_release_pages()

Commit 1b151e24 ("block: Remove special-casing of compound pages")
caused a change in behaviour when releasing the pages if the buffer does
not start at the beginning of the page.  This was because the calculation
of the number of pages to release was incorrect.  This was fixed by commit
38b43539 ("block: Fix page refcounts for unaligned buffers in
__bio_release_pages()").

We pin the user buffer during direct I/O writes.  If this buffer is a
hugepage, bio_release_page() will unpin it and decrement all references
and pin counts at ->bi_end_io.  However, if any references to the hugepage
remain post-I/O, the hugepage will not be freed upon unmap, leading to a
memory leak.

This patch verifies that a hugepage, used as a user buffer for DIO
operations, is correctly freed upon unmapping, regardless of whether the
offsets are aligned or unaligned w.r.t page boundary.

Test Result  Fail Scenario (Without the fix)
--------------------------------------------------------
[]# ./hugetlb_dio
TAP version 13
1..4
No. Free pages before allocation : 7
No. Free pages after munmap : 7
ok 1 : Huge pages freed successfully !
No. Free pages before allocation : 7
No. Free pages after munmap : 7
ok 2 : Huge pages freed successfully !
No. Free pages before allocation : 7
No. Free pages after munmap : 7
ok 3 : Huge pages freed successfully !
No. Free pages before allocation : 7
No. Free pages after munmap : 6
not ok 4 : Huge pages not freed!
Totals: pass:3 fail:1 xfail:0 xpass:0 skip:0 error:0

Test Result  PASS Scenario (With the fix)
---------------------------------------------------------
[]#./hugetlb_dio
TAP version 13
1..4
No. Free pages before allocation : 7
No. Free pages after munmap : 7
ok 1 : Huge pages freed successfully !
No. Free pages before allocation : 7
No. Free pages after munmap : 7
ok 2 : Huge pages freed successfully !
No. Free pages before allocation : 7
No. Free pages after munmap : 7
ok 3 : Huge pages freed successfully !
No. Free pages before allocation : 7
No. Free pages after munmap : 7
ok 4 : Huge pages freed successfully !
Totals: pass:4 fail:0 xfail:0 xpass:0 skip:0 error:0

[donettom@linux.ibm.com: address review comments from Muhammad]
  Link: https://lkml.kernel.org/r/20240604132801.23377-1-donettom@linux.ibm.com
[donettom@linux.ibm.com: add this test to run_vmtests.sh]
  Link: https://lkml.kernel.org/r/20240607182000.6494-1-donettom@linux.ibm.com
Link: https://lkml.kernel.org/r/20240523063905.3173-1-donettom@linux.ibm.com


Fixes: 38b43539 ("block: Fix page refcounts for unaligned buffers in __bio_release_pages()")
Signed-off-by: Donet Tom <donettom@linux.ibm.com>
Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Co-developed-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Reviewed-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Mike Rapoport (IBM) <rppt@kernel.org>
Cc: Muchun Song <songmuchun@bytedance.com>
Cc: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Tony Battersby <tonyb@cybernetics.com>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>