linux-kernel - [PATCH v2 0/2] mm/damon/vaddr: Fix issue in damon_va_evenly_split

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Message-Id: <20241022083927.3592237-1-zhengyejian@huaweicloud.com>
Date: Tue, 22 Oct 2024 16:39:25 +0800
From: Zheng Yejian <zhengyejian@...weicloud.com>
To: sj@...nel.org
Cc: akpm@...ux-foundation.org,
	damon@...ts.linux.dev,
	foersleo@...zon.de,
	linux-kernel@...r.kernel.org,
	linux-mm@...ck.org,
	shakeel.butt@...ux.dev,
	sieberf@...zon.com,
	yeweihua4@...wei.com,
	zhengyejian@...weicloud.com
Subject: [PATCH v2 0/2] mm/damon/vaddr: Fix issue in damon_va_evenly_split_region()

According to the logic of damon_va_evenly_split_region(), currently
following split case would not meet the expectation:

  Suppose DAMON_MIN_REGION=0x1000,
  Case: Split [0x0, 0x3000) into 2 pieces, then the result would be
        acutually 3 regions:
          [0x0, 0x1000), [0x1000, 0x2000), [0x2000, 0x3000)
        but NOT the expected 2 regions:
          [0x0, 0x1000), [0x1000, 0x3000) !!!

The root cause is that when calculating size of each split piece in
damon_va_evenly_split_region():

  `sz_piece = ALIGN_DOWN(sz_orig / nr_pieces, DAMON_MIN_REGION);`

both the dividing and the ALIGN_DOWN may cause loss of precision,
then each time split one piece of size 'sz_piece' from origin 'start' to
'end' would cause more pieces are split out than expected!!!

To fix it, count for each piece split and make sure no more than
'nr_pieces'. In addition, add above case into damon_test_split_evenly().

And add 'nr_piece == 1' check in damon_va_evenly_split_region()
for better code readability and add a corresponding kunit testcase.

Zheng Yejian (2):
  mm/damon/vaddr: Fix issue in damon_va_evenly_split_region()
  mm/damon/vaddr: Check 'nr_piece == 1' case in
    damon_va_evenly_split_region()

 mm/damon/tests/vaddr-kunit.h | 2 ++
 mm/damon/vaddr.c             | 7 +++++--
 2 files changed, 7 insertions(+), 2 deletions(-)

-- 
2.25.1