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Mon, 11 May 2026 00:35:23 -0700 (PDT) Message-ID: <4e3d0c1b-af33-470d-acb0-1e6540ba312a@gmail.com> Date: Mon, 11 May 2026 15:35:16 +0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2] mm/khugepaged: avoid underflow in madvise_collapse for sub-PMD MADV_COLLAPSE To: "David Hildenbrand (Arm)" , linux-mm@kvack.org, linux-kernel@vger.kernel.org Cc: akpm@linux-foundation.org, ljs@kernel.org, ziy@nvidia.com, baolin.wang@linux.alibaba.com, liam@infradead.org, npache@redhat.com, ryan.roberts@arm.com, dev.jain@arm.com, baohua@kernel.org, lance.yang@linux.dev References: <20260511065701.799006-1-chenwandun@lixiang.com> Content-Language: en-US From: Wandun In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 5/11/26 15:17, David Hildenbrand (Arm) wrote: > On 5/11/26 08:57, Wandun Chen wrote: >> From: Chen Wandun >> >> madvise_collapse() computes the THP-aligned window: >> >> hstart = ALIGN(start, HPAGE_PMD_SIZE); /* round up */ >> hend = ALIGN_DOWN(end, HPAGE_PMD_SIZE); /* round down */ >> >> The following case will cause hstart > hend, and result in underflow >> in the return statement, avoid it by returning -EINVAL early when >> hstart > hend. >> >> madvise(PMD-aligned + PAGE_SIZE, PAGE_SIZE, MADV_COLLAPSE); > Ok, so providing a PMD-aligned address as start will result in 0 and a > non-aligned address will result in -EINVAL. > > Didn't Lorenzo agree that just returning 0 in both cases would be clearer? But I > might have misunderstood it. Lorenzo suggested retuern -EINVAL for both case at the beginning, Later, Lorenzo add an correction, suggested should return 0 for compatibilty reasons for hstart == hend case. (If I haven't missed any information) Best regards, Wandun > > That would also make the code easier ... > >> In addition, kmalloc_obj(), mmgrab() and lru_add_drain_all() are >> unnecessary when hstart == hend, so skip these operations by >> returning early too. >> >> Signed-off-by: Chen Wandun >> >> --- >> v1 --> v2: >> - Rebase and resolve code conflict. >> - Return -EINVAL when hstart > hend, suggested by Lorenzo. >> - Drop Fixes tag, suggested by David and Lorenzo. >> - Updated commit message to be more explicit, suggested by Lorenzo. >> --- >> mm/khugepaged.c | 12 +++++++++--- >> 1 file changed, 9 insertions(+), 3 deletions(-) >> >> diff --git a/mm/khugepaged.c b/mm/khugepaged.c >> index 28a843f30b32..36baab17f098 100644 >> --- a/mm/khugepaged.c >> +++ b/mm/khugepaged.c >> @@ -2837,6 +2837,15 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start, >> if (!thp_vma_allowable_order(vma, vma->vm_flags, TVA_FORCED_COLLAPSE, PMD_ORDER)) >> return -EINVAL; >> >> + hstart = ALIGN(start, HPAGE_PMD_SIZE); >> + hend = ALIGN_DOWN(end, HPAGE_PMD_SIZE); >> + >> + if (hstart > hend) >> + return -EINVAL; >> + >> + if (hstart == hend) >> + return 0; >> + > ... as it would simply be: > > /* Nothing to collapse. */ > if (hstart >= hend) > return 0; > @Lorenzo, or do we want to keep the (questionable : ) ) existing behavior? >