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From: Chengming Zhou In-Reply-To: <20251220202926.318366-1-sashal@kernel.org> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Migadu-Flow: FLOW_OUT X-Rspam-User: X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: 35FE3A000B X-Stat-Signature: m5bc79uwiauj68aydbyf1tmeosr567pa X-HE-Tag: 1766391815-33328 X-HE-Meta: U2FsdGVkX18yVQ27PgCVu0y+5hn52IipegQ7crqWafrPHzDI0QMiVKzaNEXVC/SpDwUSBMEN6j9R3UQS/tPQBOve4Q+S9JR3P2PXn7Rb7PTzk3x9SisvR9jAgXPQWiPxcYDrvqDwX8EIOzWnDcmlV2DCPYmkWqI+sacEmds8UP4WUkAEtH1BD/ZQl+Y3TtAMpTRkEfYsQqEMj1aevyF56UF4fHIYbC3GSqHQqtxOuYKC4xtH3vxZVKkJgHF3D0oTPfhO83Y/mlGD6AFrJPzDKp4xeJ5e14jlFS7jh8wZtVlGdUfFm+2qIF/F8f+0mSGnun0zkbiMSqEJPq5dlXrrhaQ18oeNDkkuxd4kURmMzZbKyCLDocyAj4Pov8EbeKLvk7Td2wJwivvKq76wep+Qaw0BnIDyAGp1WYzOEQvz+PfRWBuqr7zynJDJGJNNrQBAdMJZ7+1R3y16FiYcq3SAZvbnORioi6f8mVc3HRCZ5FAH7GbIHJ1/ztjJY27Y+0WiaNZx0utVb/g5RUeFbduBTF3SuJy75ezRy8Q3vKaXgnlR4AGmIIhWTCiB3bSfY8nTcyfk31mODNf/6umYN7MfGAVCed5Vd7ssg84X5G1sKYx2i/byT0B0d1Y9J43yZvC6n4gkNYsOqpc6uDTbcEWDxEG8yjB1rPh/86Qr+QlVjvavh7uaVxzzCF4I894D9DL7bO6OgNDvwob4qjWIApf8hgbSlC5n0UwFpRwaDrzDH2UBqfmC9JwAozvFBoWDVhQm3UihcYjLlCDz/VrOLSmMpO2vo9V6x/uMttfxlhjboUFh+8Ww8Q/CcG1Pepcq97BvQI0Blbs017vLykfdMBK28WCgMJQHYCAIXyenlqdHwMGNJNb8a0v0kxzgwooj5tXbYPupkGDMpRmkj7IPGZw1l/aLS/qVUNAJwzePpmfsTFFDn5/SlZZE9wyiq14N0X6kCDxSEVW9ATJptv0NF72 H6bE0l9p mtLz57OU/Dmsi/TjV5hl3LwmEk4ym4j9jvMjL4YE8FeLb0uwliy5Fzd36QiUgpELDkDSu5kXR7ajjk6aHYyCDIPxsdgelVm7JCCg4H0WgvJevg+c6PlyjbKBFBvWohyr08jYrSKiXQE+C/4R3Kk6TgVo2vO0kGZdsiXbU7CeHKfU+m1bIAtI/s41e0WKi2I6luod1UUe/3o2D++PKnyvyTWaz+0YiTcVktY27H3gXAE4yq1eUdM3/ViTsalGLYbaz4JLCzimwU5nwxBFuR31R0JQQRQ== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 2025/12/21 04:29, Sasha Levin wrote: > On ARM32 with HIGHMEM/HIGHPTE, break_ksm_pmd_entry() triggers a BUG > during KSM unmerging because pte_unmap_unlock() is passed a pointer > that may be beyond the mapped PTE page. > > The issue occurs when the PTE iteration loop completes without finding > a KSM page. After the loop, 'ptep' has been incremented past the last > PTE entry. On ARM32 LPAE with 512 PTEs per page (512 * 8 = 4096 bytes), > this means ptep points to the next page, outside the kmap'd region. > > When pte_unmap_unlock(ptep, ptl) calls kunmap_local(ptep), it unmaps > the wrong page address, leaving the original kmap slot still mapped. > The next kmap_local then finds this slot unexpectedly occupied: > > WARNING: mm/highmem.c:622 kunmap_local_indexed (address mismatch) > kernel BUG at mm/highmem.c:564 __kmap_local_pfn_prot (slot not empty) > > Fix this by passing start_ptep to pte_unmap_unlock(), which always > points within the originally mapped PTE page. > > Reproducer: Run LTP ksm03 test on ARM32 with HIGHMEM enabled. The test > triggers KSM merging followed by unmerging (writing 0 then 2 to > /sys/kernel/mm/ksm/run), which exercises break_ksm_pmd_entry(). > > Fixes: 5d4939fc2258 ("ksm: perform a range-walk in break_ksm") > Assisted-by: claude-opus-4-5-20251101 > Signed-off-by: Sasha Levin Reviewed-by: Chengming Zhou Thanks. > --- > mm/ksm.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/mm/ksm.c b/mm/ksm.c > index cfc182255c7b..2d89a7c8b4eb 100644 > --- a/mm/ksm.c > +++ b/mm/ksm.c > @@ -650,7 +650,7 @@ static int break_ksm_pmd_entry(pmd_t *pmdp, unsigned long addr, unsigned long en > } > } > out_unlock: > - pte_unmap_unlock(ptep, ptl); > + pte_unmap_unlock(start_ptep, ptl); > return found; > } >