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From: Muchun Song In-Reply-To: <7f22c46c-2119-4de6-9d58-efcab05b5751@bytedance.com> Date: Thu, 5 Sep 2024 15:18:05 +0800 Cc: David Hildenbrand , Hugh Dickins , Matthew Wilcox , "Vlastimil Babka (SUSE)" , Andrew Morton , Mike Rapoport , Vishal Moola , Peter Xu , Ryan Roberts , christophe.leroy2@cs-soprasteria.com, LKML , Linux Memory Management List , linux-arm-kernel@lists.infradead.org, linuxppc-dev@lists.ozlabs.org Content-Transfer-Encoding: quoted-printable Message-Id: <69F12D50-BCA9-4874-B558-71008EF82674@linux.dev> References: <24be821f-a95f-47f1-879a-c392a79072cc@linux.dev> <05955456-8743-448A-B7A4-BC45FABEA628@linux.dev> <7f22c46c-2119-4de6-9d58-efcab05b5751@bytedance.com> To: Qi Zheng X-Migadu-Flow: FLOW_OUT X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: D5C04C000A X-Stat-Signature: f6dhmy31mrrgrc4rrah78akuec5kf1mu X-Rspam-User: X-HE-Tag: 1725520726-765496 X-HE-Meta: U2FsdGVkX1/cFuGb3t2s6wNyNWn6eoc6VfoQp51Lcie37gXQoVhS2Xu/k6M3ZfiICUzsuoBHWVKCeF8gizengC54yta/jf01V+ZmZLXAXBqvcObKpq9NXrAv+/abnPP4VPT1R9O2mwJBy/myQpQE7Ai5WMp1MECPF56iCIrq6Qt76xIU0brKJ99dhCVeSyl1q36Ian7sVP6aEWFVsQViYhhjc1+N5+vPeZkGYdP+DvAjX7B1feYUiE7ShxJOqj3JtufdEVajsqfujLZKz7pySg7Huc22vsPCsgK8ruxqpeFNNpWCVzVTE7oFdbfEWufAXLLqiRhmoQOKOQtl2JcwBuMO/y1SRbDabHoR1qVhXqmizf2GUJdEamNJKVOHEYlH+7U7gfO6slMqp/OlNyx4aLVPerY0wgueM2xTjkWxGa9jnX5NWdRyFvU0xGKyslXDIMkLpM9OGUYhJ+HP2cQGCg9xkIuJNNTXYJmaE+WtCM5TU4b9WtyMl6uJTMZQ2As0KhyrUwle0cKvod2epnCD4jbOhQOtQmpLS9EBFGQQZae6remdAvcqoH6BgQuJzmurAmYp7LKhmdXv4sr/hSU6kTVRUnVfvO/BeAOVXzw+we668R4xl/y5f4h0RY4kpH+6pzE4wXUSkRG0eBBJVyIwEVlvwqYdh2BHW2WcKQ5f/z7Lo/iQQysOGZ112N63+euQD7RUEQbAXE+ajuq1Nb0HYa1mVVToX/rmp2fXaZr8eV1fHcuevdOTq/48YImRqDMkAYcYzLmbGFAEcdqtqqVITM3tCNHbe2lKEJ5aNuJTrB8lOaB5dztizDGJ+xCp9W0fwv+J/WpwFi8rd8+kImFkgesZia3E0jZ1U1F1IdLbuZC0W+KmfjoksvYQpNOn8x904XpbXSyfdstfapuRufSkfk4dLDW5imm9RWvwUFB/TYuuJawAYQ08G3vzueQ1XngYKkWQhhry5S6IYsAYv7i SLahkbj4 jk+BmIoJxndZImsjtJLdkKbQ7GxZNZ+mWGXJop4eqfswY9+AKF6cjQLayFBLch6k59P6tWz0I9NDxNK5SHlBTNRp5/4XcnALCXZClt6+i4bFeCsYg6vUHnVdfwiZjTqkZtXME0R5Xj9Wl2z1xWnKigWgyDts/19mHqzBeflfWUMmC2zD/QRYOgc6xIKYSjTrHk6PcxkKAMEp2T59WHTFXGhtG9FxwMIOqOXN7OxOvPYct9EXPxvMwAgd9XorzrYHRcOa8QrfXOWi2+Cg6LwqnWonhhObiDYXa8VCUvGlRlWbuPjnaCPK1pBsuluYXiE34o6Ng7zxhsF+e6nM5LkwDHI3QZkWNr43JXkENPTiHXO0A8ww= 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 Sep 5, 2024, at 14:41, Qi Zheng wrote: >=20 >=20 >=20 > On 2024/9/5 14:32, Muchun Song wrote: >>> On Aug 30, 2024, at 14:54, Qi Zheng = wrote: >>>=20 >>>=20 >>>=20 >>> On 2024/8/29 16:10, Muchun Song wrote: >>>> On 2024/8/22 15:13, Qi Zheng wrote: >>>>> In collapse_pte_mapped_thp(), we may modify the pte and pmd entry = after >>>>> acquring the ptl, so convert it to using = pte_offset_map_rw_nolock(). At >>>>> this time, the write lock of mmap_lock is not held, and the = pte_same() >>>>> check is not performed after the PTL held. So we should get = pgt_pmd and do >>>>> pmd_same() check after the ptl held. >>>>>=20 >>>>> For the case where the ptl is released first and then the pml is = acquired, >>>>> the PTE page may have been freed, so we must do pmd_same() check = before >>>>> reacquiring the ptl. >>>>>=20 >>>>> Signed-off-by: Qi Zheng >>>>> --- >>>>> mm/khugepaged.c | 16 +++++++++++++++- >>>>> 1 file changed, 15 insertions(+), 1 deletion(-) >>>>>=20 >>>>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c >>>>> index 53bfa7f4b7f82..15d3f7f3c65f2 100644 >>>>> --- a/mm/khugepaged.c >>>>> +++ b/mm/khugepaged.c >>>>> @@ -1604,7 +1604,7 @@ int collapse_pte_mapped_thp(struct mm_struct = *mm, unsigned long addr, >>>>> if (userfaultfd_armed(vma) && !(vma->vm_flags & VM_SHARED)) >>>>> pml =3D pmd_lock(mm, pmd); >>>>> - start_pte =3D pte_offset_map_nolock(mm, pmd, haddr, &ptl); >>>>> + start_pte =3D pte_offset_map_rw_nolock(mm, pmd, haddr, = &pgt_pmd, &ptl); >>>>> if (!start_pte) /* mmap_lock + page lock should = prevent this */ >>>>> goto abort; >>>>> if (!pml) >>>>> @@ -1612,6 +1612,9 @@ int collapse_pte_mapped_thp(struct mm_struct = *mm, unsigned long addr, >>>>> else if (ptl !=3D pml) >>>>> spin_lock_nested(ptl, SINGLE_DEPTH_NESTING); >>>>> + if (unlikely(!pmd_same(pgt_pmd, pmdp_get_lockless(pmd)))) >>>>> + goto abort; >>>>> + >>>>> /* step 2: clear page table and adjust rmap */ >>>>> for (i =3D 0, addr =3D haddr, pte =3D start_pte; >>>>> i < HPAGE_PMD_NR; i++, addr +=3D PAGE_SIZE, pte++) { >>>>> @@ -1657,6 +1660,16 @@ int collapse_pte_mapped_thp(struct = mm_struct *mm, unsigned long addr, >>>>> /* step 4: remove empty page table */ >>>>> if (!pml) { >>>>> pml =3D pmd_lock(mm, pmd); >>>>> + /* >>>>> + * We called pte_unmap() and release the ptl before = acquiring >>>>> + * the pml, which means we left the RCU critical section, = so the >>>>> + * PTE page may have been freed, so we must do pmd_same() = check >>>>> + * before reacquiring the ptl. >>>>> + */ >>>>> + if (unlikely(!pmd_same(pgt_pmd, pmdp_get_lockless(pmd)))) = { >>>>> + spin_unlock(pml); >>>>> + goto pmd_change; >>>> Seems we forget to flush TLB since we've cleared some pte entry? >>>=20 >>> See comment above the ptep_clear(): >>>=20 >>> /* >>> * Must clear entry, or a racing truncate may re-remove it. >>> * TLB flush can be left until pmdp_collapse_flush() does it. >>> * PTE dirty? Shmem page is already dirty; file is read-only. >>> */ >>>=20 >>> The TLB flush was handed over to pmdp_collapse_flush(). If a >> But you skipped pmdp_collapse_flush(). >=20 > I skip it only in !pmd_same() case, at which time it must be cleared > by other thread, which will be responsible for flushing TLB: WOW! AMAZING! You are right. >=20 > CPU 0 CPU 1 > pmd_clear > spin_unlock > flushing tlb > spin_lock > if (!pmd_same)=20 > goto pmd_change; > pmdp_collapse_flush >=20 > Did I miss something? >=20 >>> concurrent thread free the PTE page at this time, the TLB will >>> also be flushed after pmd_clear(). >>>=20 >>>>> + } >>>>> if (ptl !=3D pml) >>>>> spin_lock_nested(ptl, SINGLE_DEPTH_NESTING); >>>>> } >>>>> @@ -1688,6 +1701,7 @@ int collapse_pte_mapped_thp(struct mm_struct = *mm, unsigned long addr, >>>>> pte_unmap_unlock(start_pte, ptl); >>>>> if (pml && pml !=3D ptl) >>>>> spin_unlock(pml); >>>>> +pmd_change: >>>>> if (notified) >>>>> mmu_notifier_invalidate_range_end(&range); >>>>> drop_folio: