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Mon, 19 Jul 2021 05:56:06 -0700 (PDT) Received: from [10.200.196.235] ([139.177.225.251]) by smtp.gmail.com with ESMTPSA id a18sm19970441pfi.6.2021.07.19.05.56.03 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 19 Jul 2021 05:56:06 -0700 (PDT) Subject: Re: [PATCH 5/7] mm: free user PTE page table pages To: =?UTF-8?Q?Mika_Penttil=c3=a4?= , akpm@linux-foundation.org, tglx@linutronix.de, hannes@cmpxchg.org, mhocko@kernel.org, vdavydov.dev@gmail.com Cc: linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, songmuchun@bytedance.com References: <20210718043034.76431-1-zhengqi.arch@bytedance.com> <20210718043034.76431-6-zhengqi.arch@bytedance.com> <9c3c87d5-e64e-f13f-ef36-b438e4de1e66@nextfour.com> From: Qi Zheng Message-ID: <80b7d7fc-9d6d-0d1b-a333-b0ccd856e7c1@bytedance.com> Date: Mon, 19 Jul 2021 20:56:01 +0800 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:78.0) Gecko/20100101 Thunderbird/78.12.0 MIME-Version: 1.0 In-Reply-To: <9c3c87d5-e64e-f13f-ef36-b438e4de1e66@nextfour.com> Content-Type: text/plain; charset=utf-8; format=flowed Authentication-Results: imf04.hostedemail.com; dkim=pass header.d=bytedance-com.20150623.gappssmtp.com header.s=20150623 header.b=Gie9aHoU; spf=pass (imf04.hostedemail.com: domain of zhengqi.arch@bytedance.com designates 209.85.216.52 as permitted sender) smtp.mailfrom=zhengqi.arch@bytedance.com; dmarc=pass (policy=none) header.from=bytedance.com X-Rspamd-Server: rspam03 X-Rspamd-Queue-Id: E8BA950000A2 X-Stat-Signature: s847qe1b7nbnjazdyf4zofzcn3b7ckx9 X-HE-Tag: 1626699367-630778 Content-Transfer-Encoding: quoted-printable 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: On 7/18/21 2:19 PM, Mika Penttil=C3=A4 wrote: >> + >> +/* >> + * returns true if the pmd has been populated with PTE page table, >> + * or false for all other cases. >> + */ >> +bool pte_install_try_get(struct mm_struct *mm, pmd_t *pmd, pgtable_t=20 >> *pte) >> +{ >> +=C2=A0=C2=A0=C2=A0 spinlock_t *ptl; >> +=C2=A0=C2=A0=C2=A0 bool retval =3D true; >> + >> +retry: >> +=C2=A0=C2=A0=C2=A0 ptl =3D pmd_lock(mm, pmd); >> +=C2=A0=C2=A0=C2=A0 if (likely(pmd_none(*pmd))) { >> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 __pte_install(mm, pmd, pte= ); >> +=C2=A0=C2=A0=C2=A0 } else if (pmd_leaf(*pmd) || !pmd_present(*pmd)) { >> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 retval =3D false; >> +=C2=A0=C2=A0=C2=A0 } else if (!pte_get_unless_zero(pmd)) { >> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 spin_unlock(ptl); >> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 goto retry; >> +=C2=A0=C2=A0=C2=A0 } >> +=C2=A0=C2=A0=C2=A0 spin_unlock(ptl); >> +=C2=A0=C2=A0=C2=A0 return retval; >> +} >> + >=20 > Can pte_get_unless_zero() return true above? Can the pmd have been by=20 > populated by others? In that case the ref count is wrongly incremented. >=20 Here we only have mmap_read_lock(mm), so the pmd can be populated with other PTE page table page after a page fault in a different thread B of=20 this mm. In this case, thread B already hold a pte_refcount of the PTE=20 page table page populated in the pmd, so pte_get_unless_zero() can return true above. Similarly, if THP is enabled, the pmd also can be populated with a THP=20 page, we can see more detail in comment in handle_pte_fault(). The pmd_leaf() above is to detect this situation.