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[142.68.57.212]) by smtp.gmail.com with ESMTPSA id k43sm4150456qtk.67.2020.04.24.07.10.22 (version=TLS1_2 cipher=ECDHE-ECDSA-CHACHA20-POLY1305 bits=256/256); Fri, 24 Apr 2020 07:10:22 -0700 (PDT) Received: from jgg by mlx.ziepe.ca with local (Exim 4.90_1) (envelope-from ) id 1jRz25-0008Fk-JP; Fri, 24 Apr 2020 11:10:21 -0300 Date: Fri, 24 Apr 2020 11:10:21 -0300 From: Jason Gunthorpe To: Li Xinhai Cc: Mike Kravetz , "linux-mm@kvack.org" , akpm , Punit Agrawal , Longpeng Subject: Re: [PATCH] mm/hugetlb: avoid unnecessary check on pud and pmd entry in huge_pte_offset Message-ID: <20200424141021.GG26002@ziepe.ca> References: <1587646154-26276-1-git-send-email-lixinhai.lxh@gmail.com> <20200423183835.GA13640@mellanox.com> <202004241207494655904@gmail.com> <20200424125753.GK13640@mellanox.com> <2020042421333861801820@gmail.com> <20200424134206.GM13640@mellanox.com> <2020042422071809487733@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <2020042422071809487733@gmail.com> User-Agent: Mutt/1.9.4 (2018-02-28) 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 Fri, Apr 24, 2020 at 10:07:19PM +0800, Li Xinhai wrote: > On 2020-04-24=C2=A0at 21:42=C2=A0Jason Gunthorpe=C2=A0wrote: > >On Fri, Apr 24, 2020 at 09:33:40PM +0800, Li Xinhai wrote: > >> > > >> >> , so when sz =3D=3D PMD_SIZE, pmd_offset() only called with a val= id PUD > >> >> entry which point to PMD page table. > >> > > >> >But what prevents pud_huge? > >> > > >> if sz =3D=3D PUD_SIZE, the 'return (pte_t*)pud' alrady end the funct= ion, which cover > >> pud_huge() and pud_none(), because we the mapping is for PUD_SIZE hu= ge page. > >> > >> So, there is no possibility for pmd_offset() been called with invali= d pud entry. > >> Below is the code I used for test which has BUG_ON, that should give= more > >> clear idea about the semantics of code path: > >> > >> ... > >> pud =3D pud_offset(p4d, addr); > >> if (sz =3D=3D PUD_SIZE) { > >> /* must be pud_huge or pud_none */ > >> BUG_ON(!pud_huge(*pud) && !pud_none(*pud)); > >> return (pte_t *)pud; // note that return valid pointer for pud_none(= ) case, > >> // instead of NULL, that is same semantics as existing code. > >> } > >> if (!pud_present(*pud)) > >> return NULL; // note that only return NULL in case pud not present, > >> // same sematics as existing code. > >> /* must have a valid entry and size to go further */ > >> BUG_ON(sz !=3D PMD_SIZE); > >> > >> pmd =3D pmd_offset(pud, addr); > >> /* must be pmd_huge or pmd_none */ > >> BUG_ON(!pmd_huge(*pmd) && !pmd_none(*pmd)); > > > >But why is !pmd_huge() ? The prior code returned null here, is that > >dead code? Your commit message should explain all of this.. > >=20 > let's see exising code for pmd part, the reason are in comments: > ... > =C2=A0 =C2=A0 =C2=A0 =C2=A0 pmd =3D pmd_offset(pud, addr); > =C2=A0 =C2=A0 =C2=A0 =C2=A0 if (sz !=3D PMD_SIZE && pmd_none(*pmd)) > =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 return NULL; //= dead code, must sz =3D=3D PMD_SIZE > =C2=A0 =C2=A0 =C2=A0 =C2=A0 /* hugepage or swap? */ > =C2=A0 =C2=A0 =C2=A0 =C2=A0 if (pmd_huge(*pmd) || !pmd_present(*pmd)) /= / !pmd_present() also cover pmd_none(), > =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 return (pte_t *= )pmd; // so, all possible and valid value in pmd entry will reach here. >=20 > return NULL; // dead code; can we have (!pmd_huge() && pmd_present()) = and reach here?=C2=A0 > // no, because this is a hugetlb mapping. otherwise, there is invalid= value in pmd entry. > ... well if you are relying on the caller to not call this in wrong cases it would make sense to have a if (WARN_ON(!pmd_huge(*pmd))) return NULL To document the assertion Jason