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Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Thu, 6 Jun 2019 16:23:04 +0100 Received: from d06av21.portsmouth.uk.ibm.com (d06av21.portsmouth.uk.ibm.com [9.149.105.232]) by b06avi18878370.portsmouth.uk.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id x56FN3SR28508518 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Thu, 6 Jun 2019 15:23:03 GMT Received: from d06av21.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id A36A35204E; Thu, 6 Jun 2019 15:23:03 +0000 (GMT) Received: from skywalker.linux.ibm.com (unknown [9.85.84.230]) by d06av21.portsmouth.uk.ibm.com (Postfix) with ESMTP id E89575204F; Thu, 6 Jun 2019 15:23:02 +0000 (GMT) X-Mailer: emacs 26.2 (via feedmail 11-beta-1 I) From: "Aneesh Kumar K.V" To: Nicholas Piggin , linuxppc-dev@lists.ozlabs.org Subject: Re: [PATCH] powerpc/64s: Fix THP PMD collapse serialisation In-Reply-To: <20190603060531.13088-1-npiggin@gmail.com> References: <20190603060531.13088-1-npiggin@gmail.com> Date: Thu, 06 Jun 2019 20:53:01 +0530 MIME-Version: 1.0 Content-Type: text/plain X-TM-AS-GCONF: 00 x-cbid: 19060615-0028-0000-0000-00000377B067 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 19060615-0029-0000-0000-00002437911A Message-Id: <8736kmhh62.fsf@linux.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:, , definitions=2019-06-06_11:, , signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=2 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 mlxscore=0 impostorscore=0 mlxlogscore=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1810050000 definitions=main-1906060104 X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Nicholas Piggin Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" Nicholas Piggin writes: > Commit 1b2443a547f9 ("powerpc/book3s64: Avoid multiple endian conversion > in pte helpers") changed the actual bitwise tests in pte_access_permitted > by using pte_write() and pte_present() helpers rather than raw bitwise > testing _PAGE_WRITE and _PAGE_PRESENT bits. > > The pte_present change now returns true for ptes which are !_PAGE_PRESENT > and _PAGE_INVALID, which is the combination used by pmdp_invalidate to > synchronize access from lock-free lookups. pte_access_permitted is used by > pmd_access_permitted, so allowing GUP lock free access to proceed with > such PTEs breaks this synchronisation. > > This bug has been observed on HPT host, with random crashes and corruption > in guests, usually together with bad PMD messages in the host. > > Fix this by adding an explicit check in pmd_access_permitted, and > documenting the condition explicitly. > > The pte_write() change should be okay, and would prevent GUP from falling > back to the slow path when encountering savedwrite ptes, which matches > what x86 (that does not implement savedwrite) does. > I guess we are doing the find_linux_pte change in another patch. Reviewed-by: Aneesh Kumar K.V > Fixes: 1b2443a547f9 ("powerpc/book3s64: Avoid multiple endian conversion in pte helpers") > Cc: Aneesh Kumar K.V > Cc: Christophe Leroy > Signed-off-by: Nicholas Piggin > --- > arch/powerpc/include/asm/book3s/64/pgtable.h | 19 ++++++++++++++++++- > arch/powerpc/mm/book3s64/pgtable.c | 3 +++ > 2 files changed, 21 insertions(+), 1 deletion(-) > > diff --git a/arch/powerpc/include/asm/book3s/64/pgtable.h b/arch/powerpc/include/asm/book3s/64/pgtable.h > index 7dede2e34b70..aaa72aa1b765 100644 > --- a/arch/powerpc/include/asm/book3s/64/pgtable.h > +++ b/arch/powerpc/include/asm/book3s/64/pgtable.h > @@ -1092,7 +1092,24 @@ static inline int pmd_protnone(pmd_t pmd) > #define pmd_access_permitted pmd_access_permitted > static inline bool pmd_access_permitted(pmd_t pmd, bool write) > { > - return pte_access_permitted(pmd_pte(pmd), write); > + pte_t pte = pmd_pte(pmd); > + unsigned long pteval = pte_val(pte); > + > + /* > + * pmdp_invalidate sets this combination (that is not caught by > + * !pte_present() check in pte_access_permitted), to prevent > + * lock-free lookups, as part of the serialize_against_pte_lookup() > + * synchronisation. > + * > + * This check inadvertently catches the case where the PTE's hardware > + * PRESENT bit is cleared while TLB is flushed, to work around > + * hardware TLB issues. This is suboptimal, but should not be hit > + * frequently and should be harmless. > + */ > + if ((pteval & _PAGE_INVALID) && !(pteval & _PAGE_PRESENT)) > + return false; > + > + return pte_access_permitted(pte, write); > } > > #ifdef CONFIG_TRANSPARENT_HUGEPAGE > diff --git a/arch/powerpc/mm/book3s64/pgtable.c b/arch/powerpc/mm/book3s64/pgtable.c > index 16bda049187a..ff98b663c83e 100644 > --- a/arch/powerpc/mm/book3s64/pgtable.c > +++ b/arch/powerpc/mm/book3s64/pgtable.c > @@ -116,6 +116,9 @@ pmd_t pmdp_invalidate(struct vm_area_struct *vma, unsigned long address, > /* > * This ensures that generic code that rely on IRQ disabling > * to prevent a parallel THP split work as expected. > + * > + * Marking the entry with _PAGE_INVALID && ~_PAGE_PRESENT requires > + * a special case check in pmd_access_permitted. > */ > serialize_against_pte_lookup(vma->vm_mm); > return __pmd(old_pmd); > -- > 2.20.1