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Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Fri, 7 Jun 2019 06:35:38 +0100 Received: from d06av26.portsmouth.uk.ibm.com (d06av26.portsmouth.uk.ibm.com [9.149.105.62]) by b06avi18878370.portsmouth.uk.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id x575ZbLK29557098 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Fri, 7 Jun 2019 05:35:37 GMT Received: from d06av26.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 4D196AE04D; Fri, 7 Jun 2019 05:35:37 +0000 (GMT) Received: from d06av26.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id ABE6FAE045; Fri, 7 Jun 2019 05:35:36 +0000 (GMT) Received: from skywalker.linux.ibm.com (unknown [9.124.35.207]) by d06av26.portsmouth.uk.ibm.com (Postfix) with ESMTP; Fri, 7 Jun 2019 05:35:36 +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 1/2] powerpc/64s: Fix THP PMD collapse serialisation In-Reply-To: <20190607035636.5446-1-npiggin@gmail.com> References: <20190607035636.5446-1-npiggin@gmail.com> Date: Fri, 07 Jun 2019 11:05:35 +0530 MIME-Version: 1.0 Content-Type: text/plain X-TM-AS-GCONF: 00 x-cbid: 19060705-0008-0000-0000-000002F11C47 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 19060705-0009-0000-0000-0000225E0B1E Message-Id: <87zhmuez4o.fsf@linux.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:, , definitions=2019-06-07_02:, , 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-1906070039 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. > 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 > --- > > I accounted for Aneesh's and Christophe's feedback, except I couldn't > find a good way to replace the ifdef with IS_ENABLED because of > _PAGE_INVALID etc., but at least cleaned that up a bit nicer. > > Patch 1 solves a problem I can hit quite reliably running HPT/HPT KVM. > Patch 2 was noticed by Aneesh when inspecting code for similar bugs. > They should probably both be merged in stable kernels after upstream. > > arch/powerpc/include/asm/book3s/64/pgtable.h | 30 ++++++++++++++++++++ > arch/powerpc/mm/book3s64/pgtable.c | 3 ++ > 2 files changed, 33 insertions(+) > > diff --git a/arch/powerpc/include/asm/book3s/64/pgtable.h b/arch/powerpc/include/asm/book3s/64/pgtable.h > index 7dede2e34b70..ccf00a8b98c6 100644 > --- a/arch/powerpc/include/asm/book3s/64/pgtable.h > +++ b/arch/powerpc/include/asm/book3s/64/pgtable.h > @@ -876,6 +876,23 @@ static inline int pmd_present(pmd_t pmd) > return false; > } > > +static inline int pmd_is_serializing(pmd_t pmd) > +{ > + /* > + * If the pmd is undergoing a split, the _PAGE_PRESENT bit is clear > + * and _PAGE_INVALID is set (see pmd_present, pmdp_invalidate). > + * > + * This condition may also occur when flushing a pmd while flushing > + * it (see ptep_modify_prot_start), so callers must ensure this > + * case is fine as well. > + */ > + if ((pmd_raw(pmd) & cpu_to_be64(_PAGE_PRESENT | _PAGE_INVALID)) == > + cpu_to_be64(_PAGE_INVALID)) > + return true; > + > + return false; > +} > + > static inline int pmd_bad(pmd_t pmd) > { > if (radix_enabled()) > @@ -1092,6 +1109,19 @@ 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) > { > + /* > + * pmdp_invalidate sets this combination (which 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 also catches the case where the PTE's hardware PRESENT bit is > + * cleared while TLB is flushed, which is suboptimal but should not > + * be frequent. > + */ > + if (pmd_is_serializing(pmd)) > + return false; > + > return pte_access_permitted(pmd_pte(pmd), write); > } > > 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