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Thu, 25 May 2023 07:23:53 GMT Received: from smtpav02.fra02v.mail.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id B0BFA20043; Thu, 25 May 2023 07:23:53 +0000 (GMT) Received: from smtpav02.fra02v.mail.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 0955820040; Thu, 25 May 2023 07:23:53 +0000 (GMT) Received: from p-imbrenda (unknown [9.152.224.66]) by smtpav02.fra02v.mail.ibm.com (Postfix) with ESMTP; Thu, 25 May 2023 07:23:52 +0000 (GMT) Date: Thu, 25 May 2023 09:23:51 +0200 From: Claudio Imbrenda To: Hugh Dickins Cc: Andrew Morton , Mike Kravetz , Mike Rapoport , "Kirill A. Shutemov" , Matthew Wilcox , David Hildenbrand , Suren Baghdasaryan , Qi Zheng , Russell King , Catalin Marinas , Will Deacon , Geert Uytterhoeven , Greg Ungerer , Michal Simek , Thomas Bogendoerfer , Helge Deller , John David Anglin , "Aneesh Kumar K.V" , Michael Ellerman , Alexandre Ghiti , Palmer Dabbelt , Heiko Carstens , Christian Borntraeger , John Paul Adrian Glaubitz , "David S. Miller" , Chris Zankel , Max Filippov , x86@kernel.org, linux-arm-kernel@lists.infradead.org, linux-ia64@vger.kernel.org, linux-m68k@lists.linux-m68k.org, linux-mips@vger.kernel.org, linux-parisc@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-riscv@lists.infradead.org, linux-s390@vger.kernel.org, linux-sh@vger.kernel.org, sparclinux@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH 15/23] s390: allow pte_offset_map_lock() to fail Message-ID: <20230525092351.7671f2a4@p-imbrenda> In-Reply-To: References: <77a5d8c-406b-7068-4f17-23b7ac53bc83@google.com> <94aec8fe-383f-892-dcbf-d4c14e460a7@google.com> <20230517123546.672fb9b0@p-imbrenda> <4a15dbaa-1614-ce-ce1f-f73959cef895@google.com> <20230523140056.55b664b1@p-imbrenda> Organization: IBM X-Mailer: Claws Mail 4.1.1 (GTK 3.24.38; x86_64-redhat-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-TM-AS-GCONF: 00 X-Proofpoint-ORIG-GUID: qRLyp44GLiE9zXh3PageqjkrZmyh07xV X-Proofpoint-GUID: uXGxTdAMLt8o-nI3lVA1NmtMWFctokRx X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.254,Aquarius:18.0.957,Hydra:6.0.573,FMLib:17.11.176.26 definitions=2023-05-25_03,2023-05-24_01,2023-05-22_02 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 mlxscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 mlxlogscore=999 impostorscore=0 bulkscore=0 suspectscore=0 malwarescore=0 adultscore=0 phishscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2304280000 definitions=main-2305250057 X-Rspamd-Queue-Id: 34E5C140009 X-Rspam-User: X-Rspamd-Server: rspam05 X-Stat-Signature: dgfa1ptfdxpfh9mg3oqam5uty1i6scgw X-HE-Tag: 1684999492-193196 X-HE-Meta: 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 hM+a++et 7kxKH3l+T+yJT6OIyCIAFJZEq5P8AFFInOJc0w+dBH4Juw/faVR1nDd8VfcT33PilJvAKm5j47+y9y7VsmFi+q7pQ4ief9kxt0FUDnl2Im3xOKEMwjoUIWmOMHlTUqeTGQdof5adneDwri6HP+hyJp/xpJoJKKUySEtyObrTpT/izjwiFtdOLoTaxKtDh2TNqIC4bApkpJIYYaNnNOB4iuFIx1I0nXJwXbJwy9wgFhWfncVw= 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 Tue, 23 May 2023 18:49:14 -0700 (PDT) Hugh Dickins wrote: > On Tue, 23 May 2023, Claudio Imbrenda wrote: > > > > so if I understand the above correctly, pte_offset_map_lock will only > > fail if the whole page table has disappeared, and in that case, it will > > never reappear with zero pages, therefore we can safely skip (in that > > case just break). if we were to do a continue instead of a break, we > > would most likely fail again anyway. > > Yes, that's the most likely; and you hold mmap_write_lock() there, > and VM_NOHUGEPAGE on all vmas, so I think it's the only foreseeable > possibility. > > > > > in that case I would still like a small change in your patch: please > > write a short (2~3 lines max) comment about why it's ok to do things > > that way > > Sure. > > But I now see that I've disobeyed you, and gone to 4 lines (but in the > comment above the function, so as not to distract from the code itself): > is this good wording to you? I needed to research how they were stopped > from coming in afterwards, so wanted to put something greppable in there. > > And, unless I'm misunderstanding, that "after THP was enabled" was > always supposed to say "after THP was disabled" (because splitting a > huge zero page pmd inserts a a page table full of little zero ptes). indeed, thanks for noticing and fixing it > > Or would you prefer the comment in the commit message instead, > or down just above the pte_offset_map_lock() line? > > It would much better if I could find one place at the mm end, to > enforce its end of the contract; but cannot think how to do that. > > Hugh > > --- a/arch/s390/mm/gmap.c > +++ b/arch/s390/mm/gmap.c > @@ -2537,7 +2537,12 @@ static inline void thp_split_mm(struct mm_struct *mm) > * Remove all empty zero pages from the mapping for lazy refaulting > * - This must be called after mm->context.has_pgste is set, to avoid > * future creation of zero pages > - * - This must be called after THP was enabled > + * - This must be called after THP was disabled. > + * > + * mm contracts with s390, that even if mm were to remove a page table, > + * racing with the loop below and so causing pte_offset_map_lock() to fail, > + * it will never insert a page table containing empty zero pages once > + * mm_forbids_zeropage(mm) i.e. mm->context.has_pgste is set. > */ > static int __zap_zero_pages(pmd_t *pmd, unsigned long start, > unsigned long end, struct mm_walk *walk) looks good, thanks