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Date: Sat, 7 Mar 2020 08:05:09 +0100 From: Christophe Leroy <christophe.leroy@....fr> To: Anshuman Khandual <anshuman.khandual@....com>, Qian Cai <cai@....pw> Cc: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>, Mike Rapoport <rppt@...ux.ibm.com>, Vineet Gupta <vgupta@...opsys.com>, Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>, Benjamin Herrenschmidt <benh@...nel.crashing.org>, Paul Mackerras <paulus@...ba.org>, Michael Ellerman <mpe@...erman.id.au>, Heiko Carstens <heiko.carstens@...ibm.com>, Vasily Gorbik <gor@...ux.ibm.com>, Christian Borntraeger <borntraeger@...ibm.com>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, "H. Peter Anvin" <hpa@...or.com>, "Kirill A . Shutemov" <kirill@...temov.name>, Paul Walmsley <paul.walmsley@...ive.com>, Palmer Dabbelt <palmer@...belt.com>, linux-snps-arc@...ts.infradead.org, linux-arm-kernel@...ts.infradead.org, linuxppc-dev@...ts.ozlabs.org, linux-s390@...r.kernel.org, linux-riscv@...ts.infradead.org, x86@...nel.org, linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org Subject: Re: [PATCH V15] mm/debug: Add tests validating architecture page table helpers Le 07/03/2020 à 01:56, Anshuman Khandual a écrit : > > > On 03/07/2020 06:04 AM, Qian Cai wrote: >> >> >>> On Mar 6, 2020, at 7:03 PM, Anshuman Khandual <Anshuman.Khandual@....com> wrote: >>> >>> Hmm, set_pte_at() function is not preferred here for these tests. The idea >>> is to avoid or atleast minimize TLB/cache flushes triggered from these sort >>> of 'static' tests. set_pte_at() is platform provided and could/might trigger >>> these flushes or some other platform specific synchronization stuff. Just >> >> Why is that important for this debugging option? > > Primarily reason is to avoid TLB/cache flush instructions on the system > during these tests that only involve transforming different page table > level entries through helpers. Unless really necessary, why should it > emit any TLB/cache flush instructions ? What's the problem with thoses flushes ? > >> >>> wondering is there specific reason with respect to the soft lock up problem >>> making it necessary to use set_pte_at() rather than a simple WRITE_ONCE() ? >> >> Looks at the s390 version of set_pte_at(), it has this comment, >> vmaddr); >> >> /* >> * Certain architectures need to do special things when PTEs >> * within a page table are directly modified. Thus, the following >> * hook is made available. >> */ >> >> I can only guess that powerpc could be the same here. > > This comment is present in multiple platforms while defining set_pte_at(). > Is not 'barrier()' here alone good enough ? Else what exactly set_pte_at() > does as compared to WRITE_ONCE() that avoids the soft lock up, just trying > to understand. > Argh ! I didn't realise that you were writing directly into the page tables. When it works, that's only by chance I guess. To properly set the page table entries, set_pte_at() has to be used: - On powerpc 8xx, with 16k pages, the page table entry must be copied four times. set_pte_at() does it, WRITE_ONCE() doesn't. - On powerpc book3s/32 (hash MMU), the flag _PAGE_HASHPTE must be preserved among writes. set_pte_at() preserves it, WRITE_ONCE() doesn't. set_pte_at() also does a few other mandatory things, like calling pte_mkpte() So, the WRITE_ONCE() must definitely become a set_pte_at() Christophe
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