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[24.6.216.183]) by smtp.gmail.com with ESMTPSA id u19sm5481523pfk.201.2021.10.25.09.27.10 (version=TLS1_2 cipher=ECDHE-ECDSA-AES128-GCM-SHA256 bits=128/128); Mon, 25 Oct 2021 09:27:10 -0700 (PDT) Content-Type: text/plain; charset=utf-8 Mime-Version: 1.0 (Mac OS X Mail 14.0 \(3654.120.0.1.13\)) Subject: Re: [PATCH v2 5/5] mm/mprotect: do not flush on permission promotion From: Nadav Amit In-Reply-To: Date: Mon, 25 Oct 2021 09:27:08 -0700 Cc: Linux-MM , LKML , Andrea Arcangeli , Andrew Morton , Andy Lutomirski , Dave Hansen , Peter Xu , Thomas Gleixner , Will Deacon , Yu Zhao , Nick Piggin , x86@kernel.org Content-Transfer-Encoding: quoted-printable Message-Id: <94412B55-0E16-4BC1-982F-37C082A0BABD@gmail.com> References: <20211021122112.592634-1-namit@vmware.com> <20211021122112.592634-6-namit@vmware.com> To: Peter Zijlstra X-Mailer: Apple Mail (2.3654.120.0.1.13) X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: F1003D0000A6 X-Stat-Signature: azpubh98ity5xrnmpqhh19zrmepskd1q Authentication-Results: imf20.hostedemail.com; dkim=pass header.d=gmail.com header.s=20210112 header.b="fh+pd/l5"; spf=pass (imf20.hostedemail.com: domain of nadav.amit@gmail.com designates 209.85.215.180 as permitted sender) smtp.mailfrom=nadav.amit@gmail.com; dmarc=pass (policy=none) header.from=gmail.com X-HE-Tag: 1635179227-47096 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 Oct 25, 2021, at 4:12 AM, Peter Zijlstra = wrote: >=20 > On Thu, Oct 21, 2021 at 05:21:12AM -0700, Nadav Amit wrote: >> +/* >> + * pte_may_need_flush() checks whether permissions were demoted and = require a >> + * flush. It should only be used for userspace PTEs. >> + */ >> +static inline bool pte_may_need_flush(pte_t oldpte, pte_t newpte) >> +{ >> + /* new is non-present: need only if old is present */ >> + if (!pte_present(newpte)) >> + return pte_present(oldpte); >> + >> + /* old is not present: no need for flush */ >> + if (!pte_present(oldpte)) >> + return false; >=20 > Would it not be clearer to write the above like: >=20 > /* !PRESENT -> * ; no need for flush */ > if (!pte_present(oldpte)) > return false; >=20 > /* PRESENT -> !PRESENT ; needs flush */ > if (!pte_present(newpte)) > return true; >=20 > ? I will change the comment to yours. Thanks. >=20 >=20 >> diff --git a/mm/mprotect.c b/mm/mprotect.c >> index 0f5c87af5c60..6179c82ea72d 100644 >> --- a/mm/mprotect.c >> +++ b/mm/mprotect.c >> @@ -141,7 +141,8 @@ static unsigned long change_pte_range(struct = mmu_gather *tlb, >> ptent =3D pte_mkwrite(ptent); >> } >> ptep_modify_prot_commit(vma, addr, pte, oldpte, = ptent); >> - tlb_flush_pte_range(tlb, addr, PAGE_SIZE); >> + if (pte_may_need_flush(oldpte, ptent)) >> + tlb_flush_pte_range(tlb, addr, = PAGE_SIZE); >> pages++; >> } else if (is_swap_pte(oldpte)) { >> swp_entry_t entry =3D pte_to_swp_entry(oldpte); >=20 > One question on naming, "may_need" sounds a bit washy to me, either it > does or it does not. I suppose you're trying to convey the fact that = we > ought to err towards too many TLBi rather than too few, but that's > always true. >=20 > That is, would "needs" not be a better name? The =E2=80=9Cmay=E2=80=9D is indeed intended to be clear that the = function can error towards too many TLB flushes (of any kind). For instance, in a change from (!dirty|write)->(!write), no flush is needed in theory. I was too chicken to add it, at least for now. I can change the name and indicate in the comment instead though.=