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a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1738915878; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=nMhHaL1vJmjPujjFruKDGE7fkTh57n1/fCYhxYuaV8k=; b=PoM1hGjZ3ougXQXy8Jnl7lpOzjUR/e6I5s5I+Mz6AlOEx4/UsgnONgAgYUACe8qhMy9jRW 3oiKRGGZOiuIB37rlaz+uFOUCMWzV22firM3wR1zbN83Ip01L6iBQZ387qnF9WMlvdJKSf NA8ezpmQxsDzIx0wI1lmJokvKhfUH4c= Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id D90591063; Fri, 7 Feb 2025 00:11:39 -0800 (PST) Received: from [10.162.16.89] (a077893.blr.arm.com [10.162.16.89]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 913FC3F5A1; Fri, 7 Feb 2025 00:11:11 -0800 (PST) Message-ID: <45717fc1-cd83-4e9e-8ce5-44d306037cc9@arm.com> Date: Fri, 7 Feb 2025 13:41:08 +0530 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v1 09/16] arm64/mm: Avoid barriers for invalid or userspace mappings To: Ryan Roberts , Catalin Marinas , Will Deacon , Muchun Song , Pasha Tatashin , Andrew Morton , Uladzislau Rezki , Christoph Hellwig , Mark Rutland , Ard Biesheuvel , Dev Jain , Alexandre Ghiti , Steve Capper , Kevin Brodsky Cc: linux-arm-kernel@lists.infradead.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20250205151003.88959-1-ryan.roberts@arm.com> <20250205151003.88959-10-ryan.roberts@arm.com> Content-Language: en-US From: Anshuman Khandual In-Reply-To: <20250205151003.88959-10-ryan.roberts@arm.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: D2898180005 X-Stat-Signature: s7byajc71kycpq5d84gxgdt81hfranw9 X-Rspam-User: X-HE-Tag: 1738915877-702696 X-HE-Meta: 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 yk3fSB9P BvfXMOhDOvDbqPNXQEjJcHmw5jMfg1PEkXegxo14/klt4g/B6bXso7p5sguROVMLSzVUitjJ/rykE1RtJKiidz6w7tctC8HQV95uWUs0pBQhld570c+xzbo8QfPjtyX9ryboE9KQozDVBR5pgzfqe7vY6LMVLQl2hS0X2AysfJ1z1HxC9bb5cHQViDTdeNvX7+1t1GGgz9tqTSdY= 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: List-Subscribe: List-Unsubscribe: On 2/5/25 20:39, Ryan Roberts wrote: > __set_pte_complete(), set_pmd(), set_pud(), set_p4d() and set_pgd() are > used to write entries into pgtables. And they issue barriers (currently > dsb and isb) to ensure that the written values are observed by the table > walker prior to any program-order-future memory access to the mapped > location. Right. > > Over the years some of these functions have received optimizations: In > particular, commit 7f0b1bf04511 ("arm64: Fix barriers used for page > table modifications") made it so that the barriers were only emitted for > valid-kernel mappings for set_pte() (now __set_pte_complete()). And > commit 0795edaf3f1f ("arm64: pgtable: Implement p[mu]d_valid() and check > in set_p[mu]d()") made it so that set_pmd()/set_pud() only emitted the > barriers for valid mappings. set_p4d()/set_pgd() continue to emit the > barriers unconditionally. Right. > > This is all very confusing to the casual observer; surely the rules > should be invariant to the level? Let's change this so that every level > consistently emits the barriers only when setting valid, non-user > entries (both table and leaf). Agreed. > > It seems obvious that if it is ok to elide barriers all but valid kernel > mappings at pte level, it must also be ok to do this for leaf entries at > other levels: If setting an entry to invalid, a tlb maintenance > operaiton must surely follow to synchronise the TLB and this contains s/operaiton/operation > the required barriers. If setting a valid user mapping, the previous > mapping must have been invalid and there must have been a TLB > maintenance operation (complete with barriers) to honour > break-before-make. So the worst that can happen is we take an extra > fault (which will imply the DSB + ISB) and conclude that there is > nothing to do. These are the aguments for doing this optimization at pte s/aguments/arguments > level and they also apply to leaf mappings at other levels. So user the page table updates both for the table and leaf entries remains unchanged for now regarding dsb/isb sync i.e don't do anything ? > > For table entries, the same arguments hold: If unsetting a table entry, > a TLB is required and this will emit the required barriers. If setting a > table entry, the previous value must have been invalid and the table > walker must already be able to observe that. Additionally the contents > of the pgtable being pointed to in the newly set entry must be visible > before the entry is written and this is enforced via smp_wmb() (dmb) in > the pgtable allocation functions and in __split_huge_pmd_locked(). But > this last part could never have been enforced by the barriers in > set_pXd() because they occur after updating the entry. So ultimately, > the wost that can happen by eliding these barriers for user table > entries is an extra fault. Basically nothing needs to be done while setting user page table entries. > > I observe roughly the same number of page faults (107M) with and without > this change when compiling the kernel on Apple M2. These are total page faults or only additional faults caused because there were no dsb/isb sync after the user page table update ? > > Signed-off-by: Ryan Roberts > --- > arch/arm64/include/asm/pgtable.h | 60 ++++++++++++++++++++++++++++---- > 1 file changed, 54 insertions(+), 6 deletions(-) > > diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h > index 1d428e9c0e5a..ff358d983583 100644 > --- a/arch/arm64/include/asm/pgtable.h > +++ b/arch/arm64/include/asm/pgtable.h > @@ -767,6 +767,19 @@ static inline bool in_swapper_pgdir(void *addr) > ((unsigned long)swapper_pg_dir & PAGE_MASK); > } > > +static inline bool pmd_valid_not_user(pmd_t pmd) > +{ > + /* > + * User-space table pmd entries always have (PXN && !UXN). All other > + * combinations indicate it's a table entry for kernel space. > + * Valid-not-user leaf entries follow the same rules as > + * pte_valid_not_user(). > + */ > + if (pmd_table(pmd)) > + return !((pmd_val(pmd) & (PMD_TABLE_PXN | PMD_TABLE_UXN)) == PMD_TABLE_PXN); Should not this be abstracted out as pmd_table_not_user_table() which can then be re-used in other levels as well. > + return pte_valid_not_user(pmd_pte(pmd)); > +} > + Something like. static inline bool pmd_valid_not_user_table(pmd_t pmd) { return pmd_valid(pmd) && !((pmd_val(pmd) & (PMD_PMD_TABLE_PXN | PMD_TABLE_UXN)) == PMD_TABLE_PXN); } static inline bool pmd_valid_not_user(pmd_t pmd) { if (pmd_table(pmd)) return pmd_valid_not_user_table(pmd); else return pte_valid_not_user(pmd_pte(pmd)); } > static inline void set_pmd(pmd_t *pmdp, pmd_t pmd) > { > #ifdef __PAGETABLE_PMD_FOLDED > @@ -778,7 +791,7 @@ static inline void set_pmd(pmd_t *pmdp, pmd_t pmd) > > WRITE_ONCE(*pmdp, pmd); > > - if (pmd_valid(pmd)) { > + if (pmd_valid_not_user(pmd)) { > dsb(ishst); > isb(); > } > @@ -836,6 +849,17 @@ static inline unsigned long pmd_page_vaddr(pmd_t pmd) > > static inline bool pgtable_l4_enabled(void); > > + > +static inline bool pud_valid_not_user(pud_t pud) > +{ > + /* > + * Follows the same rules as pmd_valid_not_user(). > + */ > + if (pud_table(pud)) > + return !((pud_val(pud) & (PUD_TABLE_PXN | PUD_TABLE_UXN)) == PUD_TABLE_PXN); > + return pte_valid_not_user(pud_pte(pud)); > +} This can be expressed in terms of pmd_valid_not_user() itself. #define pud_valid_not_user() pmd_valid_not_user(pud_pmd(pud)) > + > static inline void set_pud(pud_t *pudp, pud_t pud) > { > if (!pgtable_l4_enabled() && in_swapper_pgdir(pudp)) { > @@ -845,7 +869,7 @@ static inline void set_pud(pud_t *pudp, pud_t pud) > > WRITE_ONCE(*pudp, pud); > > - if (pud_valid(pud)) { > + if (pud_valid_not_user(pud)) { > dsb(ishst); > isb(); > } > @@ -917,6 +941,16 @@ static inline bool mm_pud_folded(const struct mm_struct *mm) > #define p4d_bad(p4d) (pgtable_l4_enabled() && !(p4d_val(p4d) & P4D_TABLE_BIT)) > #define p4d_present(p4d) (!p4d_none(p4d)) > > +static inline bool p4d_valid_not_user(p4d_t p4d) > +{ > + /* > + * User-space table p4d entries always have (PXN && !UXN). All other > + * combinations indicate it's a table entry for kernel space. p4d block > + * entries are not supported. > + */ > + return !((p4d_val(p4d) & (P4D_TABLE_PXN | P4D_TABLE_UXN)) == P4D_TABLE_PXN); > +} Instead #define p4d_valid_not_user_able() pmd_valid_not_user_able(p4d_pmd(p4d)) > + > static inline void set_p4d(p4d_t *p4dp, p4d_t p4d) > { > if (in_swapper_pgdir(p4dp)) { > @@ -925,8 +959,11 @@ static inline void set_p4d(p4d_t *p4dp, p4d_t p4d) > } > > WRITE_ONCE(*p4dp, p4d); > - dsb(ishst); > - isb(); > + > + if (p4d_valid_not_user(p4d)) { Check p4d_valid_not_user_able() instead. > + dsb(ishst); > + isb(); > + } > } > > static inline void p4d_clear(p4d_t *p4dp) > @@ -1044,6 +1081,14 @@ static inline bool mm_p4d_folded(const struct mm_struct *mm) > #define pgd_bad(pgd) (pgtable_l5_enabled() && !(pgd_val(pgd) & PGD_TABLE_BIT)) > #define pgd_present(pgd) (!pgd_none(pgd)) > > +static inline bool pgd_valid_not_user(pgd_t pgd) > +{ > + /* > + * Follows the same rules as p4d_valid_not_user(). > + */ > + return !((pgd_val(pgd) & (PGD_TABLE_PXN | PGD_TABLE_UXN)) == PGD_TABLE_PXN); > +} Similarly #define pgd_valid_not_user_able() pmd_valid_not_user_able(pgd_pmd(pgd)) > + > static inline void set_pgd(pgd_t *pgdp, pgd_t pgd) > { > if (in_swapper_pgdir(pgdp)) { > @@ -1052,8 +1097,11 @@ static inline void set_pgd(pgd_t *pgdp, pgd_t pgd) > } > > WRITE_ONCE(*pgdp, pgd); > - dsb(ishst); > - isb(); > + > + if (pgd_valid_not_user(pgd)) { Check pgd_valid_not_user_able() instead. > + dsb(ishst); > + isb(); > + } > } > > static inline void pgd_clear(pgd_t *pgdp)