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Received: from localhost ([::1] helo=merlin.infradead.org) by merlin.infradead.org with esmtp (Exim 4.92.3 #3 (Red Hat Linux)) id 1jsrda-0002Tu-B4; Tue, 07 Jul 2020 17:44:10 +0000 Received: from mail.kernel.org ([198.145.29.99]) by merlin.infradead.org with esmtps (Exim 4.92.3 #3 (Red Hat Linux)) id 1jsrdX-0002Sw-Tx for linux-arm-kernel@lists.infradead.org; Tue, 07 Jul 2020 17:44:08 +0000 Received: from gaia (unknown [95.146.230.158]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id D3D4A20675; Tue, 7 Jul 2020 17:44:05 +0000 (UTC) Date: Tue, 7 Jul 2020 18:44:03 +0100 From: Catalin Marinas To: Anshuman Khandual Subject: Re: [RFC V2 1/2] arm64/mm: Change THP helpers per generic memory semantics Message-ID: <20200707174403.GB32331@gaia> References: <1592226918-26378-1-git-send-email-anshuman.khandual@arm.com> <1592226918-26378-2-git-send-email-anshuman.khandual@arm.com> <20200702121135.GD22241@gaia> <48fd53ad-03a8-eb76-46a2-b65bd75a28d6@arm.com> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <48fd53ad-03a8-eb76-46a2-b65bd75a28d6@arm.com> User-Agent: Mutt/1.10.1 (2018-07-13) X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20200707_134408_088605_25DDEF9B X-CRM114-Status: GOOD ( 41.89 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: mark.rutland@arm.com, Suzuki Poulose , Marc Zyngier , linux-kernel@vger.kernel.org, linux-mm@kvack.org, ziy@nvidia.com, will@kernel.org, linux-arm-kernel@lists.infradead.org Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Mon, Jul 06, 2020 at 09:27:04AM +0530, Anshuman Khandual wrote: > On 07/02/2020 05:41 PM, Catalin Marinas wrote: > > On Mon, Jun 15, 2020 at 06:45:17PM +0530, Anshuman Khandual wrote: > >> --- a/arch/arm64/include/asm/pgtable.h > >> +++ b/arch/arm64/include/asm/pgtable.h > >> @@ -353,15 +353,92 @@ static inline int pmd_protnone(pmd_t pmd) > >> } > >> #endif > >> > >> +#define pmd_table(pmd) ((pmd_val(pmd) & PMD_TYPE_MASK) == PMD_TYPE_TABLE) > >> +#define pmd_sect(pmd) ((pmd_val(pmd) & PMD_TYPE_MASK) == PMD_TYPE_SECT) > >> + > >> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE > >> /* > >> - * THP definitions. > >> + * PMD Level Encoding (THP Enabled) > >> + * > >> + * 0b00 - Not valid Not present NA > >> + * 0b10 - Not valid Present Huge (Splitting) > >> + * 0b01 - Valid Present Huge (Mapped) > >> + * 0b11 - Valid Present Table (Mapped) > >> */ > > > > I wonder whether it would be easier to read if we add a dedicated > > PMD_SPLITTING bit, only when bit 0 is cleared. This bit can be high (say > > 59), it doesn't really matter as the entry is not valid. > > Could make (PMD[0b00] = 0b10) be represented as PMD_SPLITTING just for > better reading purpose. But if possible, IMHO it is efficient and less > vulnerable to use HW defined PTE attribute bit positions including SW > usable ones than the reserved bits, for a PMD state representation. > > Earlier proposal used PTE_SPECIAL (bit 56) instead. Using PMD_TABLE_BIT > helps save bit 56 for later. Thinking about it again, would not these > unused higher bits [59..63] create any problem ? For example while > enabling THP swapping without split via ARCH_WANTS_THP_SWAP or something > else later when these higher bits might be required. I am not sure, just > speculating. The swap encoding goes to bit 57, so going higher shouldn't be an issue. > But, do you see any particular problem with PMD_TABLE_BIT ? No. Only that we have some precedent like PTE_PROT_NONE (bit 58) and wondering whether we could use a high bit as well here. If we can get them to overlap, it simplifies this patch further. > > The only doubt I have is that pmd_mkinvalid() is used in other contexts > > when it's not necessarily splitting a pmd (search for the > > pmdp_invalidate() calls). So maybe a better name like PMD_PRESENT with a > > comment that pmd_to_page() is valid (i.e. no migration or swap entry). > > Feel free to suggest a better name. > > PMD_INVALID_PRESENT sounds better ? No strong opinion either way. Yours is clearer. > >> +static inline pmd_t pmd_mksplitting(pmd_t pmd) > >> +{ > >> + unsigned long val = pmd_val(pmd); > >> > >> -#ifdef CONFIG_TRANSPARENT_HUGEPAGE > >> -#define pmd_trans_huge(pmd) (pmd_val(pmd) && !(pmd_val(pmd) & PMD_TABLE_BIT)) > >> + return __pmd((val & ~PMD_TYPE_MASK) | PMD_TABLE_BIT); > >> +} > >> + > >> +static inline pmd_t pmd_clrsplitting(pmd_t pmd) > >> +{ > >> + unsigned long val = pmd_val(pmd); > >> + > >> + return __pmd((val & ~PMD_TYPE_MASK) | PMD_TYPE_SECT); > >> +} > >> + > >> +static inline bool pmd_splitting(pmd_t pmd) > >> +{ > >> + unsigned long val = pmd_val(pmd); > >> + > >> + if ((val & PMD_TYPE_MASK) == PMD_TABLE_BIT) > >> + return true; > >> + return false; > >> +} > >> + > >> +static inline bool pmd_mapped(pmd_t pmd) > >> +{ > >> + return pmd_sect(pmd); > >> +} > >> + > >> +static inline pmd_t pmd_mkinvalid(pmd_t pmd) > >> +{ > >> + /* > >> + * Invalidation should not have been invoked on > >> + * a PMD table entry. Just warn here otherwise. > >> + */ > >> + WARN_ON(pmd_table(pmd)); > >> + return pmd_mksplitting(pmd); > >> +} > > > > And here we wouldn't need t worry about table checks. > > This is just a temporary sanity check validating the assumption > that a table entry would never be called with pmdp_invalidate(). > This can be dropped later on if required. You could use a VM_WARN_ON. > >> +static inline int pmd_present(pmd_t pmd); > >> + > >> +static inline int pmd_trans_huge(pmd_t pmd) > >> +{ > >> + if (!pmd_present(pmd)) > >> + return 0; > >> + > >> + if (!pmd_val(pmd)) > >> + return 0; > >> + > >> + if (pmd_mapped(pmd)) > >> + return 1; > >> + > >> + if (pmd_splitting(pmd)) > >> + return 1; > >> + return 0; > > > > Doesn't your new pmd_present() already check for splitting? I think > > I actually meant pte_present() here instead, my bad. > > > checking for bit 0 and the new PMD_PRESENT. That would be similar to > > what we do with PTE_PROT_NONE. Actually, you could use the same bit for > > both. > > IIUC PROT NONE is supported at PMD level as well. Hence with valid bit > cleared, there is a chance for misinterpretation between pmd_protnone() > and pmd_splitting() if the same bit (PTE_PROT_NONE) is used. We can indeed have a PROT_NONE pmd but does it matter? All you need is that pmdp_invalidate() returns the original (present pmd) and writes a value that is still pmd_present() while invalid. You never modify the new value again AFAICT (only the old one to rebuild the pmd). It is indeed a problem if set_pmd_at() clears the new PMD_INVALID_PRESENT bit but my understanding is that it doesn't need to (see below). > >> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c > >> index 990929c8837e..337519031115 100644 > >> --- a/arch/arm64/mm/mmu.c > >> +++ b/arch/arm64/mm/mmu.c > >> @@ -22,6 +22,8 @@ > >> #include > >> #include > >> #include > >> +#include > >> +#include > >> > >> #include > >> #include > >> @@ -1483,3 +1485,21 @@ static int __init prevent_bootmem_remove_init(void) > >> } > >> device_initcall(prevent_bootmem_remove_init); > >> #endif > >> + > >> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE > >> +void set_pmd_at(struct mm_struct *mm, unsigned long addr, > >> + pmd_t *pmdp, pmd_t pmd) > >> +{ > >> + /* > >> + * PMD migration entries need to retain splitting PMD > >> + * representation created with pmdp_invalidate(). But > >> + * any non-migration entry which just might have been > >> + * invalidated previously, still need be a normal huge > >> + * page. Hence selectively clear splitting entries. > >> + */ > >> + if (!is_migration_entry(pmd_to_swp_entry(pmd))) > >> + pmd = pmd_clrsplitting(pmd); > >> + > >> + set_pte_at(mm, addr, (pte_t *)pmdp, pmd_pte(pmd)); > >> +} > >> +#endif > > > > So a pmdp_invalidate() returns the old pmd. Do we ever need to rebuild a > > pmd based on the actual bits in the new invalidated pmdp? Wondering how > > the table bit ends up here that we need to pmd_clrsplitting(). > > Yes, a pmd is always rebuilt via set_pmd_at() with the old value as > returned from an earlier pmdp_invalidate() but which may have been > changed with standard page table entry transformations. Basically, > it will not be created afresh from the pfn and VMA flags. My point is that pmdp_invalidate() is never called on an already invalid pmd. A valid pmd should never have the PMD_INVALID_PRESENT bit set. Therefore, set_pmd_at() does not need to clear any such bit as it wasn't in the old value returned by pmdp_invalidate(). > Any additional bit set in PMD via pmdp_invalidate() needs to be > cleared off in set_pmd_at(), unless it is a migration entry. I'm not convinced we need to unless we nest pmdp_invalidate() calls (have you seen any evidence of this?). -- Catalin _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel