From: Peter Zijlstra <peterz@infradead.org>
To: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: Laurent Dufour <ldufour@linux.vnet.ibm.com>,
paulmck@linux.vnet.ibm.com, akpm@linux-foundation.org,
ak@linux.intel.com, mhocko@kernel.org, dave@stgolabs.net,
jack@suse.cz, Matthew Wilcox <willy@infradead.org>,
benh@kernel.crashing.org, mpe@ellerman.id.au, paulus@samba.org,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>,
hpa@zytor.com, Will Deacon <will.deacon@arm.com>,
linux-kernel@vger.kernel.org, linux-mm@kvack.org,
haren@linux.vnet.ibm.com, khandual@linux.vnet.ibm.com,
npiggin@gmail.com, bsingharora@gmail.com,
Tim Chen <tim.c.chen@linux.intel.com>,
linuxppc-dev@lists.ozlabs.org, x86@kernel.org
Subject: Re: [PATCH v2 14/20] mm: Provide speculative fault infrastructure
Date: Mon, 28 Aug 2017 11:37:27 +0200 [thread overview]
Message-ID: <20170828093727.5wldedputadanssh@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20170827001823.n5wgkfq36z6snvf2@node.shutemov.name>
On Sun, Aug 27, 2017 at 03:18:23AM +0300, Kirill A. Shutemov wrote:
> On Fri, Aug 18, 2017 at 12:05:13AM +0200, Laurent Dufour wrote:
> > + /*
> > + * Can't call vm_ops service has we don't know what they would do
> > + * with the VMA.
> > + * This include huge page from hugetlbfs.
> > + */
> > + if (vma->vm_ops)
> > + goto unlock;
>
> I think we need to have a way to white-list safe ->vm_ops.
Either that, or simply teach all ->fault() callbacks about speculative
faults. Shouldn't be too hard, just 'work'.
> > +
> > + if (unlikely(!vma->anon_vma))
> > + goto unlock;
>
> It deserves a comment.
Yes, that was very much not intended. It wrecks most of the fun. This
really _should_ work for file maps too.
> > + /*
> > + * Do a speculative lookup of the PTE entry.
> > + */
> > + local_irq_disable();
> > + pgd = pgd_offset(mm, address);
> > + if (pgd_none(*pgd) || unlikely(pgd_bad(*pgd)))
> > + goto out_walk;
> > +
> > + p4d = p4d_alloc(mm, pgd, address);
> > + if (p4d_none(*p4d) || unlikely(p4d_bad(*p4d)))
> > + goto out_walk;
> > +
> > + pud = pud_alloc(mm, p4d, address);
> > + if (pud_none(*pud) || unlikely(pud_bad(*pud)))
> > + goto out_walk;
> > +
> > + pmd = pmd_offset(pud, address);
> > + if (pmd_none(*pmd) || unlikely(pmd_bad(*pmd)))
> > + goto out_walk;
> > +
> > + /*
> > + * The above does not allocate/instantiate page-tables because doing so
> > + * would lead to the possibility of instantiating page-tables after
> > + * free_pgtables() -- and consequently leaking them.
> > + *
> > + * The result is that we take at least one !speculative fault per PMD
> > + * in order to instantiate it.
> > + */
>
>
> Doing all this job and just give up because we cannot allocate page tables
> looks very wasteful to me.
>
> Have you considered to look how we can hand over from speculative to
> non-speculative path without starting from scratch (when possible)?
So we _can_ in fact allocate and install page-tables, but we have to be
very careful about it. The interesting case is where we race with
free_pgtables() and install a page that was just taken out.
But since we already have the VMA I think we can do something like:
if (p*g_none()) {
p*d_t *new = p*d_alloc_one(mm, address);
spin_lock(&mm->page_table_lock);
if (!vma_changed_or_dead(vma,seq)) {
if (p*d_none())
p*d_populate(mm, p*d, new);
else
p*d_free(new);
new = NULL;
}
spin_unlock(&mm->page_table_lock);
if (new) {
p*d_free(new);
goto out_walk;
}
}
I just never bothered with that, figured we ought to get the basics
working before trying to be clever.
> > + /* Transparent huge pages are not supported. */
> > + if (unlikely(pmd_trans_huge(*pmd)))
> > + goto out_walk;
>
> That's looks like a blocker to me.
>
> Is there any problem with making it supported (besides plain coding)?
Not that I can remember, but I never really looked at THP, I don't think
we even had that when I did the first versions.
next prev parent reply other threads:[~2017-08-28 9:37 UTC|newest]
Thread overview: 61+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-08-17 22:04 [PATCH v2 00/20] Speculative page faults Laurent Dufour
2017-08-17 22:05 ` [PATCH v2 01/20] mm: Dont assume page-table invariance during faults Laurent Dufour
2017-08-17 22:05 ` [PATCH v2 02/20] mm: Prepare for FAULT_FLAG_SPECULATIVE Laurent Dufour
2017-08-17 22:05 ` [PATCH v2 03/20] mm: Introduce pte_spinlock " Laurent Dufour
2017-08-17 22:05 ` [PATCH v2 04/20] mm: VMA sequence count Laurent Dufour
2017-08-17 22:05 ` [PATCH v2 05/20] mm: Protect VMA modifications using " Laurent Dufour
2017-08-17 22:05 ` [PATCH v2 06/20] mm: RCU free VMAs Laurent Dufour
2017-08-17 22:05 ` [PATCH v2 07/20] mm: Cache some VMA fields in the vm_fault structure Laurent Dufour
2017-08-17 22:05 ` [PATCH v2 08/20] mm: Protect SPF handler against anon_vma changes Laurent Dufour
2017-08-17 22:05 ` [PATCH v2 09/20] mm/migrate: Pass vm_fault pointer to migrate_misplaced_page() Laurent Dufour
2017-08-17 22:05 ` [PATCH v2 10/20] mm: Introduce __lru_cache_add_active_or_unevictable Laurent Dufour
2017-08-17 22:05 ` [PATCH v2 11/20] mm: Introduce __maybe_mkwrite() Laurent Dufour
2017-08-17 22:05 ` [PATCH v2 12/20] mm: Introduce __vm_normal_page() Laurent Dufour
2017-08-17 22:05 ` [PATCH v2 13/20] mm: Introduce __page_add_new_anon_rmap() Laurent Dufour
2017-08-17 22:05 ` [PATCH v2 14/20] mm: Provide speculative fault infrastructure Laurent Dufour
2017-08-20 12:11 ` Sergey Senozhatsky
2017-08-25 8:52 ` Laurent Dufour
2017-08-27 0:18 ` Kirill A. Shutemov
2017-08-28 9:37 ` Peter Zijlstra [this message]
2017-08-28 21:14 ` Benjamin Herrenschmidt
2017-08-28 22:35 ` Andi Kleen
2017-08-29 8:15 ` Peter Zijlstra
2017-08-29 8:33 ` Peter Zijlstra
2017-08-29 11:27 ` Peter Zijlstra
2017-08-29 21:19 ` Benjamin Herrenschmidt
2017-08-30 6:13 ` Peter Zijlstra
2017-08-29 7:59 ` Laurent Dufour
2017-08-29 12:04 ` Peter Zijlstra
2017-08-29 13:18 ` Laurent Dufour
2017-08-29 13:45 ` Peter Zijlstra
2017-08-30 5:03 ` Anshuman Khandual
2017-08-30 5:58 ` Peter Zijlstra
2017-08-30 9:32 ` Laurent Dufour
2017-08-31 6:55 ` Anshuman Khandual
2017-08-31 7:31 ` Peter Zijlstra
2017-08-30 9:53 ` Laurent Dufour
2017-08-30 3:48 ` Anshuman Khandual
2017-08-30 5:25 ` Anshuman Khandual
2017-08-30 8:56 ` Laurent Dufour
2017-08-17 22:05 ` [PATCH v2 15/20] mm: Try spin lock in speculative path Laurent Dufour
2017-08-17 22:05 ` [PATCH v2 16/20] mm: Adding speculative page fault failure trace events Laurent Dufour
2017-08-17 22:05 ` [PATCH v2 17/20] perf: Add a speculative page fault sw event Laurent Dufour
2017-08-21 8:55 ` Anshuman Khandual
2017-08-22 1:46 ` Michael Ellerman
2017-08-17 22:05 ` [PATCH v2 18/20] perf tools: Add support for the SPF perf event Laurent Dufour
2017-08-21 8:48 ` Anshuman Khandual
2017-08-25 8:53 ` Laurent Dufour
2017-08-17 22:05 ` [PATCH v2 19/20] x86/mm: Add speculative pagefault handling Laurent Dufour
2017-08-21 7:29 ` Anshuman Khandual
2017-08-29 14:50 ` Laurent Dufour
2017-08-29 14:58 ` Laurent Dufour
2017-08-17 22:05 ` [PATCH v2 20/20] powerpc/mm: Add speculative page fault Laurent Dufour
2017-08-21 6:58 ` Anshuman Khandual
2017-08-29 15:13 ` Laurent Dufour
2017-08-21 2:26 ` [PATCH v2 00/20] Speculative page faults Sergey Senozhatsky
2017-09-08 9:24 ` Laurent Dufour
2017-09-11 0:45 ` Sergey Senozhatsky
2017-09-11 6:28 ` Laurent Dufour
2017-08-21 6:28 ` Anshuman Khandual
2017-08-22 0:41 ` Paul E. McKenney
2017-08-25 9:41 ` Laurent Dufour
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