linux-kernel - Re: [PATCH v4 16/36] mips: Implement the new page table range API

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Message-ID: <20230321113015.GA11292@alpha.franken.de>
Date:   Tue, 21 Mar 2023 12:30:15 +0100
From:   Thomas Bogendoerfer <tsbogend@...ha.franken.de>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     linux-arch@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-mips@...r.kernel.org
Subject: Re: [PATCH v4 16/36] mips: Implement the new page table range API

On Sun, Mar 19, 2023 at 08:16:36PM +0000, Matthew Wilcox wrote:
> On Sun, Mar 19, 2023 at 07:45:36PM +0100, Thomas Bogendoerfer wrote:
> > On Fri, Mar 17, 2023 at 04:29:20PM +0100, Thomas Bogendoerfer wrote:
> > > hmm, not sure if that would help. R4k style TLB has two PTEs mapped
> > > per TLB entry. So by advancing per page __update_tlb() is called more
> > > often than needed.
> > 
> > btw. how big is nr going to be ? There are MIPS SoCs out there, which
> > just have 16 TLBs...
> 
> Oof.  The biggest we're going to see for now is one less than PTRS_PER_PMD
> (that'd be a PMD-sized allocation that's mapped askew with 1 page in
> one PMD and n-1 pages in the adjacent PMD).  That'd be 511 on x86 and
> I presume something similar on MIPS.  More than 16, for sure.

biggest TLB I could find is 256 entries, which can map 512 pages.

> Now, this isn't a new problem with this patchset.  With fault-around,
> we already call set_pte_at() N times.  And we don't say which ones are
> speculative entries vs the one actually faulted in.

ic

> But let's see if we can fix it.  What if we passed in the vmf?  That would
> give you the actual faulting address, so you'd know to only put the PTE
> into the Linux page tables and not go as far as putting it into the TLB.
> Open to other ideas.

that would help to optimize the case. But update_mmu_cache_range needs to
do __update_tlb() for every page to avoid stale data in TLB. If I understood
correctly only the way how TLB updates are done changed, so there shouldn't
be performance regressions. And optimizing like moving the looping over
the pages into __update_tlb() could be done in a second step.

Thomas.

-- 
Crap can work. Given enough thrust pigs will fly, but it's not necessarily a
good idea.                                                [ RFC1925, 2.3 ]