linux-kernel - Re: [PATCH 01/13] mm: Update ptep_get

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Date:   Thu, 27 Oct 2022 09:27:43 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Nadav Amit <nadav.amit@...il.com>
Cc:     Jann Horn <jannh@...gle.com>, John Hubbard <jhubbard@...dia.com>,
        X86 ML <x86@...nel.org>, Matthew Wilcox <willy@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        jroedel@...e.de, ubizjak@...il.com
Subject: Re: [PATCH 01/13] mm: Update ptep_get_lockless()s comment

On Wed, Oct 26, 2022 at 10:43:21PM +0300, Nadav Amit wrote:
> On Oct 25, 2022, at 6:06 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> 
> > 			if (!force_flush && !tlb->fullmm && details &&
> > +			    details->zap_flags & ZAP_FLAG_FORCE_FLUSH)
> > +				force_flush = 1;
> 
> Isn’t it too big of a hammer?

It is the obvious hammer :-) TLB invalidate under pte_lock when
clearing.

> At the same time, the whole reasoning about TLB flushes is not getting any
> simpler. We had cases in which MADV_DONTNEED and another concurrent
> operation that effectively zapped PTEs (e.g., another MADV_DONTNEED) caused
> the zap_pte_range() to skip entries since pte_none() was true. To resolve
> these cases we relied on tlb_finish_mmu() to flush the range when needed
> (i.e., flush the whole range when mm_tlb_flush_nested()).

Yeah, whoever thought that allowing concurrency there was a great idea :/

And I must admit to hating the pending thing with a passion. And that
mm_tlb_flush_nested() thing in tlb_finish_mmu() is a giant hack at the
best of times.

Also; I feel it's part of the problem here; it violates the basic rules
we've had for a very long time.

> Now, I do not have a specific broken scenario in mind following this change,
> but it is all sounds to me a bit dangerous and at same time can potentially
> introduce new overheads.

I'll take correctness over being fast. As you say, this whole TLB thing
is getting out of hand.

> One alternative may be using mm_tlb_flush_pending() when setting a new PTE
> to check for pending flushes and flushing the TLB if that is the case. This
> is somewhat similar to what ptep_clear_flush() does. Anyhow, I guess this
> might induce some overheads. As noted before, it is possible to track
> pending TLB flushes in VMA/page-table granularity, with different tradeoffs
> of overheads.

Right; I just don't believe in VMAs for this, they're *waaay* to big.