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

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Message-Id: <A3677D1D-3631-4E57-8C5C-6C202B47700C@gmail.com>
Date:   Wed, 26 Oct 2022 22:43:21 +0300
From:   Nadav Amit <nadav.amit@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
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 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?

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()).

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.

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.