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Subject: Re: [PATCH 01/13] mm: Update ptep_get_lockless()s comment
From:   Nadav Amit <nadav.amit@gmail.com>
In-Reply-To: <Y1f7YvKuwOl1XEwU@hirez.programming.kicks-ass.net>
Date:   Wed, 26 Oct 2022 22:43:21 +0300
Cc:     Jann Horn <jannh@google.com>, John Hubbard <jhubbard@nvidia.com>,
        X86 ML <x86@kernel.org>, Matthew Wilcox <willy@infradead.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        kernel list <linux-kernel@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>,
        Andrea Arcangeli <aarcange@redhat.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        jroedel@suse.de, ubizjak@gmail.com
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To:     Peter Zijlstra <peterz@infradead.org>
Precedence: bulk

On Oct 25, 2022, at 6:06 PM, Peter Zijlstra <peterz@infradead.org> =
wrote:

> 			if (!force_flush && !tlb->fullmm && details &&
> +			    details->zap_flags & ZAP_FLAG_FORCE_FLUSH)
> +				force_flush =3D 1;

Isn=E2=80=99t 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.