linux-kernel - Re: [RFC PATCH 11/14] mm/slub: allocate slabs from virtual memory

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Date:   Fri, 15 Sep 2023 14:57:20 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Matteo Rizzo <matteorizzo@...gle.com>, cl@...ux.com,
        penberg@...nel.org, rientjes@...gle.com, iamjoonsoo.kim@....com,
        akpm@...ux-foundation.org, vbabka@...e.cz,
        roman.gushchin@...ux.dev, 42.hyeyoo@...il.com,
        keescook@...omium.org, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-mm@...ck.org,
        linux-hardening@...r.kernel.org, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
        x86@...nel.org, hpa@...or.com, corbet@....net, luto@...nel.org,
        peterz@...radead.org
Cc:     jannh@...gle.com, evn@...gle.com, poprdi@...gle.com,
        jordyzomer@...gle.com
Subject: Re: [RFC PATCH 11/14] mm/slub: allocate slabs from virtual memory

On 9/15/23 03:59, Matteo Rizzo wrote:
> +	spin_lock_irqsave(&slub_kworker_lock, irq_flags);
> +	list_splice_init(&slub_tlbflush_queue, &local_queue);
> +	list_for_each_entry(slab, &local_queue, flush_list_elem) {
> +		unsigned long start = (unsigned long)slab_to_virt(slab);
> +		unsigned long end = start + PAGE_SIZE *
> +			(1UL << oo_order(slab->oo));
> +
> +		if (start < addr_start)
> +			addr_start = start;
> +		if (end > addr_end)
> +			addr_end = end;
> +	}
> +	spin_unlock_irqrestore(&slub_kworker_lock, irq_flags);
> +
> +	if (addr_start < addr_end)
> +		flush_tlb_kernel_range(addr_start, addr_end);

I assume that the TLB flushes in the queue are going to be pretty sparse
on average.

At least on x86, flush_tlb_kernel_range() falls back pretty quickly from
individual address invalidation to just doing a full flush.  It might
not even be worth tracking the address ranges, and just do a full flush
every time.

I'd be really curious to see how often actual ranged flushes are
triggered from this code.  I expect it would be very near zero.