linux-kernel - Re: [PATCH v2 2/2] vmalloc: Remove work as from vfree path

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Date:   Tue, 21 May 2019 09:17:32 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Rick Edgecombe <rick.p.edgecombe@...el.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        sparclinux <sparclinux@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        Network Development <netdev@...r.kernel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Nadav Amit <namit@...are.com>,
        "David S. Miller" <davem@...emloft.net>,
        Rick Edgecombe <redgecombe.lkml@...il.com>,
        Meelis Roos <mroos@...ux.ee>, Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH v2 2/2] vmalloc: Remove work as from vfree path

On Mon, May 20, 2019 at 4:39 PM Rick Edgecombe
<rick.p.edgecombe@...el.com> wrote:
>
> From: Rick Edgecombe <redgecombe.lkml@...il.com>
>
> Calling vm_unmap_alias() in vm_remove_mappings() could potentially be a
> lot of work to do on a free operation. Simply flushing the TLB instead of
> the whole vm_unmap_alias() operation makes the frees faster and pushes
> the heavy work to happen on allocation where it would be more expected.
> In addition to the extra work, vm_unmap_alias() takes some locks including
> a long hold of vmap_purge_lock, which will make all other
> VM_FLUSH_RESET_PERMS vfrees wait while the purge operation happens.
>
> Lastly, page_address() can involve locking and lookups on some
> configurations, so skip calling this by exiting out early when
> !CONFIG_ARCH_HAS_SET_DIRECT_MAP.

Hmm.  I would have expected that the major cost of vm_unmap_aliases()
would be the flush, and at least informing the code that the flush
happened seems valuable.  So would guess that this patch is actually a
loss in throughput.

--Andy