linux-kernel - Re: [PATCH] mm: Add new vma flag VM_LOCAL

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Date:   Mon, 14 May 2018 14:49:01 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Boaz Harrosh <boazh@...app.com>
Cc:     Jeff Moyer <jmoyer@...hat.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, <x86@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "Rik van Riel" <riel@...hat.com>, Jan Kara <jack@...e.cz>,
        Matthew Wilcox <mawilcox@...rosoft.com>,
        Amit Golander <Amit.Golander@...app.com>
Subject: Re: [PATCH] mm: Add new vma flag VM_LOCAL_CPU

On Mon, 14 May 2018 20:28:01 +0300 Boaz Harrosh <boazh@...app.com> wrote:

> On a call to mmap an mmap provider (like an FS) can put
> this flag on vma->vm_flags.
> 
> The VM_LOCAL_CPU flag tells the Kernel that the vma will be used
> from a single-core only, and therefore invalidation (flush_tlb) of
> PTE(s) need not be a wide CPU scheduling.
> 
> The motivation of this flag is the ZUFS project where we want
> to optimally map user-application buffers into a user-mode-server
> execute the operation and efficiently unmap.
> 
> In this project we utilize a per-core server thread so everything
> is kept local. If we use the regular zap_ptes() API All CPU's
> are scheduled for the unmap, though in our case we know that we
> have only used a single core. The regular zap_ptes adds a very big
> latency on every operation and mostly kills the concurrency of the
> over all system. Because it imposes a serialization between all cores

I'd have thought that in this situation, only the local CPU's bit is
set in the vma's mm_cpumask() and the remote invalidations are not
performed.  Is that a misunderstanding, or is all that stuff not working
correctly?