linux-kernel - Re: [PATCH 1/4] mm: Trial do_wp

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Date:   Wed, 3 Feb 2021 14:43:58 +0200
From:   Gal Pressman <galpress@...zon.com>
To:     <jgg@...pe.ca>
CC:     <aarcange@...hat.com>, <akpm@...ux-foundation.org>,
        <gokhale2@...l.gov>, <hch@....de>, <jack@...e.cz>,
        <jannh@...gle.com>, <jhubbard@...dia.com>, <kirill@...temov.name>,
        <ktkhai@...tuozzo.com>, <leonro@...dia.com>,
        <linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>,
        <mcfadden8@...l.gov>, <oleg@...hat.com>, <peterx@...hat.com>,
        <torvalds@...ux-foundation.org>, <wzam@...zon.com>,
        <yang.shi@...ux.alibaba.com>
Subject: Re: [PATCH 1/4] mm: Trial do_wp_page() simplification

> On Tue, Feb 02, 2021 at 12:05:36PM -0500, Peter Xu wrote:
> 
>> > Gal, you could also MADV_DONTFORK this range if you are explicitly
>> > allocating them via special mmap.
>> 
>> Yeah I wanted to mention this one too but I just forgot when reply: the issue
>> thread previously pasted smells like some people would like to drop
>> MADV_DONTFORK, but if it's able to still be applied I don't know why
>> not..
> 
> I want to drop the MADV_DONTFORK for dynamic data memory allocated by
> the application layers (eg with malloc) without knowledge of how they
> will be used.
> 
> This case is a buffer internal to the communication system that we
> know at allocation time how it will be used; so an explicit,
> deliberate, MADV_DONTFORK is fine

We are referring to libfabric's bounce buffers, correct?
Libfabric could be considered as the "app" here, it's not clear why these
buffers should be DONTFORK'd before ibv_reg_mr() but others don't.

Anyway, it should be simple enough to madvise them after allocation, although I
think it's part of libfabric's generic code (which isn't necessarily used on
top of rdma-core).

I'll take this discussion with the libfabric guys.
Thanks Peter and Jason.