linux-kernel - Re: [PATCH 2/2] mm: adds NOSIGBUS extension for out-of-band shmem read

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Message-Id: <1FD047D2-F5F3-4AC6-A4E4-DB8FB1568821@amacapital.net>
Date:   Thu, 3 Jun 2021 12:24:56 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Hugh Dickins <hughd@...gle.com>, Ming Lin <mlin@...nel.org>,
        Simon Ser <contact@...rsion.fr>, Linux-MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH 2/2] mm: adds NOSIGBUS extension for out-of-band shmem read



> On Jun 3, 2021, at 12:14 PM, Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> 
> On Thu, Jun 3, 2021 at 12:07 PM Hugh Dickins <hughd@...gle.com> wrote:
>> 
>> But the point that we've arrived at, that I'm actually now fairly
>> happy with, is do *not* permit MAP_NOSIGBUS on MAP_SHARED mappings.
> 
> Yeah, if that's sufficient, then that original patch should just work as-is.
> 
> But there was some reason why people didn't like that patch
> originally, and I think it was literally about how it only worked on
> private mappings (the "we don't have a flag for it in the vm_flags"
> part was just a small detail.
> 
> I guess that objection ended up changing over time.
> 
> 

I don’t understand the use case well enough to comment on whether MAP_PRIVATE is sufficient, but I’m with Hugh: if this feature is implemented for MAP_SHARED, it should be fully coherent.