linux-kernel - Re: [LSF/MM TOPIC] Discuss least bad options for resolving longterm-GUP usage by RDMA

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CAPcyv4hdVwn0L8060_wxqDV0XBOwiUojSYjF+u+ugzLoQpcHzw@mail.gmail.com>
Date:   Mon, 11 Feb 2019 10:15:33 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     Jan Kara <jack@...e.cz>, Dave Chinner <david@...morbit.com>,
        Christopher Lameter <cl@...ux.com>,
        Doug Ledford <dledford@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Ira Weiny <ira.weiny@...el.com>,
        lsf-pc@...ts.linux-foundation.org,
        linux-rdma <linux-rdma@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        John Hubbard <jhubbard@...dia.com>,
        Jerome Glisse <jglisse@...hat.com>,
        Michal Hocko <mhocko@...nel.org>
Subject: Re: [LSF/MM TOPIC] Discuss least bad options for resolving
 longterm-GUP usage by RDMA

On Mon, Feb 11, 2019 at 10:07 AM Jason Gunthorpe <jgg@...pe.ca> wrote:
>
> On Mon, Feb 11, 2019 at 09:22:58AM -0800, Dan Williams wrote:
>
> > I honestly don't like the idea that random subsystems can pin down
> > file blocks as a side effect of gup on the result of mmap. Recall that
> > it's not just RDMA that wants this guarantee. It seems safer to have
> > the file be in an explicit block-allocation-immutable-mode so that the
> > fallocate man page can describe this error case. Otherwise how would
> > you describe the scenarios under which FALLOC_FL_PUNCH_HOLE fails?
>
> I rather liked CL's version of this - ftruncate/etc is simply racing
> with a parallel pwrite - and it doesn't fail.
>
> But it also doesnt' trucate/create a hole. Another thread wrote to it
> right away and the 'hole' was essentially instantly reallocated. This
> is an inherent, pre-existing, race in the ftrucate/etc APIs.

If options are telling the truth with a potentially unexpected error,
or lying that operation succeeded when it will be immediately undone,
I'd choose the former.