linux-ext4 - Re: [PATCH RFC 00/10] RDMA/FS DAX truncate proposal

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Date:   Thu, 6 Jun 2019 16:46:52 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Ira Weiny <ira.weiny@...el.com>
Cc:     John Hubbard <jhubbard@...dia.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Jan Kara <jack@...e.cz>, Theodore Ts'o <tytso@....edu>,
        Jeff Layton <jlayton@...nel.org>,
        Dave Chinner <david@...morbit.com>,
        Matthew Wilcox <willy@...radead.org>,
        linux-xfs@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jérôme Glisse <jglisse@...hat.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-nvdimm@...ts.01.org, linux-ext4@...r.kernel.org,
        linux-mm@...ck.org, linux-rdma@...r.kernel.org
Subject: Re: [PATCH RFC 00/10] RDMA/FS DAX truncate proposal

On Thu, Jun 06, 2019 at 10:11:58AM -0700, Ira Weiny wrote:

> 2) This is a bit more subtle and something I almost delayed sending these out
>    for.  Currently the implementation of a lease break actually removes the
>    lease from the file.  I did not want this to happen and I was thinking of
>    delaying this patch set to implement something which keeps the lease around
>    but I figured I should get something out for comments.  Jan has proposed
>    something along these lines and I agree with him so I'm going to ask you to
>    read my response to him about the details.
>
> 
>    Anyway so the key here is that currently an app needs the SIGIO to retake
>    the lease if they want to map the file again or in parts based on usage.
>    For example, they may only want to map some of the file for when they are
>    using it and then map another part later.  Without the SIGIO they would lose
>    their lease or would have to just take the lease for each GUP pin (which
>    adds overhead).  Like I said I did not like this but I left it to get
>    something which works out.

So to be clear.. 

Even though the lease is broken the GUP remains, the pages remain
pined, and truncate/etc continues to fail? 

I like Jan's take on this actually.. see other email.

Jason