linux-ext4 - Re: [RFC PATCH v2 00/19] RDMA/FS DAX truncate proposal V1,000,002 ;-)

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Date:   Wed, 28 Aug 2019 20:27:23 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Ira Weiny <ira.weiny@...el.com>, Dave Chinner <david@...morbit.com>
CC:     Jason Gunthorpe <jgg@...pe.ca>, Jan Kara <jack@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Matthew Wilcox <willy@...radead.org>,
        Theodore Ts'o <tytso@....edu>, Michal Hocko <mhocko@...e.com>,
        <linux-xfs@...r.kernel.org>, <linux-rdma@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <linux-fsdevel@...r.kernel.org>,
        <linux-nvdimm@...ts.01.org>, <linux-ext4@...r.kernel.org>,
        <linux-mm@...ck.org>
Subject: Re: [RFC PATCH v2 00/19] RDMA/FS DAX truncate proposal V1,000,002 ;-)

On 8/28/19 7:02 PM, Ira Weiny wrote:
> On Mon, Aug 26, 2019 at 03:55:10PM +1000, Dave Chinner wrote:
>> On Fri, Aug 23, 2019 at 10:08:36PM -0700, Ira Weiny wrote:
>>> On Sat, Aug 24, 2019 at 10:11:24AM +1000, Dave Chinner wrote:
>>>> On Fri, Aug 23, 2019 at 09:04:29AM -0300, Jason Gunthorpe wrote:
...
>>
>> Sure, that part works because the struct file is passed. It doesn't
>> end up with the same fd number in the other process, though.
>>
>> The issue is that layout leases need to notify userspace when they
>> are broken by the kernel, so a lease stores the owner pid/tid in the
>> file->f_owner field via __f_setown(). It also keeps a struct fasync
>> attached to the file_lock that records the fd that the lease was
>> created on.  When a signal needs to be sent to userspace for that
>> lease, we call kill_fasync() and that walks the list of fasync
>> structures on the lease and calls:
>>
>> 	send_sigio(fown, fa->fa_fd, band);
>>
>> And it does for every fasync struct attached to a lease. Yes, a
>> lease can track multiple fds, but it can only track them in a single
>> process context. The moment the struct file is shared with another
>> process, the lease is no longer capable of sending notifications to
>> all the lease holders.
>>
>> Yes, you can change the owning process via F_SETOWNER, but that's
>> still only a single process context, and you can't change the fd in
>> the fasync list. You can add new fd to an existing lease by calling
>> F_SETLEASE on the new fd, but you still only have a single process
>> owner context for signal delivery.
>>
>> As such, leases that require callbacks to userspace are currently
>> only valid within the process context the lease was taken in.
> 
> But for long term pins we are not requiring callbacks.
> 

Hi Ira,

If "require callbacks to userspace" means sending SIGIO, then actually
FOLL_LONGTERM *does* require those callbacks. Because we've been, so
far, equating FOLL_LONGTERM with the vaddr_pin struct and with a lease.

What am I missing here?

thanks,
-- 
John Hubbard
NVIDIA