linux-kernel - Re: [PATCH v3 0/2] iov_iter: allow iov_iter_get_pages

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Date:   Tue, 7 Feb 2017 07:19:09 +0000
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Miklos Szeredi <miklos@...redi.hu>
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Linux NFS list <linux-nfs@...r.kernel.org>,
        ceph-devel@...r.kernel.org, lustre-devel@...ts.lustre.org,
        v9fs-developer@...ts.sourceforge.net,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jan Kara <jack@...e.cz>,
        Chris Wilson <chris@...is-wilson.co.uk>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Jeff Layton <jlayton@...hat.com>
Subject: Re: [PATCH v3 0/2] iov_iter: allow iov_iter_get_pages_alloc to
 allocate more pages per call

On Mon, Feb 06, 2017 at 03:18:42PM +0100, Miklos Szeredi wrote:

> But I think it's breakable in the same way: if the deadlocked request
> is aborted, the fault will release the page lock as well as mmap_sem,
> and from there things will resolve themselves.

Right you are - original holder of ->mmap_sem is waiting for request and
abort will wake it up...

> But you are definitely right about needing to clean up that mess in
> fuse/dev.c and doing so by fixing up the arg refcounting for just the
> read and write requests is going to be a lot simpler than having to do
> that for all of them (which was my original plan).

Speaking of refcounting - what's going on with fuse_file one?  My reading
of that code is that you have 4 states of that thing:
	* new (just created, fallback request allocated, use fuse_file_free()
to kill).  Refcount is 0.
	* intermediate - in fact it's already opened, but still not
put into ->private_data.  Refcount is still 0.  Use fuse_sync_release() to kill.
	* live - normal refcounting (fuse_file_get()/fuse_file_put()).
	* shutdown - refcount has reached 0.  Can't happen until ->release()
(obviously - ->private_data holds a counting reference), some pieces of
fuse_sync_release() correspond to some stuff in fuse_release_common(), some -
to final fuse_file_put().

To make it even more convoluted, cuse is using fuse_sync_release() and
apparently relies upon no references being grabbed after fuse_do_open(),
so that thing can be called with refcount 0 *or* refcount 1.

Another thing: what guarantees that places in writepages-related paths
where we store a reference into req->ff won't hit a request with already
non-NULL ->ff?