linux-kernel - Re: [PATCH 0/4] fs: allow userland tasks to use delayed

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Message-ID: <20150914144837.GA4332@fieldses.org>
Date:	Mon, 14 Sep 2015 10:48:37 -0400
From:	"J. Bruce Fields" <bfields@...ldses.org>
To:	Jeff Layton <jlayton@...chiereds.net>
Cc:	Al Viro <viro@...IV.linux.org.uk>, linux-nfs@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/4] fs: allow userland tasks to use delayed_fput
 infrastructure

On Mon, Sep 14, 2015 at 09:45:51AM -0400, Jeff Layton wrote:
> I'm breaking this piece out of the open file cache work for nfsd to see
> if we can get this piece settled before I re-post the whole set. If this
> looks like a reasonable approach we can sort out how it should be merged
> (either by you directly, or via Bruce's tree with the rest of the open
> file cache patches).
> 
> For those just joining in, some background:
> 
> We want to add an open file cache for nfsd to reduce the open/close
> overhead on READ/WRITE RPCs, and so we can eliminate the raparm cache.
> The basic idea is to keep a cache of open files, and close them down on
> certain sorts of activity -- primarily, after an unlink that takes the
> link count to 0, or before setting a lease.
> 
> The setlease part is problematic though. The plan is to have a notifier
> callback into nfsd from vfs_setlease that will tell nfsd to close any
> open files that are associated with the inode so we don't block lease
> attempts solely due to cached but otherwise idle nfsd files. That means
> that we need to be able to close out the files and ensure that the final
> __fput runs before we try to set a lease.

I think I probably asked something similar before, but just to be sure I
understand....  Do leases really need to be 100% reliable, or can we get
away with saying "sorry, I don't feel like granting one right now".  An
entry in the filehandle cache suggests we're likely to recall the thing
soon anyway.  We use that option to get out of corner cases in the
delegation case, but I don't know if it makes sense for oplocks.

--b.

> 
> My latest pass involved making __fput_sync available to userland tasks,
> but Al had concerns that that could lead to stack blowouts. This
> patchset is an alternative approach that allows userland tasks to use
> the delayed_fput infrastructure instead. The idea is that we'd have the
> pre-setlease notifier do a fput_queue() and then call flush_delayed_fput
> to ensure that any queued __fput() calls complete before setting the
> lease.
> 
> There's also a fix for a potential race in flush_delayed_fput in here
> and some doc comment cleanups.
> 
> Al, does this look more acceptable than using __fput_sync?
> 
> Jeff Layton (4):
>   fs: have flush_delayed_fput flush the workqueue job
>   fs: add a kerneldoc header to fput
>   fs: add fput_queue
>   fs: export flush_delayed_fput
> 
>  fs/file_table.c      | 57 +++++++++++++++++++++++++++++++++++++++++-----------
>  include/linux/file.h |  1 +
>  2 files changed, 46 insertions(+), 12 deletions(-)
> 
> -- 
> 2.4.3
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