linux-kernel - Re: [PATCH vfs 1/2] lib: implement ptrset

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Message-Id: <20141113144041.23bff773808562c699507621@linux-foundation.org>
Date:	Thu, 13 Nov 2014 14:40:41 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Tejun Heo <tj@...nel.org>
Cc:	Jens Axboe <axboe@...nel.dk>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Christoph Hellwig <hch@...radead.org>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH vfs 1/2] lib: implement ptrset

On Thu, 13 Nov 2014 17:27:36 -0500 Tejun Heo <tj@...nel.org> wrote:

> Hello, Andrew.
> 
> On Thu, Nov 13, 2014 at 02:23:33PM -0800, Andrew Morton wrote:
> > This seems rather slow and bloaty.  Why not
> > 
> > struct tjpointer {
> > 	struct list_head list;
> > 	void *pointer;
> > };
> > 
> > And then callers do things like
> > 
> > 	struct tjpointer *tjp;
> > 
> > 	lock();
> > 
> > 	for_each_tjpointer(tjp, &my_tjpointer_list) {
> > 		foo(tjp->ptr);
> > 	}
> > 
> > 	tjpointer_del(tjp);
> > 
> > 	unlock();
> > 
> > That's less storage, vastly less support code, insertion and removal
> > are O(1) and it doesn't need the ghastly preload thing.
> 
> The goal is moving the memory necessary for indexing to the indexer
> instead of the indexees.  In the above case, the indexee would have to
> either embed tjpointer inside it or at least have a pointer pointing
> at it.

In that case tjpointer_add() would need to do a kmalloc() for each inode
which is added to the bdev/cdev, just as ptrset_add() is doing.

That might require a nasty preload thing.  But really, for just two
known callers it would be better to require the caller to create the
storage.


	struct tjpointer *new_tpj;

	new_tpj = kmalloc(...);
	lock();
	tjpointer_add(&my_tjp_list, new_tjp, my_pointer);
	unlock();

Basically what I'm saying is nuke the rbtree and use lists.
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