linux-kernel - Re: [RFC PATCH v2 14/14] dcache: Implement object migration

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Date:   Wed, 3 Apr 2019 18:19:21 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     "Tobin C. Harding" <tobin@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Roman Gushchin <guro@...com>,
        Alexander Viro <viro@....linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>,
        Pekka Enberg <penberg@...helsinki.fi>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Christopher Lameter <cl@...ux.com>,
        Matthew Wilcox <willy@...radead.org>,
        Miklos Szeredi <mszeredi@...hat.com>,
        Andreas Dilger <adilger@...ger.ca>,
        Waiman Long <longman@...hat.com>,
        Tycho Andersen <tycho@...ho.ws>, Theodore Ts'o <tytso@....edu>,
        Andi Kleen <ak@...ux.intel.com>,
        David Chinner <david@...morbit.com>,
        Nick Piggin <npiggin@...il.com>,
        Rik van Riel <riel@...hat.com>,
        Hugh Dickins <hughd@...gle.com>, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [RFC PATCH v2 14/14] dcache: Implement object migration

On Wed, Apr 03, 2019 at 06:08:11PM +0100, Al Viro wrote:

> Oh, *brilliant*
> 
> Let's do d_invalidate() on random dentries and hope they go away.
> With convoluted and brittle logics for deciding which ones to
> spare, which is actually wrong.  This will pick mountpoints
> and tear them out, to start with.
> 
> NAKed-by: Al Viro <viro@...iv.linux.org.uk>
> 
> And this is a NAK for the entire approach; if it has a positive refcount,
> LEAVE IT ALONE.  Period.  Don't play this kind of games, they are wrong.
> d_invalidate() is not something that can be done to an arbitrary dentry.

PS: "try to evict what can be evicted out of this set" can be done, but
you want something like
	start with empty list
	go through your array of references
		grab dentry->d_lock
		if dentry->d_lockref.count is not zero
			unlock and continue
		if dentry->d_flags & DCACHE_SHRINK_LIST
			ditto, it's not for us to play with
                if (dentry->d_flags & DCACHE_LRU_LIST)
                        d_lru_del(dentry);
		d_shrink_add(dentry, &list);
		unlock

on the collection phase and
	if the list is not empty by the end of that loop
		shrink_dentry_list(&list);
on the disposal.