linux-kernel - Re: [PATCH 2/2] radix-tree: Fix optimisation problem

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Date:   Sun, 25 Sep 2016 00:04:43 +0300
From:   "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Matthew Wilcox <mawilcox@...rosoft.com>,
        Matthew Wilcox <mawilcox@...uxonhyperv.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Konstantin Khlebnikov <koct9i@...il.com>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH 2/2] radix-tree: Fix optimisation problem

On Sat, Sep 24, 2016 at 01:21:36PM -0700, Linus Torvalds wrote:
> On Fri, Sep 23, 2016 at 1:16 PM, Matthew Wilcox <mawilcox@...rosoft.com> wrote:
> >
> >  #ifdef CONFIG_RADIX_TREE_MULTIORDER
> >         if (radix_tree_is_internal_node(entry)) {
> > -               unsigned long siboff = get_slot_offset(parent, entry);
> > +               unsigned long siboff = get_slot_offset(parent,
> > +                                               (void **)entry_to_node(entry));
> 
> I feel that it is *this* part that I think needs a huge honking comment.
> 
> If you are going to make get_slot_offset() different, then you could
> just rewrite get_slot_offset() to do
> 
>         unsigned long diff = (unsigned long) slot - (unsigned
> long)parent->slots;
>         return diff / sizeof(void *);
> 
> and add a comment to say "don't do this as a pointer diff, because
> 'slot' may not be an aligned pointer". No BUG_ON() necessary, because
> it "just works".
> 
> At that point, gcc should just generate the right code, because it
> doesn't see it as a pointer subtraction followed by a pointer
> addition.
> 
> And yes, that crazy " (void **)entry_to_node(entry)" fixes it *too*,
> but it needs a *comment*.
> 
> Why is that special, when all the other uses of get_slot_offset()
> don't have that? *That* is what should be explained. Not some internal
> detail.
> 
> That said, if this code isn't even used, as Konstantin says (THP
> selects it - doesn't THP use it?), then the fix really should be to
> just remove the odd code instead of adding to it.
> 
> Looking around for uses that set "order" to anything but zero, I
> really don't see it. So maybe we should just do *that* trivial thing
> instead, and remove CONFIG_RADIX_TREE_MULTIORDER, since it's appears
> to be buggy and always has been.

Well, my ext4-with-huge-pages patchset[1] uses multi-order entries.
It also converts shmem-with-huge-pages and hugetlb to them.

I'm okay with converting it to other mechanism, but I need something.
(I looked into Konstantin's RFC patchset[2]. It looks okay, but I don't
feel myself qualified to review it as I don't know much about radix-tree
internals.)

[1] http://lkml.kernel.org/r/20160915115523.29737-1-kirill.shutemov@linux.intel.com
[2] http://lkml.kernel.org/r/147230727479.9957.1087787722571077339.stgit@zurg

-- 
 Kirill A. Shutemov