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From:   Pengfei Li <lpf.vector@gmail.com>
Date:   Fri, 12 Jul 2019 23:09:00 +0800
Message-ID: <CAD7_sbEoGRUOJdcHnfUTzP7GfUhCdhfo8uBpUFZ9HGwS36VkSg@mail.gmail.com>
Subject: Re: [PATCH v4 2/2] mm/vmalloc.c: Modify struct vmap_area to reduce
 its size
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Uladzislau Rezki <urezki@gmail.com>, rpenyaev@suse.de,
        peterz@infradead.org, guro@fb.com, rick.p.edgecombe@intel.com,
        rppt@linux.ibm.com, aryabinin@virtuozzo.com, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
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On Fri, Jul 12, 2019 at 9:49 PM Matthew Wilcox <willy@infradead.org> wrote:
>
> On Fri, Jul 12, 2019 at 08:02:13PM +0800, Pengfei Li wrote:
>
> I don't think you need struct union struct union.  Because llist_node
> is just a pointer, you can get the same savings with just:
>
>         union {
>                 struct llist_node purge_list;
>                 struct vm_struct *vm;
>                 unsigned long subtree_max_size;
>         };
>

Thanks for your comments.

As you said, I did this in v3.
https://patchwork.kernel.org/patch/11031507/

The reason why I use struct union struct in v4 is that I want to
express "in the tree" and "in the purge list" are two completely
isolated cases.

struct vmap_area {
        union {
                struct {        /* Case A: In the tree */
                        ...
                };

                struct {        /* Case B: In the purge list */
                        ...
                };
        };
};

The "rb_node" and "list" should also not be used when va is in
the purge list

what do you think of this idea?