linux-kernel - Re: [PATCH] mm: avoid slub allocation while holding list

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Date:   Mon, 9 Sep 2019 20:16:00 -0600
From:   Yu Zhao <yuzhao@...gle.com>
To:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc:     "Kirill A. Shutemov" <kirill@...temov.name>,
        Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: avoid slub allocation while holding list_lock

On Tue, Sep 10, 2019 at 10:41:31AM +0900, Tetsuo Handa wrote:
> Yu Zhao wrote:
> > I think we can safely assume PAGE_SIZE is unsigned long aligned and
> > page->objects is non-zero. But if you don't feel comfortable with these
> > assumptions, I'd be happy to ensure them explicitly.
> 
> I know PAGE_SIZE is unsigned long aligned. If someone by chance happens to
> change from "dynamic allocation" to "on stack", get_order() will no longer
> be called and the bug will show up.
> 
> I don't know whether __get_free_page(GFP_ATOMIC) can temporarily consume more
> than 4096 bytes, but if it can, we might want to avoid "dynamic allocation".

With GFP_ATOMIC and ~~__GFP_HIGHMEM, it shouldn't.

> By the way, if "struct kmem_cache_node" is object which won't have many thousands
> of instances, can't we embed that buffer into "struct kmem_cache_node" because
> max size of that buffer is only 4096 bytes?

It seems to me allocation in error path is better than always keeping
a page around. But the latter may still be acceptable given it's done
only when debug is on and, of course, on a per-node scale.