linux-kernel - Re: [PATCHv3, RESEND 5/8] khugepaged: Allow to collapse a page shared across fork

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Message-ID: <20200414213526.ecn3zcgnqxoveddq@box>
Date:   Wed, 15 Apr 2020 00:35:26 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        akpm@...ux-foundation.org, Andrea Arcangeli <aarcange@...hat.com>,
        Zi Yan <ziy@...dia.com>, Yang Shi <yang.shi@...ux.alibaba.com>,
        Ralph Campbell <rcampbell@...dia.com>,
        William Kucharski <william.kucharski@...cle.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCHv3, RESEND 5/8] khugepaged: Allow to collapse a page
 shared across fork

On Mon, Apr 13, 2020 at 01:48:22PM -0700, John Hubbard wrote:

[Thanks for all your suggestions and corrections]

> > +	if (IS_ENABLED(CONFIG_DEBUG_VM) && expected_refcount > refcount) {
> > +		pr_err("expected_refcount: %d, refcount: %d\n",
> > +				expected_refcount, refcount);
> > +		dump_page(page, "Unexpected refcount");
> 
> 
> I see two issues with the pr_err() and the dump_page() call:
> 
> 1. You probably want to rate limit this, otherwise you'll have a big
> problem if lots of pages are pinned!

Nope. Only if kernel is buggy. See below.

> 2. Actually, I don't think you'd want to print anything at all here, even with
> rate limiting, because doing so presumes that "unexpected" means "wrong". And I
> think this patch doesn't expect to have GUP pins (or pin_user_pages() pins, ha),
> but that doesn't mean that they're wrong to have.

See condition. We only do it if refcount is *below* expected refcount. It
should never happen. Pinned page would have refcount above expected.

-- 
 Kirill A. Shutemov