linux-kernel - Re: [PATCH -V6 06/21] swap: Support PMD swap mapping when splitting huge PMD

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Message-ID: <20181025150044.urvklakbzd6jauyb@ca-dmjordan1.us.oracle.com>
Date:   Thu, 25 Oct 2018 08:00:44 -0700
From:   Daniel Jordan <daniel.m.jordan@...cle.com>
To:     "Huang, Ying" <ying.huang@...el.com>
Cc:     Daniel Jordan <daniel.m.jordan@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Michal Hocko <mhocko@...nel.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Shaohua Li <shli@...nel.org>, Hugh Dickins <hughd@...gle.com>,
        Minchan Kim <minchan@...nel.org>,
        Rik van Riel <riel@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        Zi Yan <zi.yan@...rutgers.edu>
Subject: Re: [PATCH -V6 06/21] swap: Support PMD swap mapping when splitting
 huge PMD

On Thu, Oct 25, 2018 at 08:54:16AM +0800, Huang, Ying wrote:
> Daniel Jordan <daniel.m.jordan@...cle.com> writes:
> 
> > On Wed, Oct 10, 2018 at 03:19:09PM +0800, Huang Ying wrote:
> >> +#ifdef CONFIG_THP_SWAP
> >> +/*
> >> + * The corresponding page table shouldn't be changed under us, that
> >> + * is, the page table lock should be held.
> >> + */
> >> +int split_swap_cluster_map(swp_entry_t entry)
> >> +{
> >> +	struct swap_info_struct *si;
> >> +	struct swap_cluster_info *ci;
> >> +	unsigned long offset = swp_offset(entry);
> >> +
> >> +	VM_BUG_ON(!IS_ALIGNED(offset, SWAPFILE_CLUSTER));
> >> +	si = _swap_info_get(entry);
> >> +	if (!si)
> >> +		return -EBUSY;
> >
> > I think this return value doesn't get used anywhere?
> 
> Yes.  And the error is only possible if page table is corrupted.  So
> maybe add a VM_BUG_ON() in it caller __split_huge_swap_pmd()?

Taking a second look at this, I see we'd get some nice pr_err message in this
case, so VM_BUG_ON doesn't seem necessary.

Still odd there's an unchecked return value, but it could be useful to future
callers.  Just my nitpick, feel free to leave as is.