linux-kernel - Re: [PATCH -mm -V3 03/21] mm, THP, swap: Support PMD swap mapping in swap

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Date:   Tue, 12 Jun 2018 05:04:30 -0700
From:   Daniel Jordan <daniel.m.jordan@...cle.com>
To:     "Huang, Ying" <ying.huang@...el.com>
Cc:     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@...e.com>,
        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 -mm -V3 03/21] mm, THP, swap: Support PMD swap mapping in
 swap_duplicate()

On Tue, Jun 12, 2018 at 09:23:19AM +0800, Huang, Ying wrote:
> Daniel Jordan <daniel.m.jordan@...cle.com> writes:
> > #2:  We've masked off SWAP_HAS_CACHE and COUNT_CONTINUED, and already checked
> > for SWAP_MAP_BAD, so I think condition #2 always fails and can just be removed.
> 
> I think this is used to check some software bug.  For example,
> SWAP_MAP_SHMEM will yield true here.

So it does!  And so __swap_duplicate returns -EINVAL in that case, which
swap_shmem_alloc just ignores.  Confusing, and an explicit check for
SWAP_MAP_SHMEM would be cleaner, but why fix what isn't broken.

> 
> >> +#ifdef CONFIG_THP_SWAP
> >> +static int __swap_duplicate_cluster(swp_entry_t *entry, unsigned char usage)
> > ...
> >> +	} else {
> >> +		for (i = 0; i < SWAPFILE_CLUSTER; i++) {
> >> +retry:
> >> +			err = __swap_duplicate_locked(si, offset + i, 1);
> >
> > I guess usage is assumed to be 1 at this point (__swap_duplicate_locked makes
> > the same assumption).  Maybe make this explicit with
> >
> > 			err = __swap_duplicate_locked(si, offset + i, usage);
> >
> > , use 'usage' in cluster_set_count and __swap_entry_free too, and then
> > earlier have a
> >
> >        VM_BUG_ON(usage != SWAP_HAS_CACHE && usage != 1);
> >
> > ?
> 
> Yes.  I will fix this.  And we can just check it in
> __swap_duplicate_locked() and all these will be covered.

I'll respond to your other mail.

> > Not related to your changes, but while we're here, the comment with
> > SWAP_HAS_CONT in swap_count() could be deleted: I don't think there ever was a
> > SWAP_HAS_CONT.
> 
> Yes.  We should correct this.  Because this should go to a separate patch,
> would you mind to submit a patch to fix it?

Sure, I'll do that.

Daniel