Date: Wed, 19 Jun 2024 02:03:05 -0700
From: Chris Li <chrisl@...nel.org>
To: "Huang, Ying" <ying.huang@...el.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, Kairui Song <kasong@...cent.com>, 
	Ryan Roberts <ryan.roberts@....com>, Kalesh Singh <kaleshsingh@...gle.com>, 
	linux-kernel@...r.kernel.org, linux-mm@...ck.org, 
	Barry Song <baohua@...nel.org>
Subject: Re: [PATCH v2 1/2] mm: swap: swap cluster switch to double link list

On Wed, Jun 19, 2024 at 12:53 AM Huang, Ying <ying.huang@...el.com> wrote:
>
> Chris Li <chrisl@...nel.org> writes:
>
> > On Tue, Jun 18, 2024 at 12:56 AM Huang, Ying <ying.huang@...el.com> wrote:
> >>
> >> Chris Li <chrisl@...nel.org> writes:
> >>
> >> > On Sun, Jun 16, 2024 at 11:21 PM Huang, Ying <ying.huang@...el.com> wrote:
> >> >>
> >> >> Hi, Chris,
> >> >>
> >> >> Chris Li <chrisl@...nel.org> writes:
>
> [snip]
>
> >> >> > diff --git a/include/linux/swap.h b/include/linux/swap.h
> >> >> > index 3df75d62a835..cd9154a3e934 100644
> >> >> > --- a/include/linux/swap.h
> >> >> > +++ b/include/linux/swap.h
> >> >> > @@ -242,23 +242,22 @@ enum {
> >> >> >   * space with SWAPFILE_CLUSTER pages long and naturally aligns in disk. All
> >> >> >   * free clusters are organized into a list. We fetch an entry from the list to
> >> >> >   * get a free cluster.
> >> >> > - *
> >> >> > - * The data field stores next cluster if the cluster is free or cluster usage
> >> >> > - * counter otherwise. The flags field determines if a cluster is free. This is
> >> >> > - * protected by swap_info_struct.lock.
> >> >> >   */
> >> >> >  struct swap_cluster_info {
> >> >> >       spinlock_t lock;        /*
> >> >> > -                              * Protect swap_cluster_info fields
> >> >> > -                              * and swap_info_struct->swap_map
> >> >> > +                              * Protect swap_cluster_info count and state
> >> >>
> >> >> Protect swap_cluster_info fields except 'list' ?
> >> >
> >> > I change it to protect the swap_cluster_info bitfields in the second patch.
> >>
> >> Although I still prefer my version, I will not insist on that.
> >
> > Sure, I actually don't have a strong preference about that. It is just comments.
> >
> >>
> >> >>
> >> >> > +                              * field and swap_info_struct->swap_map
> >> >> >                                * elements correspond to the swap
> >> >> >                                * cluster
> >> >> >                                */
> >> >> > -     unsigned int data:24;
> >> >> > -     unsigned int flags:8;
> >> >> > +     unsigned int count:12;
> >> >> > +     unsigned int state:3;
> >> >>
> >> >> I still prefer normal data type over bit fields.  How about
> >> >>
> >> >>         u16 usage;
> >> >>         u8  state;
> >> >
> >> > I don't mind the "count" rename to "usage". That is probably a better
> >> > name. However I have another patch intended to add more bit fields in
> >> > the cluster info struct. The second patch adds "order" and the later
> >> > patch will add more. That is why I choose bitfield to be more condense
> >> > with bits.
> >>
> >> We still have space for another "u8" for "order".  It appears trivial to
> >> change it to bit fields when necessary in the future.
> >
> > We can, I don't see it necessary to change from bit field to u8 and
> > back to bit field in the future. It is more of a personal preference
> > issue.
>
> I have to say that I don't think that it's just a personal preference.
> IMO, if it's unnecessary, we shouldn't use bit fields.  You cannot
> guarantee that your future changes will be merged in its current state.
> So, I still think that it's better to avoid bit fields for now.

That is surprising to hear, I am not dependent on any hardware
physical bit location.
Anyway, not too big a deal for me. I changed it to u16/u8.

> >> > When you get the cluster from the list, you have a cluster pointer. I
> >> > feel it is unnecessary to convert to index then back convert to
> >> > cluster pointer inside lock_cluster(). I actually feel using indexes
> >> > to refer to the cluster is error prone because we also have offset.
> >>
> >> I don't think so, it's common to use swap offset.
> >
> > Swap offset is not an issue, it is all over the place. The cluster
> > index(offset/512) is the one I try to avoid.
> > I have made some mistakes myself on offset vs index.
>
> Yes.  That's not good, but it's hard to be avoided too.  Can we make the
> variable name more consistent?  index: cluster index, offset: swap
> offset.
>
> And, in fact, swap offset is the parameter of lock_cluster() instead of
> cluster index.

Right, when you get the cluster pointer from the list, it can't
directly use lock_cluster().

>
> >> >
> >> >>
> >> >> > +             __free_cluster(si, ci);
> >> >> >               memset(si->swap_map + idx * SWAPFILE_CLUSTER,
> >> >> >                               0, SWAPFILE_CLUSTER);
> >> >> >               unlock_cluster(ci);
> >> >> > @@ -521,20 +412,19 @@ static void swap_users_ref_free(struct percpu_ref *ref)
> >> >> >       complete(&si->comp);
> >> >> >  }
> >> >> >
>
> [snip]
>
> >> >> > @@ -611,10 +497,10 @@ scan_swap_map_ssd_cluster_conflict(struct swap_info_struct *si,
> >> >> >  {
> >> >> >       struct percpu_cluster *percpu_cluster;
> >> >> >       bool conflict;
> >> >> > -
> >> >>
> >> >> Usually we use one blank line after local variable declaration.
> >> > Ack.
> >> >
> >> >>
> >> >> > +     struct swap_cluster_info *first = list_first_entry(&si->free_clusters, struct swap_cluster_info, list);
> >> >> >       offset /= SWAPFILE_CLUSTER;
> >> >> > -     conflict = !cluster_list_empty(&si->free_clusters) &&
> >> >> > -             offset != cluster_list_first(&si->free_clusters) &&
> >> >> > +     conflict = !list_empty(&si->free_clusters) &&
> >> >> > +             offset !=  first - si->cluster_info &&
> >> >> >               cluster_is_free(&si->cluster_info[offset]);
> >> >> >
> >> >> >       if (!conflict)
> >> >> > @@ -655,10 +541,14 @@ static bool scan_swap_map_try_ssd_cluster(struct swap_info_struct *si,
> >> >> >       cluster = this_cpu_ptr(si->percpu_cluster);
> >> >> >       tmp = cluster->next[order];
> >> >> >       if (tmp == SWAP_NEXT_INVALID) {
> >> >> > -             if (!cluster_list_empty(&si->free_clusters)) {
> >> >> > -                     tmp = cluster_next(&si->free_clusters.head) *
> >> >> > -                                     SWAPFILE_CLUSTER;
> >> >> > -             } else if (!cluster_list_empty(&si->discard_clusters)) {
> >> >> > +             if (!list_empty(&si->free_clusters)) {
> >> >> > +                     ci = list_first_entry(&si->free_clusters, struct swap_cluster_info, list);
> >> >> > +                     list_del(&ci->list);
> >> >>
> >> >> The free cluster is deleted from si->free_clusters now.  But later you
> >> >> will call scan_swap_map_ssd_cluster_conflict() and may abandon the
> >> >> cluster.  And in alloc_cluster() later, it may be deleted again.
> >> >
> >> > Yes, that is a bug. Thanks for catching that.
> >> >
> >> >>
> >> >> > +                     spin_lock(&ci->lock);
> >> >> > +                     ci->state = CLUSTER_STATE_PER_CPU;
> >> >>
> >> >> Need to change ci->state when move a cluster off the percpu_cluster.
> >> >
> >> > In the next patch. This patch does not use the off state yet.
> >>
> >> But that is confusing to use wrong state name, the really meaning is
> >> something like CLUSTER_STATE_NON_FREE.  But as I suggested above, we can
> >
> > It can be FREE and on the per cpu pointer as well. That is the tricky part.
> > It can happen on the current code as well.
>
> cluster_set_count_flag(0, 0) is called in alloc_cluster().  So, it's not
> an issue in current code.  If you need more, that shouldn't be done in
> this patch.

Revert to V1 like using the flags.

Chris

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