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Message-ID: <CAMgjq7CE29aR6kQtBid-NGJM-B+dZZFTZ=ta142xoRi06zs2FQ@mail.gmail.com>
Date: Tue, 10 Dec 2024 16:15:36 +0800
From: Kairui Song <ryncsn@...il.com>
To: Yosry Ahmed <yosryahmed@...gle.com>
Cc: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
Chris Li <chrisl@...nel.org>, Hugh Dickins <hughd@...gle.com>,
"Huang, Ying" <ying.huang@...el.com>, Roman Gushchin <roman.gushchin@...ux.dev>,
Shakeel Butt <shakeel.butt@...ux.dev>, Johannes Weiner <hannes@...xchg.org>,
Barry Song <baohua@...nel.org>, Michal Hocko <mhocko@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/4] mm/swap_cgroup: simplify swap cgroup definitions
On Tue, Dec 3, 2024 at 3:26 AM Yosry Ahmed <yosryahmed@...gle.com> wrote:
>
> On Mon, Dec 2, 2024 at 10:42 AM Kairui Song <ryncsn@...il.com> wrote:
> >
> > From: Kairui Song <kasong@...cent.com>
> >
> > Remove the intermediate struct swap_cgroup, it just a unsigned short
> > wrapper, simplify the code.
>
> Did you actually remove the struct? It doesn't seem like it.
Oops, I forgot to drop these lines of code indeed, I'll send V2
merging this into patch 4/4 so this commit can be ignored for now.
>
> >
> > Also zero the map on initialization to prevent unexpected behaviour as
> > swap cgroup helpers are suppose to return 0 on error.
>
> All the callers lookup the id of an already swapped out page, so it
> should never be uninitialized. Maybe we should WARN if the result of
> the lookup is 0 in this case?
Yes, just a fallback for robustness.
Lookup returning 0 is expected in some cases, eg. Cgroup V1 will call
mem_cgroup_swapin_uncharge_swap early when the folio is added to swap
cache, and erase the record. Then the mem_cgroup_uncharge_swap in
swapfile.c (called when the swap cache is being dropped and entry is
freed) won't double uncharge it. So we can't WARN on that.
>
> >
> > Signed-off-by: Kairui Song <kasong@...cent.com>
> > ---
> > mm/swap_cgroup.c | 45 +++++++++++++++++++--------------------------
> > 1 file changed, 19 insertions(+), 26 deletions(-)
> >
> > diff --git a/mm/swap_cgroup.c b/mm/swap_cgroup.c
> > index 1770b076f6b7..a76afdc3666a 100644
> > --- a/mm/swap_cgroup.c
> > +++ b/mm/swap_cgroup.c
> > @@ -12,14 +12,12 @@ struct swap_cgroup {
> > };
> >
> > struct swap_cgroup_ctrl {
> > - struct swap_cgroup *map;
> > + unsigned short *map;
> > spinlock_t lock;
> > };
> >
> > static struct swap_cgroup_ctrl swap_cgroup_ctrl[MAX_SWAPFILES];
> >
> > -#define SC_PER_PAGE (PAGE_SIZE/sizeof(struct swap_cgroup))
> > -
> > /*
> > * SwapCgroup implements "lookup" and "exchange" operations.
> > * In typical usage, this swap_cgroup is accessed via memcg's charge/uncharge
> > @@ -33,18 +31,6 @@ static struct swap_cgroup_ctrl swap_cgroup_ctrl[MAX_SWAPFILES];
> > *
> > * TODO: we can push these buffers out to HIGHMEM.
> > */
> > -static struct swap_cgroup *lookup_swap_cgroup(swp_entry_t ent,
> > - struct swap_cgroup_ctrl **ctrlp)
> > -{
> > - pgoff_t offset = swp_offset(ent);
> > - struct swap_cgroup_ctrl *ctrl;
> > -
> > - ctrl = &swap_cgroup_ctrl[swp_type(ent)];
> > - if (ctrlp)
> > - *ctrlp = ctrl;
> > - return &ctrl->map[offset];
> > -}
> > -
> > /**
> > * swap_cgroup_record - record mem_cgroup for a set of swap entries
> > * @ent: the first swap entry to be recorded into
> > @@ -58,20 +44,21 @@ unsigned short swap_cgroup_record(swp_entry_t ent, unsigned short id,
> > unsigned int nr_ents)
> > {
> > struct swap_cgroup_ctrl *ctrl;
> > - struct swap_cgroup *sc;
> > + unsigned short *map;
> > unsigned short old;
> > unsigned long flags;
> > pgoff_t offset = swp_offset(ent);
> > pgoff_t end = offset + nr_ents;
> >
> > - sc = lookup_swap_cgroup(ent, &ctrl);
> > + ctrl = &swap_cgroup_ctrl[swp_type(ent)];
> > + map = ctrl->map;
> >
> > spin_lock_irqsave(&ctrl->lock, flags);
> > - old = sc->id;
> > - for (; offset < end; offset++, sc++) {
> > - VM_BUG_ON(sc->id != old);
> > - sc->id = id;
> > - }
> > + old = map[offset];
> > + do {
> > + VM_BUG_ON(map[offset] != old);
> > + map[offset] = id;
> > + } while (++offset != end);
>
> Why did you change the for loop here?
>
> > spin_unlock_irqrestore(&ctrl->lock, flags);
> >
> > return old;
> > @@ -85,20 +72,26 @@ unsigned short swap_cgroup_record(swp_entry_t ent, unsigned short id,
> > */
> > unsigned short lookup_swap_cgroup_id(swp_entry_t ent)
> > {
> > + struct swap_cgroup_ctrl *ctrl;
> > +
> > if (mem_cgroup_disabled())
> > return 0;
> > - return lookup_swap_cgroup(ent, NULL)->id;
> > +
> > + ctrl = &swap_cgroup_ctrl[swp_type(ent)];
> > + pgoff_t offset = swp_offset(ent);
> > +
> > + return READ_ONCE(ctrl->map[offset]);
>
> The READ_ONCE() does not exist today in lookup_swap_cgroup(). Why is it needed?
>
> > }
> >
> > int swap_cgroup_swapon(int type, unsigned long max_pages)
> > {
> > - struct swap_cgroup *map;
> > + void *map;
> > struct swap_cgroup_ctrl *ctrl;
> >
> > if (mem_cgroup_disabled())
> > return 0;
> >
> > - map = vcalloc(max_pages, sizeof(struct swap_cgroup));
> > + map = vzalloc(max_pages * sizeof(unsigned short));
> > if (!map)
> > goto nomem;
> >
> > @@ -117,7 +110,7 @@ int swap_cgroup_swapon(int type, unsigned long max_pages)
> >
> > void swap_cgroup_swapoff(int type)
> > {
> > - struct swap_cgroup *map;
> > + void *map;
>
> Why void?
>
> > struct swap_cgroup_ctrl *ctrl;
> >
> > if (mem_cgroup_disabled())
> > --
> > 2.47.0
> >
>
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