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Date: Fri, 25 Feb 2022 09:50:14 +0000
From: Hyeonggon Yoo <42.hyeyoo@...il.com>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: linux-mm@...ck.org, Roman Gushchin <guro@...com>,
Andrew Morton <akpm@...ux-foundation.org>,
linux-kernel@...r.kernel.org, Joonsoo Kim <iamjoonsoo.kim@....com>,
David Rientjes <rientjes@...gle.com>,
Christoph Lameter <cl@...ux.com>,
Pekka Enberg <penberg@...nel.org>
Subject: Re: [PATCH 5/5] mm/slub: Refactor deactivate_slab()
On Fri, Feb 25, 2022 at 09:34:09AM +0000, Hyeonggon Yoo wrote:
> On Thu, Feb 24, 2022 at 07:16:11PM +0100, Vlastimil Babka wrote:
> > On 2/21/22 11:53, Hyeonggon Yoo wrote:
> > > Simply deactivate_slab() by removing variable 'lock' and replacing
> > > 'l' and 'm' with 'mode'. Instead, remove slab from list and unlock
> > > n->list_lock when cmpxchg_double() fails, and then retry.
> > >
> > > One slight functional change is releasing and taking n->list_lock again
> > > when cmpxchg_double() fails. This is not harmful because SLUB avoids
> > > deactivating slabs as much as possible.
> > >
> > > Signed-off-by: Hyeonggon Yoo <42.hyeyoo@...il.com>
> >
> > Hm I wonder if we could simplify even a bit more. Do we have to actually
> > place the slab on a partial (full) list before the cmpxchg, only to remove
> > it when cmpxchg fails? Seems it's to avoid anyone else seeing the slab
> > un-frozen, but not on the list, which would be unexpected. However if anyone
> > sees such slab, they have to take the list_lock first to start working with
> > the slab... so this should be safe, because we hold the list_lock here, and
> > will place the slab on the list before we release it. But it thus shouldn't
> > matter if the placement happens before or after a successful cmpxchg, no? So
> > we can only do it once after a successful cmpxchg and need no undo's?
> >
>
> My thought was similar. But after testing I noticed that &n->list_lock prevents
> race between __slab_free() and deactivate_slab().
>
> > Specifically AFAIK the only possible race should be with a __slab_free()
> > which might observe !was_frozen after we succeed an unfreezing cmpxchg and
> > go through the
> > "} else { /* Needs to be taken off a list */"
> > branch but then it takes the list_lock as the first thing, so will be able
> > to proceed only after the slab is actually on the list.
> >
> > Do I miss anything or would you agree?
> >
>
> It's so tricky.
>
> I tried to simplify more as you said. Seeing frozen slab on list was not
> problem. But the problem was that something might interfere between
> cmpxchg_double() and taking spinlock.
>
> This is what I faced:
>
> CPU A CPU B
> deactivate_slab() { __slab_free() {
> /* slab is full */
> slab.frozen = 0;
> cmpxchg_double();
> /* Hmm...
> slab->frozen == 0 &&
> slab->freelist != NULL?
> Oh This must be on the list.. */
Oh this is wrong.
slab->freelist must be
NULL because it's full
slab.
It's more complex
than I thought...
> spin_lock_irqsave();
> cmpxchg_double();
> /* Corruption: slab
> * was not yet inserted to
> * list but try removing */
> remove_full();
> spin_unlock_irqrestore();
> }
> spin_lock_irqsave();
> add_full();
> spin_unlock_irqrestore();
> }
So it was...
CPU A CPU B
deactivate_slab() { __slab_free() {
/* slab is full */
slab.frozen = 0;
cmpxchg_double();
/*
Hmm...
!was_frozen &&
prior == NULL?
Let's freeze this!
*/
put_cpu_partial();
}
spin_lock_irqsave();
add_full();
/* It's now frozen by CPU B and at the same time on full list */
spin_unlock_irqrestore();
And &n->list_lock prevents such a race.
>
> I think it's quite confusing because it's protecting code, not data.
>
> Would you have an idea to solve it, or should we add a comment for this?
>
> > > ---
> > > mm/slub.c | 74 +++++++++++++++++++++++++------------------------------
> > > 1 file changed, 33 insertions(+), 41 deletions(-)
> > >
> > > diff --git a/mm/slub.c b/mm/slub.c
> > > index a4964deccb61..2d0663befb9e 100644
> > > --- a/mm/slub.c
> > > +++ b/mm/slub.c
> > > @@ -2350,8 +2350,8 @@ static void deactivate_slab(struct kmem_cache *s, struct slab *slab,
> > > {
> > > enum slab_modes { M_NONE, M_PARTIAL, M_FULL, M_FREE };
> > > struct kmem_cache_node *n = get_node(s, slab_nid(slab));
> > > - int lock = 0, free_delta = 0;
> > > - enum slab_modes l = M_NONE, m = M_NONE;
> > > + int free_delta = 0;
> > > + enum slab_modes mode = M_NONE;
> > > void *nextfree, *freelist_iter, *freelist_tail;
> > > int tail = DEACTIVATE_TO_HEAD;
> > > unsigned long flags = 0;
> > > @@ -2420,57 +2420,49 @@ static void deactivate_slab(struct kmem_cache *s, struct slab *slab,
> > > new.frozen = 0;
> > >
> > > if (!new.inuse && n->nr_partial >= s->min_partial)
> > > - m = M_FREE;
> > > + mode = M_FREE;
> > > else if (new.freelist) {
> > > - m = M_PARTIAL;
> > > - if (!lock) {
> > > - lock = 1;
> > > - /*
> > > - * Taking the spinlock removes the possibility that
> > > - * acquire_slab() will see a slab that is frozen
> > > - */
> > > - spin_lock_irqsave(&n->list_lock, flags);
> > > - }
> > > - } else {
> > > - m = M_FULL;
> > > - if (kmem_cache_debug_flags(s, SLAB_STORE_USER) && !lock) {
> > > - lock = 1;
> > > - /*
> > > - * This also ensures that the scanning of full
> > > - * slabs from diagnostic functions will not see
> > > - * any frozen slabs.
> > > - */
> > > - spin_lock_irqsave(&n->list_lock, flags);
> > > - }
> > > + mode = M_PARTIAL;
> > > + /*
> > > + * Taking the spinlock removes the possibility that
> > > + * acquire_slab() will see a slab that is frozen
> > > + */
> > > + spin_lock_irqsave(&n->list_lock, flags);
> > > + add_partial(n, slab, tail);
> > > + } else if (kmem_cache_debug_flags(s, SLAB_STORE_USER)) {
> > > + mode = M_FULL;
> > > + /*
> > > + * This also ensures that the scanning of full
> > > + * slabs from diagnostic functions will not see
> > > + * any frozen slabs.
> > > + */
> > > + spin_lock_irqsave(&n->list_lock, flags);
> > > + add_full(s, n, slab);
> > > }
> > >
> > > - if (l != m) {
> > > - if (l == M_PARTIAL)
> > > - remove_partial(n, slab);
> > > - else if (l == M_FULL)
> > > - remove_full(s, n, slab);
> > >
> > > - if (m == M_PARTIAL)
> > > - add_partial(n, slab, tail);
> > > - else if (m == M_FULL)
> > > - add_full(s, n, slab);
> > > - }
> > > -
> > > - l = m;
> > > if (!cmpxchg_double_slab(s, slab,
> > > old.freelist, old.counters,
> > > new.freelist, new.counters,
> > > - "unfreezing slab"))
> > > + "unfreezing slab")) {
> > > + if (mode == M_PARTIAL) {
> > > + remove_partial(n, slab);
> > > + spin_unlock_irqrestore(&n->list_lock, flags);
> > > + } else if (mode == M_FULL) {
> > > + remove_full(s, n, slab);
> > > + spin_unlock_irqrestore(&n->list_lock, flags);
> > > + }
> > > goto redo;
> > > + }
> > >
> > > - if (lock)
> > > - spin_unlock_irqrestore(&n->list_lock, flags);
> > >
> > > - if (m == M_PARTIAL)
> > > + if (mode == M_PARTIAL) {
> > > + spin_unlock_irqrestore(&n->list_lock, flags);
> > > stat(s, tail);
> > > - else if (m == M_FULL)
> > > + } else if (mode == M_FULL) {
> > > + spin_unlock_irqrestore(&n->list_lock, flags);
> > > stat(s, DEACTIVATE_FULL);
> > > - else if (m == M_FREE) {
> > > + } else if (mode == M_FREE) {
> > > stat(s, DEACTIVATE_EMPTY);
> > > discard_slab(s, slab);
> > > stat(s, FREE_SLAB);
> >
>
> --
> Thank you, You are awesome!
> Hyeonggon :-)
--
Thank you, You are awesome!
Hyeonggon :-)
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