linux-kernel - Re: [PATCH v2 2/2] mm: zswap: disable migration while using per-CPU acomp

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Message-ID: <CAGsJ_4z-5bsKT_6byG9ms1Ycfm4XX0gZ2LcCW=jA-umsHO=6eg@mail.gmail.com>
Date: Wed, 8 Jan 2025 18:06:30 +1300
From: Barry Song <21cnbao@...il.com>
To: Nhat Pham <nphamcs@...il.com>
Cc: Yosry Ahmed <yosryahmed@...gle.com>, Johannes Weiner <hannes@...xchg.org>, 
	Andrew Morton <akpm@...ux-foundation.org>, Chengming Zhou <chengming.zhou@...ux.dev>, 
	Vitaly Wool <vitalywool@...il.com>, Sam Sun <samsun1006219@...il.com>, 
	"linux-mm@...ck.org" <linux-mm@...ck.org>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, 
	"stable@...r.kernel.org" <stable@...r.kernel.org>, 
	"Sridhar, Kanchana P" <kanchana.p.sridhar@...el.com>
Subject: Re: [PATCH v2 2/2] mm: zswap: disable migration while using per-CPU acomp_ctx

On Wed, Jan 8, 2025 at 5:46 PM Nhat Pham <nphamcs@...il.com> wrote:
>
> On Wed, Jan 8, 2025 at 9:34 AM Yosry Ahmed <yosryahmed@...gle.com> wrote:
> >
> >
> > Actually, using the mutex to protect against CPU hotunplug is not too
> > complicated. The following diff is one way to do it (lightly tested).
> > Johannes, Nhat, any preferences between this patch (disabling
> > migration) and the following diff?
>
> I mean if this works, this over migration diasbling any day? :)
>
> >
> > diff --git a/mm/zswap.c b/mm/zswap.c
> > index f6316b66fb236..4d6817c679a54 100644
> > --- a/mm/zswap.c
> > +++ b/mm/zswap.c
> > @@ -869,17 +869,40 @@ static int zswap_cpu_comp_dead(unsigned int cpu,
> > struct hlist_node *node)
> >         struct zswap_pool *pool = hlist_entry(node, struct zswap_pool, node);
> >         struct crypto_acomp_ctx *acomp_ctx = per_cpu_ptr(pool->acomp_ctx, cpu);
> >
> > +       mutex_lock(&acomp_ctx->mutex);
> >         if (!IS_ERR_OR_NULL(acomp_ctx)) {
> >                 if (!IS_ERR_OR_NULL(acomp_ctx->req))
> >                         acomp_request_free(acomp_ctx->req);
> > +               acomp_ctx->req = NULL;
> >                 if (!IS_ERR_OR_NULL(acomp_ctx->acomp))
> >                         crypto_free_acomp(acomp_ctx->acomp);
> >                 kfree(acomp_ctx->buffer);
> >         }
> > +       mutex_unlock(&acomp_ctx->mutex);
> >
> >         return 0;
> >  }
> >
> > +static struct crypto_acomp_ctx *acomp_ctx_get_cpu_locked(
> > +               struct crypto_acomp_ctx __percpu *acomp_ctx)
> > +{
> > +       struct crypto_acomp_ctx *ctx;
> > +
> > +       for (;;) {
> > +               ctx = raw_cpu_ptr(acomp_ctx);
> > +               mutex_lock(&ctx->mutex);
>
> I'm a bit confused. IIUC, ctx is per-cpu right? What's protecting this
> cpu-local data (including the mutex) from being invalidated under us
> while we're sleeping and waiting for the mutex?
>
> If it is somehow protected, then yeah this seems quite elegant :)

thought about this again. Could it be the following?

bool cpus_is_read_locked(void)
{
        return percpu_is_read_locked(&cpu_hotplug_lock);
}

in zswap:

bool locked = cpus_is_read_locked();

if (!locked)
     cpus_read_lock();

.... // do our job

if (!locked)
     cpus_read_unlock();

This seems to resolve all three problems:
1. if our context has held read lock, we won't hold it again;
2. if other contexts are holding write lock, we wait for the
completion of cpuhotplug
by acquiring read lock
3. if our context hasn't held a read lock, we hold it.

>
> > +               if (likely(ctx->req))
> > +                       return ctx;
> > +               /* Raced with zswap_cpu_comp_dead() on CPU hotunplug */
> > +               mutex_unlock(&ctx->mutex);
> > +       }
> > +}
> > +
> > +static void acomp_ctx_put_unlock(struct crypto_acomp_ctx *ctx)
> > +{
> > +       mutex_unlock(&ctx->mutex);
> > +}
> > +
> >  static bool zswap_compress(struct page *page, struct zswap_entry *entry,
> >                            struct zswap_pool *pool)
> >  {
> > @@ -893,10 +916,7 @@ static bool zswap_compress(struct page *page,
> > struct zswap_entry *entry,
> >         gfp_t gfp;
> >         u8 *dst;
> >
> > -       acomp_ctx = raw_cpu_ptr(pool->acomp_ctx);
> > -
> > -       mutex_lock(&acomp_ctx->mutex);
> > -
> > +       acomp_ctx = acomp_ctx_get_cpu_locked(pool->acomp_ctx);
> >         dst = acomp_ctx->buffer;
> >         sg_init_table(&input, 1);
> >         sg_set_page(&input, page, PAGE_SIZE, 0);
> > @@ -949,7 +969,7 @@ static bool zswap_compress(struct page *page,
> > struct zswap_entry *entry,
> >         else if (alloc_ret)
> >                 zswap_reject_alloc_fail++;
> >
> > -       mutex_unlock(&acomp_ctx->mutex);
> > +       acomp_ctx_put_unlock(acomp_ctx);
> >         return comp_ret == 0 && alloc_ret == 0;
> >  }
> >
> > @@ -960,9 +980,7 @@ static void zswap_decompress(struct zswap_entry
> > *entry, struct folio *folio)
> >         struct crypto_acomp_ctx *acomp_ctx;
> >         u8 *src;
> >
> > -       acomp_ctx = raw_cpu_ptr(entry->pool->acomp_ctx);
> > -       mutex_lock(&acomp_ctx->mutex);
> > -
> > +       acomp_ctx = acomp_ctx_get_cpu_locked(entry->pool->acomp_ctx);
> >         src = zpool_map_handle(zpool, entry->handle, ZPOOL_MM_RO);
> >         /*

Thanks
Barry