linux-kernel - Re: [PATCH] zsmalloc: Fix races between modifications of fullness and isolated

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Date:   Wed, 26 Jul 2023 20:31:47 +0900
From:   Sergey Senozhatsky <senozhatsky@...omium.org>
To:     Andrew Yang (楊智強) 
        <Andrew.Yang@...iatek.com>
Cc:     "senozhatsky@...omium.org" <senozhatsky@...omium.org>,
        "bigeasy@...utronix.de" <bigeasy@...utronix.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mediatek@...ts.infradead.org" 
        <linux-mediatek@...ts.infradead.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        wsd_upstream <wsd_upstream@...iatek.com>,
        Casper Li (李中榮) <casper.li@...iatek.com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "minchan@...nel.org" <minchan@...nel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "matthias.bgg@...il.com" <matthias.bgg@...il.com>,
        "angelogioacchino.delregno@...labora.com" 
        <angelogioacchino.delregno@...labora.com>
Subject: Re: [PATCH] zsmalloc: Fix races between modifications of fullness
 and isolated

On (23/07/26 06:59), Andrew Yang (楊智強) wrote:
> On Wed, 2023-07-26 at 12:18 +0900, Sergey Senozhatsky wrote:
> >
> > External email : Please do not click links or open attachments until
> > you have verified the sender or the content.
> >  On (23/07/21 14:37), Andrew Yang wrote:
> > >
> > > Since fullness and isolated share the same unsigned int,
> > > modifications of them should be protected by the same lock.
> > >
> > > Signed-off-by: Andrew Yang <andrew.yang@...iatek.com>
> > > Fixes: c4549b871102 ("zsmalloc: remove zspage isolation for
> > migration")
> >
> > Have you observed issues in real life? That commit is more than a
> > year
> > and a half old, so I wonder.
> >
> Yes, we encountered many kernel exceptions of
> VM_BUG_ON(zspage->isolated == 0) in dec_zspage_isolation() and
> BUG_ON(!pages[1]) in zs_unmap_object() lately.

Got it.

> This issue only occurs when migration and reclamation occur at the
> same time. With our memory stress test, we can reproduce this issue
> several times a day. We have no idea why no one else encountered
> this issue. BTW, we switched to the new kernel version with this
> defect a few months ago.

Yeah, pretty curious myself.

> > > @@ -1858,8 +1860,8 @@ static int zs_page_migrate(struct page
> > *newpage, struct page *page,
> > >   * Since we complete the data copy and set up new zspage
> > structure,
> > >   * it's okay to release the pool's lock.
> > >   */
> >
> > This comment should be moved too, because this is not where we unlock
> > the
> > pool anymore.
> >
> Okay, I will submit a new patch later.

Thank you!