linux-kernel - Re: [PATCH 01/16] mm/page_alloc: ensure kswapd doesn't accidentally go to sleep

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Message-ID: <YyA4Pz8hnnTsomUh@casper.infradead.org>
Date:   Tue, 13 Sep 2022 08:58:55 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Anshuman Khandual <anshuman.khandual@....com>
Cc:     Miaohe Lin <linmiaohe@...wei.com>, akpm@...ux-foundation.org,
        david@...hat.com, osalvador@...e.de, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 01/16] mm/page_alloc: ensure kswapd doesn't accidentally
 go to sleep

On Tue, Sep 13, 2022 at 12:32:50PM +0530, Anshuman Khandual wrote:
> 
> 
> On 9/9/22 14:54, Miaohe Lin wrote:
> > If ALLOC_KSWAPD is set, wake_all_kswapds() will be called to ensure
> > kswapd doesn't accidentally go to sleep. But when reserve_flags is
> > set, alloc_flags will be overwritten and ALLOC_KSWAPD is thus lost.
> > Preserve the ALLOC_KSWAPD flag in alloc_flags to ensure kswapd won't
> > go to sleep accidentally.
> 
> Currently wake_all_kswapds() gets skipped subsequently if ALLOC_KSWAPD
> is lost, but this only happens when the 'retry:' loops is taken ?

Right, but see the comment:

        /* Ensure kswapd doesn't accidentally go to sleep as long as we loop */

and that is not currently true.  I think that was an inadvertent change.
Particularly since the changelog for 0a79cdad5eb2 says "No functional
change".

> > 
> > Fixes: 0a79cdad5eb2 ("mm: use alloc_flags to record if kswapd can wake")
> > Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
> > ---
> >  mm/page_alloc.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index ba5c9402a5cb..4b97a03fa2dd 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -5147,7 +5147,8 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
> >  
> >  	reserve_flags = __gfp_pfmemalloc_flags(gfp_mask);
> >  	if (reserve_flags)
> > -		alloc_flags = gfp_to_alloc_flags_cma(gfp_mask, reserve_flags);
> > +		alloc_flags = gfp_to_alloc_flags_cma(gfp_mask, reserve_flags) |
> > +					  (alloc_flags & ALLOC_KSWAPD);
> >  
> >  	/*
> >  	 * Reset the nodemask and zonelist iterators if memory policies can be
>