From: Mel Gorman <mgorman@techsingularity.net>
To: Vlastimil Babka <vbabka@suse.cz>
Cc: Andrew Morton <akpm@linux-foundation.org>,
Yury Norov <yury.norov@gmail.com>,
Andrea Arcangeli <aarcange@redhat.com>,
David Rientjes <rientjes@google.com>,
Michal Hocko <mhocko@kernel.org>,
Will Deacon <will.deacon@arm.com>,
Catalin Marinas <catalin.marinas@arm.com>,
linux-kernel@vger.kernel.org,
linux-arm-kernel@lists.infradead.org, linux-mm@kvack.org
Subject: Re: [PATCH] mm, page_alloc: Fix a division by zero error when boosting watermarks
Date: Wed, 13 Feb 2019 14:15:18 +0000 [thread overview]
Message-ID: <20190213141518.GS9565@techsingularity.net> (raw)
In-Reply-To: <295be99c-d09a-5572-fa49-2673a62c295b@suse.cz>
On Wed, Feb 13, 2019 at 02:42:36PM +0100, Vlastimil Babka wrote:
> On 2/13/19 2:19 PM, Mel Gorman wrote:
> > Yury Norov reported that an arm64 KVM instance could not boot since after
> > v5.0-rc1 and could addressed by reverting the patches
> >
> > 1c30844d2dfe272d58c ("mm: reclaim small amounts of memory when an external
> > 73444bc4d8f92e46a20 ("mm, page_alloc: do not wake kswapd with zone lock held")
> >
> > The problem is that a division by zero error is possible if boosting occurs
> > either very early in boot or if the high watermark is very small. This
> > patch checks for the conditions and avoids boosting in those cases.
>
> Hmm is it really a division by zero? The following line sets max_boost to
> pageblock_nr_pages if it's zero. And where would the division happen anyway?
>
> So I wonder what's going on, your patch should AFAICS only take effect when
> zone->_watermark[WMARK_HIGH] is 0 or 1 to begin with, otherwise max_boost is at
> least 2?
>
The issue can occur if pageblock_nr_pages is also zero or not yet
initialised. It means the changelog is misleading because it has to
trigger very early in boot as happened with Yury.
> Also upon closer look, I think that (prior to the patch), boost_watermark()
> could be reduced (thanks to the max+min capping) to
>
> zone->watermark_boost = pageblock_nr_pages
>
I don't think it's worth being fancy about it if we're hitting
fragmentation issues that early in boot.
--
Mel Gorman
SUSE Labs
WARNING: multiple messages have this Message-ID (diff)
From: Mel Gorman <mgorman@techsingularity.net>
To: Vlastimil Babka <vbabka@suse.cz>
Cc: Andrea Arcangeli <aarcange@redhat.com>,
Yury Norov <yury.norov@gmail.com>,
Catalin Marinas <catalin.marinas@arm.com>,
Will Deacon <will.deacon@arm.com>,
linux-kernel@vger.kernel.org, Michal Hocko <mhocko@kernel.org>,
linux-mm@kvack.org, David Rientjes <rientjes@google.com>,
Andrew Morton <akpm@linux-foundation.org>,
linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH] mm, page_alloc: Fix a division by zero error when boosting watermarks
Date: Wed, 13 Feb 2019 14:15:18 +0000 [thread overview]
Message-ID: <20190213141518.GS9565@techsingularity.net> (raw)
In-Reply-To: <295be99c-d09a-5572-fa49-2673a62c295b@suse.cz>
On Wed, Feb 13, 2019 at 02:42:36PM +0100, Vlastimil Babka wrote:
> On 2/13/19 2:19 PM, Mel Gorman wrote:
> > Yury Norov reported that an arm64 KVM instance could not boot since after
> > v5.0-rc1 and could addressed by reverting the patches
> >
> > 1c30844d2dfe272d58c ("mm: reclaim small amounts of memory when an external
> > 73444bc4d8f92e46a20 ("mm, page_alloc: do not wake kswapd with zone lock held")
> >
> > The problem is that a division by zero error is possible if boosting occurs
> > either very early in boot or if the high watermark is very small. This
> > patch checks for the conditions and avoids boosting in those cases.
>
> Hmm is it really a division by zero? The following line sets max_boost to
> pageblock_nr_pages if it's zero. And where would the division happen anyway?
>
> So I wonder what's going on, your patch should AFAICS only take effect when
> zone->_watermark[WMARK_HIGH] is 0 or 1 to begin with, otherwise max_boost is at
> least 2?
>
The issue can occur if pageblock_nr_pages is also zero or not yet
initialised. It means the changelog is misleading because it has to
trigger very early in boot as happened with Yury.
> Also upon closer look, I think that (prior to the patch), boost_watermark()
> could be reduced (thanks to the max+min capping) to
>
> zone->watermark_boost = pageblock_nr_pages
>
I don't think it's worth being fancy about it if we're hitting
fragmentation issues that early in boot.
--
Mel Gorman
SUSE Labs
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http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
next prev parent reply other threads:[~2019-02-13 14:15 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-02-13 13:19 [PATCH] mm, page_alloc: Fix a division by zero error when boosting watermarks Mel Gorman
2019-02-13 13:19 ` Mel Gorman
2019-02-13 13:20 ` Will Deacon
2019-02-13 13:20 ` Will Deacon
2019-02-13 13:42 ` Vlastimil Babka
2019-02-13 13:42 ` Vlastimil Babka
2019-02-13 14:15 ` Mel Gorman [this message]
2019-02-13 14:15 ` Mel Gorman
2019-02-13 14:30 ` [PATCH] mm, page_alloc: Fix a division by zero error when boosting watermarks v2 Mel Gorman
2019-02-13 14:30 ` Mel Gorman
2019-02-13 14:31 ` Vlastimil Babka
2019-02-13 14:31 ` Vlastimil Babka
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