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Date:   Tue, 6 Jun 2023 12:07:33 -0400
From:   "Liam R. Howlett" <Liam.Howlett@Oracle.com>
To:     "Yin, Fengwei" <fengwei.yin@intel.com>
Cc:     Hugh Dickins <hughd@google.com>,
        Peng Zhang <zhangpeng.00@bytedance.com>,
        maple-tree@lists.infradead.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, "Liu, Yujie" <yujie.liu@intel.com>,
        Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH 00/14] Reduce preallocations for maple tree
Message-ID: <20230606160733.3bljcadb3j2zc4ei@revolver>
Mail-Followup-To: "Liam R. Howlett" <Liam.Howlett@Oracle.com>,
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        Peng Zhang <zhangpeng.00@bytedance.com>,
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        linux-kernel@vger.kernel.org, "Liu, Yujie" <yujie.liu@intel.com>,
        Andrew Morton <akpm@linux-foundation.org>
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* Yin, Fengwei <fengwei.yin@intel.com> [230605 23:11]:
> 
> 
> On 6/6/2023 11:08 AM, Hugh Dickins wrote:
> > On Tue, 6 Jun 2023, Yin, Fengwei wrote:
> >> On 6/6/2023 10:41 AM, Hugh Dickins wrote:
> >>> On Mon, 5 Jun 2023, Liam R. Howlett wrote:
> >>>>
> >>>> You mean "mm: update validate_mm() to use vma iterator" here I guess.  I
> >>>> have it as a different commit id in my branch.
> >>>>
> >>>> I 'restored' some of the checking because I was able to work around not
> >>>> having the mt_dump() definition with the vma iterator.  I'm now
> >>>> wondering how wide spread CONFIG_DEBUG_VM is used and if I should not
> >>>> have added these extra checks.
> >>>
> >>> Most CONFIG_DEBUG_VM checks are quite cheap, mostly VM_BUG_ONs for
> >> Indeed. I had CONFIG_DEBUG_VM enabled and didn't see surprise perf report.
> >>
> >>
> >>> easily checked conditions.  If validate_mm() is still the kind of thing
> >>> it used to be, checking through every vma on every mmap operation, please
> >>> don't bring that into CONFIG_DEBUG_VM - it distorts performance too much,
> >>> so always used to be under a separate CONFIG_DEBUG_VM_RB instead.

Okay, I will update my patch to use CONFIG_DEBUG_VM_MAPLE_TREE for
validate_mm().

> >> So does this mean CONFIG_DEBUG_VM is allowed to be enabled for performance
> >> testing? Thanks.
> > 
> > I was going to say:
> > No, I did not mean that: I just meant that even developers not doing
> > strict performance testing still like to keep a rough eye on performance
> > changes; and historically CONFIG_DEBUG_VM has not distorted very much.
> > 
> > But then I wonder about certain distros which (wrongly or rightly) turn
> > CONFIG_DEBUG_VM on: I expect they do performance testing on their kernels.
> Fair enough. Thanks for explanation.
> 

Thanks for looking at this everyone.

Regards,
Liam