Re: [PATCH v3 0/6] hugetlb_cgroup: Add hugetlb_cgroup reservation limits

From: Mina Almasry <almasrymina@google.com>
To: Mike Kravetz <mike.kravetz@oracle.com>
Cc: "Michal Hocko" <mhocko@kernel.org>, shuah <shuah@kernel.org>,
	"David Rientjes" <rientjes@google.com>,
	"Shakeel Butt" <shakeelb@google.com>,
	"Greg Thelen" <gthelen@google.com>,
	"Andrew Morton" <akpm@linux-foundation.org>,
	khalid.aziz@oracle.com,
	"open list" <linux-kernel@vger.kernel.org>,
	linux-mm@kvack.org, linux-kselftest@vger.kernel.org,
	cgroups@vger.kernel.org,
	"Aneesh Kumar" <aneesh.kumar@linux.vnet.ibm.com>,
	"Michal Koutný" <mkoutny@suse.com>, "Tejun Heo" <tj@kernel.org>,
	"Johannes Weiner" <hannes@cmpxchg.org>,
	"Li Zefan" <lizefan@huawei.com>
Subject: Re: [PATCH v3 0/6] hugetlb_cgroup: Add hugetlb_cgroup reservation limits
Date: Thu, 5 Sep 2019 12:55:11 -0700	[thread overview]
Message-ID: <CAHS8izP=8WDvZvTjenX5CtdKfYTbOO+bU7oK1Nx=r7QZrBjpaw@mail.gmail.com> (raw)
In-Reply-To: <cb7ebcce-05c5-3384-5632-2bbac9995c15@oracle.com>

On Tue, Sep 3, 2019 at 10:58 AM Mike Kravetz <mike.kravetz@oracle.com> wrote:
>
> On 8/29/19 12:18 AM, Michal Hocko wrote:
> > [Cc cgroups maintainers]
> >
> > On Wed 28-08-19 10:58:00, Mina Almasry wrote:
> >> On Wed, Aug 28, 2019 at 4:23 AM Michal Hocko <mhocko@kernel.org> wrote:
> >>>
> >>> On Mon 26-08-19 16:32:34, Mina Almasry wrote:
> >>>>  mm/hugetlb.c                                  | 493 ++++++++++++------
> >>>>  mm/hugetlb_cgroup.c                           | 187 +++++--
> >>>
> >>> This is a lot of changes to an already subtle code which hugetlb
> >>> reservations undoubly are.
> >>
> >> For what it's worth, I think this patch series is a net decrease in
> >> the complexity of the reservation code, especially the region_*
> >> functions, which is where a lot of the complexity lies. I removed the
> >> race between region_del and region_{add|chg}, refactored the main
> >> logic into smaller code, moved common code to helpers and deleted the
> >> duplicates, and finally added lots of comments to the hard to
> >> understand pieces. I hope that when folks review the changes they will
> >> see that! :)
> >
> > Post those improvements as standalone patches and sell them as
> > improvements. We can talk about the net additional complexity of the
> > controller much easier then.
>
> All such changes appear to be in patch 4 of this series.  The commit message
> says "region_add() and region_chg() are heavily refactored to in this commit
> to make the code easier to understand and remove duplication.".  However, the
> modifications were also added to accommodate the new cgroup reservation
> accounting.  I think it would be helpful to explain why the existing code does
> not work with the new accounting.  For example, one change is because
> "existing code coalesces resv_map entries for shared mappings.  new cgroup
> accounting requires that resv_map entries be kept separate for proper
> uncharging."
>
> I am starting to review the changes, but it would help if there was a high
> level description.  I also like Michal's idea of calling out the region_*
> changes separately.  If not a standalone patch, at least the first patch of
> the series.  This new code will be exercised even if cgroup reservation
> accounting not enabled, so it is very important than no subtle regressions
> be introduced.
>

Yep, seems I'm not calling out these changes as clearly as I should.
I'll look into breaking them into separate patches. I'll probably put
them as a separate patch or right behind current patchset 4, since
they are really done to make removing the coalescing a bit easier. Let
me look into that.

> --
> Mike Kravetz