linux-kernel - Re: [PATCH v12 4/9] hugetlb: disable region_add file

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Date:   Sat, 15 Feb 2020 17:25:13 -0800 (PST)
From:   David Rientjes <rientjes@...gle.com>
To:     Mina Almasry <almasrymina@...gle.com>
cc:     mike.kravetz@...cle.com, shuah@...nel.org, shakeelb@...gle.com,
        gthelen@...gle.com, Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-kselftest@...r.kernel.org, cgroups@...r.kernel.org
Subject: Re: [PATCH v12 4/9] hugetlb: disable region_add file_region
 coalescing

On Tue, 11 Feb 2020, Mina Almasry wrote:

> A follow up patch in this series adds hugetlb cgroup uncharge info the
> file_region entries in resv->regions. The cgroup uncharge info may
> differ for different regions, so they can no longer be coalesced at
> region_add time. So, disable region coalescing in region_add in this
> patch.
> 
> Behavior change:
> 
> Say a resv_map exists like this [0->1], [2->3], and [5->6].
> 
> Then a region_chg/add call comes in region_chg/add(f=0, t=5).
> 
> Old code would generate resv->regions: [0->5], [5->6].
> New code would generate resv->regions: [0->1], [1->2], [2->3], [3->5],
> [5->6].
> 
> Special care needs to be taken to handle the resv->adds_in_progress
> variable correctly. In the past, only 1 region would be added for every
> region_chg and region_add call. But now, each call may add multiple
> regions, so we can no longer increment adds_in_progress by 1 in region_chg,
> or decrement adds_in_progress by 1 after region_add or region_abort. Instead,
> region_chg calls add_reservation_in_range() to count the number of regions
> needed and allocates those, and that info is passed to region_add and
> region_abort to decrement adds_in_progress correctly.
> 
> We've also modified the assumption that region_add after region_chg
> never fails. region_chg now pre-allocates at least 1 region for
> region_add. If region_add needs more regions than region_chg has
> allocated for it, then it may fail.
> 
> Signed-off-by: Mina Almasry <almasrymina@...gle.com>

Acked-by: David Rientjes <rientjes@...gle.com>