Re: [PATCH v4 6/9] hugetlb: disable region_add file_region coalescing

[Mina Almasry <almasrymina@google.com>]

On Mon, Sep 16, 2019 at 4:57 PM Mike Kravetz <mike.kravetz@oracle.com> wrote:
>
> On 9/10/19 4:31 PM, 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.
>
> Hate to throw more theoretical examples at you but ...
>
> Consider an existing reserv_map like [3-10]
> Then a region_chg/add call comes in region_chg/add(f=0, t=10).
> The region_chg is going to return 3 (additional reservations needed), and
> also out_regions_needed = 1 as it would want to create a region [0-3].
> Correct?
> But, there is nothing to prevent another thread from doing a region_del [5-7]
> after the region_chg and before region_add.  Correct?
> If so, it seems the region_add would need to create two regions, but there
> is only one in the cache and we would BUG in get_file_region_entry_from_cache.
> Am I reading the code correctly?
>
> The existing code wants to make sure region_add called after region_chg will
> never return error.  This is why all needed allocations were done in the
> region_chg call, and it was relatively easy to do in existing code when
> region_chg would only need one additional region at most.
>
> I'm thinking that we may have to make region_chg allocate the worst case
> number of regions (t - f)/2, OR change to the code such that region_add
> could return an error.

Yep you are right, I missed reasoning about the region_del punch hole
into the reservations case. Let me consider these 2 options.

> --
> Mike Kravetz