On Mon, Sep 16, 2019 at 4:57 PM Mike Kravetz <mike.kravetz@xxxxxxxxxx> 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
