On Wed, Mar 28, 2018 at 01:30:20PM +0300, Kirill Tkhai wrote: > On 27.03.2018 18:48, Vladimir Davydov wrote: > > On Tue, Mar 27, 2018 at 06:09:20PM +0300, Kirill Tkhai wrote: > >>>>>> diff --git a/mm/vmscan.c b/mm/vmscan.c > >>>>>> index 8fcd9f8d7390..91b5120b924f 100644 > >>>>>> --- a/mm/vmscan.c > >>>>>> +++ b/mm/vmscan.c > >>>>>> @@ -159,6 +159,56 @@ unsigned long vm_total_pages; > >>>>>> static LIST_HEAD(shrinker_list); > >>>>>> static DECLARE_RWSEM(shrinker_rwsem); > >>>>>> > >>>>>> +#if defined(CONFIG_MEMCG) && !defined(CONFIG_SLOB) > >>>>>> +static DEFINE_IDA(bitmap_id_ida); > >>>>>> +static DECLARE_RWSEM(bitmap_rwsem); > >>>>> > >>>>> Can't we reuse shrinker_rwsem for protecting the ida? > >>>> > >>>> I think it won't be better, since we allocate memory under this semaphore. > >>>> After we use shrinker_rwsem, we'll have to allocate the memory with GFP_ATOMIC, > >>>> which does not seems good. Currently, the patchset makes shrinker_rwsem be taken > >>>> for a small time, just to assign already allocated memory to maps. > >>> > >>> AFAIR it's OK to sleep under an rwsem so GFP_ATOMIC wouldn't be > >>> necessary. Anyway, we only need to allocate memory when we extend > >>> shrinker bitmaps, which is rare. In fact, there can only be a limited > >>> number of such calls, as we never shrink these bitmaps (which is fine > >>> by me). > >> > >> We take bitmap_rwsem for writing to expand shrinkers maps. If we replace > >> it with shrinker_rwsem and the memory allocation get into reclaim, there > >> will be deadlock. > > > > Hmm, AFAICS we use down_read_trylock() in shrink_slab() so no deadlock > > would be possible. We wouldn't be able to reclaim slabs though, that's > > true, but I don't think it would be a problem for small allocations. > > > > That's how I see this. We use shrinker_rwsem to protect IDR mapping > > shrink_id => shrinker (I still insist on IDR). It may allocate, but the > > allocation size is going to be fairly small so it's OK that we don't > > call shrinkers there. After we allocated a shrinker ID, we release > > shrinker_rwsem and call mem_cgroup_grow_shrinker_map (or whatever it > > will be called), which checks if per-memcg shrinker bitmaps need growing > > and if they do it takes its own mutex used exclusively for protecting > > the bitmaps and reallocates the bitmaps (we will need the mutex anyway > > to synchronize css_online vs shrinker bitmap reallocation as the > > shrinker_rwsem is private to vmscan.c and we don't want to export it > > to memcontrol.c). > > But what the profit of prohibiting reclaim during shrinker id allocation? > In case of this is a IDR, it still may require 1 page, and still may get > in after fast reclaim. If we prohibit reclaim, we'll fail to register > the shrinker. > > It's not a rare situation, when all the memory is occupied by page cache. shrinker_rwsem doesn't block page cache reclaim, only dcache reclaim. I don't think that dcache can occupy all available memory. > So, we will fail to mount something in some situation. > > What the advantages do we have to be more significant, than this disadvantage? The main advantage is code simplicity.
