On Tue, 22 Dec 2020, 03:11 Song Bao Hua (Barry Song), <song.bao.hua@xxxxxxxxxxxxx> wrote: > > > > > -----Original Message----- > > From: Song Bao Hua (Barry Song) > > Sent: Tuesday, December 22, 2020 3:03 PM > > To: 'Vitaly Wool' <vitaly.wool@xxxxxxxxxxxx> > > Cc: Shakeel Butt <shakeelb@xxxxxxxxxx>; Minchan Kim <minchan@xxxxxxxxxx>; Mike > > Galbraith <efault@xxxxxx>; LKML <linux-kernel@xxxxxxxxxxxxxxx>; linux-mm > > <linux-mm@xxxxxxxxx>; Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>; > > NitinGupta <ngupta@xxxxxxxxxx>; Sergey Senozhatsky > > <sergey.senozhatsky.work@xxxxxxxxx>; Andrew Morton > > <akpm@xxxxxxxxxxxxxxxxxxxx>; tiantao (H) <tiantao6@xxxxxxxxxxxxx> > > Subject: RE: [PATCH] zsmalloc: do not use bit_spin_lock > > > > > > > I'm still not convinced. Will kmap what, src? At this point src might become > > just a bogus pointer. > > > > As long as the memory is still there, we can kmap it by its page struct. But > > if > > it is not there anymore, we have no way. > > > > > Why couldn't the object have been moved somewhere else (due to the compaction > > mechanism for instance) > > > at the time DMA kicks in? > > > > So zs_map_object() will guarantee the src won't be moved by holding those > > preemption-disabled lock? > > If so, it seems we have to drop the MOVABLE gfp in zswap for zsmalloc case? > > > > Or we can do get_page() to avoid the movement of the page. I would like to discuss this more in zswap context than zsmalloc's. Since zsmalloc does not implement reclaim callback, using it in zswap is a corner case anyway. zswap, on the other hand, may be dealing with some new backends in future which have more chances to become mainstream. Imagine typical NUMA-like cases, i. e. a zswap pool allocated in some kind SRAM, or in unused video memory. In such a case if you try to use a pointer to an invalidated zpool mapping, you are on the way to thrash the system. So: no assumptions that the zswap pool is in regular linear RAM should be made. ~Vitaly > > > > > > > > > > > > > ~Vitaly > > > > > > > Thanks > > Barry > >
