> -----Original Message----- > From: Vitaly Wool [mailto:vitaly.wool@xxxxxxxxxxxx] > Sent: Tuesday, December 22, 2020 10:44 PM > To: Song Bao Hua (Barry Song) <song.bao.hua@xxxxxxxxxxxxx> > 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 > > 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. I see. But it seems we still need a solution for the compatibility of zsmalloc and zswap? this will require change in either zsmalloc or zswap. or do you want to make zswap depend on !ZSMALLOC? > > 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 Thanks Barry
