On Tue, 22 Dec 2020, 22:06 Song Bao Hua (Barry Song), <song.bao....@hisilicon.com> wrote: > > > > > -----Original Message----- > > From: Vitaly Wool [mailto:vitaly.w...@konsulko.com] > > Sent: Tuesday, December 22, 2020 10:44 PM > > To: Song Bao Hua (Barry Song) <song.bao....@hisilicon.com> > > Cc: Shakeel Butt <shake...@google.com>; Minchan Kim <minc...@kernel.org>; > > Mike > > Galbraith <efa...@gmx.de>; LKML <linux-kernel@vger.kernel.org>; linux-mm > > <linux...@kvack.org>; Sebastian Andrzej Siewior <bige...@linutronix.de>; > > NitinGupta <ngu...@vflare.org>; Sergey Senozhatsky > > <sergey.senozhatsky.w...@gmail.com>; Andrew Morton > > <a...@linux-foundation.org>; tiantao (H) <tiant...@hisilicon.com> > > Subject: Re: [PATCH] zsmalloc: do not use bit_spin_lock > > > > On Tue, 22 Dec 2020, 03:11 Song Bao Hua (Barry Song), > > <song.bao....@hisilicon.com> wrote: > > > > > > > > > > > > > -----Original Message----- > > > > From: Song Bao Hua (Barry Song) > > > > Sent: Tuesday, December 22, 2020 3:03 PM > > > > To: 'Vitaly Wool' <vitaly.w...@konsulko.com> > > > > Cc: Shakeel Butt <shake...@google.com>; Minchan Kim > > > > <minc...@kernel.org>; > > Mike > > > > Galbraith <efa...@gmx.de>; LKML <linux-kernel@vger.kernel.org>; linux-mm > > > > <linux...@kvack.org>; Sebastian Andrzej Siewior <bige...@linutronix.de>; > > > > NitinGupta <ngu...@vflare.org>; Sergey Senozhatsky > > > > <sergey.senozhatsky.w...@gmail.com>; Andrew Morton > > > > <a...@linux-foundation.org>; tiantao (H) <tiant...@hisilicon.com> > > > > 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?
No, I really don't think we should go that far. What if we add a flag to zpool, named like "can_sleep_mapped", and have it set for zbud/z3fold? Then zswap could go the current path if the flag is set; and if it's not set, and mutex_trylock fails, copy data from src to a temporary buffer, then unmap the handle, take the mutex, process the buffer instead of src. Not the nicest thing to do but at least it won't break anything. ~Vitaly > > 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
