linux-kernel - RE: [PATCH] zsmalloc: do not use bit_spin

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Date:   Tue, 22 Dec 2020 02:10:42 +0000
From:   "Song Bao Hua (Barry Song)" <song.bao.hua@...ilicon.com>
To:     Vitaly Wool <vitaly.wool@...sulko.com>
CC:     Shakeel Butt <shakeelb@...gle.com>,
        Minchan Kim <minchan@...nel.org>,
        "Mike Galbraith" <efault@....de>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        NitinGupta <ngupta@...are.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "tiantao (H)" <tiantao6@...ilicon.com>
Subject: RE: [PATCH] zsmalloc: do not use bit_spin_lock



> -----Original Message-----
> From: Song Bao Hua (Barry Song)
> Sent: Tuesday, December 22, 2020 3:03 PM
> To: 'Vitaly Wool' <vitaly.wool@...sulko.com>
> Cc: Shakeel Butt <shakeelb@...gle.com>; Minchan Kim <minchan@...nel.org>; Mike
> Galbraith <efault@....de>; LKML <linux-kernel@...r.kernel.org>; linux-mm
> <linux-mm@...ck.org>; Sebastian Andrzej Siewior <bigeasy@...utronix.de>;
> NitinGupta <ngupta@...are.org>; Sergey Senozhatsky
> <sergey.senozhatsky.work@...il.com>; Andrew Morton
> <akpm@...ux-foundation.org>; tiantao (H) <tiantao6@...ilicon.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.

> >
> > >
> > > ~Vitaly
> >
> 
> Thanks
> Barry