linux-kernel - Re: [PATCHv1 0/6] zsmalloc: preemptible object mapping

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Message-ID: <vbw33jnoxvwiuirh3474z7vmjfnai4xowrwzss3v7hknhpte7y@txfwwucv2ljg>
Date: Thu, 30 Jan 2025 12:13:30 +0900
From: Sergey Senozhatsky <senozhatsky@...omium.org>
To: Yosry Ahmed <yosry.ahmed@...ux.dev>
Cc: Sergey Senozhatsky <senozhatsky@...omium.org>, 
	Andrew Morton <akpm@...ux-foundation.org>, Minchan Kim <minchan@...nel.org>, 
	Johannes Weiner <hannes@...xchg.org>, Nhat Pham <nphamcs@...il.com>, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCHv1 0/6] zsmalloc: preemptible object mapping

On (25/01/29 15:53), Yosry Ahmed wrote:
> On Wed, Jan 29, 2025 at 03:43:46PM +0900, Sergey Senozhatsky wrote:
> > This is Part II of the series [1] that makes zram read() and write()
> > preemptible.  This part focuses only zsmalloc because zsmalloc imposes
> > atomicity restrictions on its users.  One notable example is object
> > mapping API, which returns with:
> > a) local CPU lock held
> > b) zspage rwlock held
> > 
> > First, zsmalloc is converted to use sleepable RW-"lock" (it's atomic_t
> > in fact) for zspage migration protection.  Second, a new handle mapping
> > is introduced which doesn't use per-CPU buffers (and hence no local CPU
> > lock), does fewer memcpy() calls, but requires users to provide a
> > pointer to temp buffer for object copy-in (when needed).  Third, zram is
> > converted to the new zsmalloc mapping API and thus zram read() becomes
> > preemptible.
> > 
> > [1] https://lore.kernel.org/linux-mm/20250127072932.1289973-1-senozhatsky@chromium.org
> > 
> > RFC -> v1:
> > - Only zspage->lock (leaf-lock for zs_map_object()) is converted
> >   to a preemptible lock.  The rest of the zspool locks remain the
> >   same (Yosry hated with passion the fact that in RFC series all
> >   zspool looks would become preemptible).
> 
> Hated is a big word here, I was merely concerned about how the locking
> changes would affect performance :P

Yeah I'm just messing around :)