Re: + mm-zsmalloc-dont-hold-locks-of-all-pages-when-free_zspage.patch added to mm-unstable branch

[Sergey Senozhatsky <senozhatsky@chromium.org>]

On (24/02/27 10:04), Andrew Morton wrote:
> The patch titled
>      Subject: mm/zsmalloc: don't hold locks of all pages when free_zspage()
> has been added to the -mm mm-unstable branch.  Its filename is
>      mm-zsmalloc-dont-hold-locks-of-all-pages-when-free_zspage.patch
> 
> This patch will shortly appear at
>      https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-zsmalloc-dont-hold-locks-of-all-pages-when-free_zspage.patch
> 
> This patch will later appear in the mm-unstable branch at
>     git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
> 
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> 
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> 
> The -mm tree is included into linux-next via the mm-everything
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> 
> ------------------------------------------------------
> From: Chengming Zhou <zhouchengming@bytedance.com>
> Subject: mm/zsmalloc: don't hold locks of all pages when free_zspage()
> Date: Tue, 27 Feb 2024 03:02:54 +0000
> 
> Patch series "mm/zsmalloc: simplify synchronization between
> zs_page_migrate() and free_zspage()".
> 
> free_zspage() has to hold locks of all pages, since zs_page_migrate() path
> rely on this page lock to protect the race between zs_free() and it, so it
> can safely get zspage from page->private.
> 
> But this way is not good and simple enough:
> 
> 1. Since zs_free() couldn't be sleepable, it can only trylock pages,
>    or has to kick_deferred_free() to defer that to a work.
> 
> 2. Even in the worker context, async_free_zspage() can't simply
>    lock all pages in lock_zspage(), it's still trylock because of
>    the race between zs_free() and zs_page_migrate(). Please see
>    the commit 2505a981114d ("zsmalloc: fix races between asynchronous
>    zspage free and page migration") for details.
> 
> Actually, all free_zspage() needs is to get zspage from page safely, we
> can use RCU to achieve it easily.  Then free_zspage() don't need to hold
> locks of all pages, so don't need the deferred free mechanism at all. 
> This patchset implements it and remove all of deferred free related code.
> 
> 
> This patch (of 2):
> 
> free_zspage() has to hold locks of all pages, since zs_page_migrate() path
> rely on this page lock to protect the race between zs_free() and it, so it
> can safely get zspage from page->private.

Andrew, please drop this series as it's NULL-deref crashing zsmalloc
(as far as I can tell).