linux-kernel - Re: [PATCH] mm/util.c: Make kvfree() safe for calling while holding spinlocks

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Date:   Thu, 27 Jan 2022 16:54:15 +0100
From:   Uladzislau Rezki <urezki@...il.com>
To:     Michal Hocko <mhocko@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Manfred Spraul <manfred@...orfullife.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Vasily Averin <vvs@...tuozzo.com>, cgel.zte@...il.com,
        shakeelb@...gle.com, rdunlap@...radead.org, dbueso@...e.de,
        unixbhaskar@...il.com, chi.minghao@....com.cn, arnd@...db.de,
        Zeal Robot <zealci@....com.cn>, linux-mm@...ck.org,
        1vier1@....de, stable@...r.kernel.org
Subject: Re: [PATCH] mm/util.c: Make kvfree() safe for calling while holding
 spinlocks

On Thu, Jan 27, 2022 at 09:25:48AM +0100, Michal Hocko wrote:
> On Thu 27-01-22 06:59:50, Manfred Spraul wrote:
> > Hi Andrew,
> > 
> > On 1/27/22 03:53, Andrew Morton wrote:
> > > On Wed, 22 Dec 2021 20:48:28 +0100 Manfred Spraul <manfred@...orfullife.com> wrote:
> > > 
> > > > One codepath in find_alloc_undo() calls kvfree() while holding a spinlock.
> > > > Since vfree() can sleep this is a bug.
> > > > 
> > > > Previously, the code path used kfree(), and kfree() is safe to be called
> > > > while holding a spinlock.
> > > > 
> > > > Minghao proposed to fix this by updating find_alloc_undo().
> > > > 
> > > > Alternate proposal to fix this: Instead of changing find_alloc_undo(),
> > > > change kvfree() so that the same rules as for kfree() apply:
> > > > Having different rules for kfree() and kvfree() just asks for bugs.
> > > > 
> > > > Disadvantage: Releasing vmalloc'ed memory will be delayed a bit.
> > > I know we've been around this loop a bunch of times and deferring was
> > > considered.   But I forget the conclusion.  IIRC, mhocko was involved?
> > 
> > I do not remember a mail from mhocko.
> 
> I do not remember either.
> 
> > 
> > Shakeel proposed to use the approach from Chi.
> > 
> > Decision: https://marc.info/?l=linux-kernel&m=164132032717757&w=2
> 
> And I would agree with Shakeel and go with the original change to the
> ipc code. That is trivial and without any other side effects like this
> one. I bet nobody has evaluated what the undconditional deferred freeing
> has. At least changelog doesn't really dive into that more than a very
> vague statement that this will happen.
>
Absolutely agree here. Especially that changing the kvfree() will not
look stable.

After applying the https://www.spinics.net/lists/linux-mm/msg282264.html
we will be able to use vfree() from atomic anyway.

--
Vlad Rezki