linux-kernel - Re: mm, slab/slub: Ensure kmem_cache_alloc

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Message-ID: <87h6vu1l6d.ffs@tglx>
Date:   Fri, 10 Feb 2023 00:19:38 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Vlastimil Babka <vbabka@...e.cz>,
        kernel test robot <oliver.sang@...el.com>,
        Shanker Donthineni <sdonthineni@...dia.com>,
        oe-lkp@...ts.linux.dev, lkp@...el.com,
        linux-kernel@...r.kernel.org, Marc Zyngier <maz@...nel.org>,
        Michael Walle <michael@...le.cc>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Hans de Goede <hdegoede@...hat.com>,
        Wolfram Sang <wsa+renesas@...g-engineering.com>,
        linux-mm@...ck.org, "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        David Rientjes <rientjes@...gle.com>,
        Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Hyeonggon Yoo <42.hyeyoo@...il.com>,
        Roman Gushchin <roman.gushchin@...ux.dev>
Subject: Re: mm, slab/slub: Ensure kmem_cache_alloc_bulk() is available early

On Thu, Feb 09 2023 at 20:28, Matthew Wilcox wrote:
> On Wed, Feb 08, 2023 at 09:46:30PM +0100, Thomas Gleixner wrote:
>> The issue is that there might be maple_tree users which depend on
>> GFP_ATOMIC, but call in from interrupt enabled context, which is
>> legitimate today.
>> 
>> Willy might have some insight on that.
>
> Not today, but eventually.  There are XArray users which modify the tree
> in interrupt context or under some other spinlock that we can't drop
> for them in order to do an allocation.  And I want to replace the radix
> tree underpinnings of the XArray with the maple tree.  In my copious
> spare time.

If any usage which you described, i.e. interrupt context or with a
spinlock held, where interrupts were disabled on acquisition of the
lock, ends up calling into kmem_cache_alloc_bulk() today, then that's
broken because kmem_cache_alloc_bulk() reenables interrupts
unconditionally.

So either such code does not exist as of today or it just gets lucky to
not run into the code path leading up to kmem_cache_alloc_bulk().

We have to clarify what the valid calling convention of
kmem_cache_alloc_bulk() is in the regular kernel context, i.e. outside
of early boot.

Thanks,

        tglx