From:   Thomas Gleixner <tglx@linutronix.de>
To:     Vlastimil Babka <vbabka@suse.cz>,
        kernel test robot <oliver.sang@intel.com>,
        Shanker Donthineni <sdonthineni@nvidia.com>
Cc:     oe-lkp@lists.linux.dev, lkp@intel.com,
        linux-kernel@vger.kernel.org, Marc Zyngier <maz@kernel.org>,
        Michael Walle <michael@walle.cc>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Hans de Goede <hdegoede@redhat.com>,
        Wolfram Sang <wsa+renesas@sang-engineering.com>,
        linux-mm@kvack.org, "Liam R. Howlett" <Liam.Howlett@oracle.com>,
        Matthew Wilcox <willy@infradead.org>,
        David Rientjes <rientjes@google.com>,
        Christoph Lameter <cl@linux.com>,
        Pekka Enberg <penberg@kernel.org>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Hyeonggon Yoo <42.hyeyoo@gmail.com>,
        Roman Gushchin <roman.gushchin@linux.dev>,
        Matthew Wilcox <willy@infradead.org>
Subject: Re: mm, slab/slub: Ensure kmem_cache_alloc_bulk() is available early
In-Reply-To: <e3a8f265-ea15-35ba-e46e-89024e83db47@suse.cz>
References: <202302011308.f53123d2-oliver.sang@intel.com>
 <87o7qdzfay.ffs@tglx> <9a682773-df56-f36c-f582-e8eeef55d7f8@suse.cz>
 <875ycdwyx6.ffs@tglx> <871qn1wofe.ffs@tglx>
 <6c0b681e-97bc-d975-a8b9-500abdaaf0bc@suse.cz>
 <8b7762c3-02be-a5c9-1c4d-507cfb51a15c@suse.cz> <87edr1uykw.ffs@tglx>
 <e3a8f265-ea15-35ba-e46e-89024e83db47@suse.cz>
Date:   Wed, 08 Feb 2023 21:46:30 +0100
Message-ID: <878rh73mxl.ffs@tglx>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

On Wed, Feb 08 2023 at 10:15, Vlastimil Babka wrote:

Cc+ Willy

> On 2/7/23 19:20, Thomas Gleixner wrote:
>> On Tue, Feb 07 2023 at 15:47, Vlastimil Babka wrote:
>>> From 340d7c7b99f3e67780f6dec480ed1d27e6f325eb Mon Sep 17 00:00:00 2001
>>> From: Vlastimil Babka <vbabka@suse.cz>
>>> Date: Tue, 7 Feb 2023 15:34:53 +0100
>>> Subject: [PATCH] mm, slab/slub: remove notes that bulk alloc/free needs
>>>  interrupts enabled
>>>
>>> The slab functions kmem_cache_[alloc|free]_bulk() have been documented
>>> as requiring interrupts to be enabled, since their addition in 2015.
>>> It's unclear whether that was a fundamental restriction, or an attempt
>>> to save some cpu cycles by not having to save and restore the irq
>>> flags.
>> 
>> I don't think so. The restriction is rather meant to avoid huge
>> allocations in atomic context which causes latencies and also might
>> deplete the atomic reserves.
>
> Fair enough.
>
>> So I rather avoid that and enforce !ATOMIC mode despite the
>> local_irq_save/restore() change which is really only to accomodate with
>> early boot.
>
> We could add some warning then? People might use the bulk alloc unknowingly
> again e.g. via maple tree. GFP_KERNEL would warn through the existing
> warning, but e.g. GFP_ATOMIC currently not.

Correct.

> Some maple tree users could use its preallocation instead outside of the
> atomic context, when possible.

Right.

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.

Thanks,

        tglx