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

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Date:   Wed, 8 Feb 2023 10:15:45 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Thomas Gleixner <tglx@...utronix.de>,
        kernel test robot <oliver.sang@...el.com>,
        Shanker Donthineni <sdonthineni@...dia.com>
Cc:     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>,
        Matthew Wilcox <willy@...radead.org>,
        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 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@...e.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.
Some maple tree users could use its preallocation instead outside of the
atomic context, when possible.

> Thanks,
> 
>         tglx