Re: [PATCH v2 1/3] nvme-hwmon: Return error on kzalloc failure

From: Keith Busch <kbusch@kernel.org>
To: Serge Semin <fancer.lancer@gmail.com>
Cc: Christoph Hellwig <hch@lst.de>,
	Serge Semin <Sergey.Semin@baikalelectronics.ru>,
	Jens Axboe <axboe@kernel.dk>, Jens Axboe <axboe@fb.com>,
	Sagi Grimberg <sagi@grimberg.me>,
	Alexey Malahov <Alexey.Malahov@baikalelectronics.ru>,
	Pavel Parkhomenko <Pavel.Parkhomenko@baikalelectronics.ru>,
	Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
	linux-nvme@lists.infradead.org, linux-block@vger.kernel.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 1/3] nvme-hwmon: Return error on kzalloc failure
Date: Fri, 30 Sep 2022 08:57:02 -0600	[thread overview]
Message-ID: <YzcDvmlslPki8gBj@kbusch-mbp.dhcp.thefacebook.com> (raw)
In-Reply-To: <20220930095247.vqtdc53rr66uaiwv@mobilestation>

On Fri, Sep 30, 2022 at 12:52:47PM +0300, Serge Semin wrote:
> On Thu, Sep 29, 2022 at 05:53:43PM -0600, Keith Busch wrote:
> > On Fri, Sep 30, 2022 at 01:46:46AM +0300, Serge Semin wrote:
> > > Inability to allocate a buffer is a critical error which shouldn't be
> > > tolerated since most likely the rest of the driver won't work correctly.
> > > Thus instead of returning the zero status let's return the -ENOMEM error
> > > if the nvme_hwmon_data structure instance couldn't be created.
> > 
> 
> > Nak for this one. The hwmon is not necessary for the rest of the driver to
> > function, so having the driver detach from the device seems a bit harsh.
> 
> Even if it is as you say, neither the method semantic nor the way it's
> called imply that. Any failures except the allocation one are
> perceived as erroneous.

This is called by nvme_init_ctrl_finish(), and returns the error to
nvme_reset_work() only if it's < 0, which indicates we can't go on and the
driver unbinds.

This particular condition for hwmon is not something that prevents us from
making forward progress.

> > The
> > driver can participate in memory reclaim, so failing on a low memory condition
> > can make matters worse.
> 
> Yes it can, so can many other places in the driver utilizing kmalloc
> with just GFP_KERNEL flag passed including on the same path as the
> nvme_hwmon_init() execution. Kmalloc will make sure the reclaim is
> either finished or executed in background anyway in all cases. 

This path is in the first initialization before we've set up a namespace that
can be used as a reclaim destination.

> Don't
> really see why memory allocation failure is less worse in this case
> than in many others in the same driver especially seeing as I said

The other initialization kmalloc's are required to make forward progress toward
setting up a namespace. This one is not required.