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Date: Thu, 6 Jun 2024 22:03:33 -0700
From: Christoph Hellwig <hch@infradead.org>
To: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Martin Oliveira <martin.oliveira@eideticom.com>,
	linux-rdma@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-pci@vger.kernel.org, linux-mm@kvack.org,
	Jason Gunthorpe <jgg@ziepe.ca>, Leon Romanovsky <leon@kernel.org>,
	Bjorn Helgaas <bhelgaas@google.com>,
	Logan Gunthorpe <logang@deltatee.com>, Tejun Heo <tj@kernel.org>,
	"Rafael J. Wysocki" <rafael@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Mike Marciniszyn <mike.marciniszyn@intel.com>,
	Michael Guralnik <michaelgur@nvidia.com>,
	Dan Williams <dan.j.williams@intel.com>,
	Ard Biesheuvel <ardb@kernel.org>,
	Valentine Sinitsyn <valesini@yandex-team.ru>,
	Lukas Wunner <lukas@wunner.de>
Subject: Re: [PATCH 1/6] kernfs: create vm_operations_struct without
 page_mkwrite()
Message-ID: <ZmKUpXQmMLpH8vf5@infradead.org>
References: <20240605192934.742369-1-martin.oliveira@eideticom.com>
 <20240605192934.742369-2-martin.oliveira@eideticom.com>
 <2024060658-ember-unblessed-4c74@gregkh>
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In-Reply-To: <2024060658-ember-unblessed-4c74@gregkh>

On Thu, Jun 06, 2024 at 10:54:06PM +0200, Greg Kroah-Hartman wrote:
> On Wed, Jun 05, 2024 at 01:29:29PM -0600, Martin Oliveira wrote:
> > The standard kernfs vm_ops installs a page_mkwrite() operator which
> > modifies the file update time on write.
> > 
> > This not always required (or makes sense), such as in the P2PDMA, which
> > uses the sysfs file as an allocator from userspace.
> 
> That's not a good idea, please don't do that.  sysfs binary files are
> "pass through", why would you want to use this as an allocator?

I think the real question is why sysfs binary files implement
page_mkwrite by default.  page_mkwrite is needed for file systems that
need to allocate space from a free space pool, which seems odd for
sysfs.