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Date: Thu, 6 Jun 2024 22:54:06 +0200
From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To: Martin Oliveira <martin.oliveira@eideticom.com>
Cc: 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: <2024060658-ember-unblessed-4c74@gregkh>
References: <20240605192934.742369-1-martin.oliveira@eideticom.com>
 <20240605192934.742369-2-martin.oliveira@eideticom.com>
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In-Reply-To: <20240605192934.742369-2-martin.oliveira@eideticom.com>

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?

> Furthermore, having the page_mkwrite() operator causes
> writable_file_mapping_allowed() to fail due to
> vma_needs_dirty_tracking() on the gup flow, which is a pre-requisite for
> enabling P2PDMA over RDMA.
> 
> Fix this by adding a new boolean on kernfs_ops to differentiate between
> the different behaviours.

This isn't going to work well.

What exactly are you wanting to do in sysfs that you feel this is
required?

thanks,

greg k-h