linux-kernel - Re: [PATCH 04/13] fs/dax: Add dax_page

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Message-ID: <20240627053343.GD14837@lst.de>
Date: Thu, 27 Jun 2024 07:33:43 +0200
From: Christoph Hellwig <hch@....de>
To: Alistair Popple <apopple@...dia.com>
Cc: dan.j.williams@...el.com, vishal.l.verma@...el.com,
	dave.jiang@...el.com, logang@...tatee.com, bhelgaas@...gle.com,
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	linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
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	jhubbard@...dia.com, hch@....de, david@...morbit.com
Subject: Re: [PATCH 04/13] fs/dax: Add dax_page_free callback

On Thu, Jun 27, 2024 at 10:54:19AM +1000, Alistair Popple wrote:
> When a fs dax page is freed it has to notify filesystems that the page
> has been unpinned/unmapped and is free. Currently this involves
> special code in the page free paths to detect a transition of refcount
> from 2 to 1 and to call some fs dax specific code.
> 
> A future change will require this to happen when the page refcount
> drops to zero. In this case we can use the existing
> pgmap->ops->page_free() callback so wire that up for all devices that
> support FS DAX (nvdimm and virtio).

Given that ->page_ffree is only called from free_zone_device_folio
and right next to a switch on the the type, can't we just do the
wake_up_var there without the somewhat confusing indirect call that
just back in common code without any driver logic?