linux-ext4 - Re: [PATCH v6 01/26] fuse: Fix dax truncate/punch

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Message-ID: <67a3fde7da328_2d2c2942b@dwillia2-xfh.jf.intel.com.notmuch>
Date: Wed, 5 Feb 2025 16:10:15 -0800
From: Dan Williams <dan.j.williams@...el.com>
To: Vivek Goyal <vgoyal@...hat.com>, Alistair Popple <apopple@...dia.com>
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 Maglione" <gmaglione@...hat.com>
Subject: Re: [PATCH v6 01/26] fuse: Fix dax truncate/punch_hole fault path

Vivek Goyal wrote:
> On Fri, Jan 10, 2025 at 05:00:29PM +1100, Alistair Popple wrote:
> > FS DAX requires file systems to call into the DAX layout prior to unlinking
> > inodes to ensure there is no ongoing DMA or other remote access to the
> > direct mapped page. The fuse file system implements
> > fuse_dax_break_layouts() to do this which includes a comment indicating
> > that passing dmap_end == 0 leads to unmapping of the whole file.
> > 
> > However this is not true - passing dmap_end == 0 will not unmap anything
> > before dmap_start, and further more dax_layout_busy_page_range() will not
> > scan any of the range to see if there maybe ongoing DMA access to the
> > range. Fix this by passing -1 for dmap_end to fuse_dax_break_layouts()
> > which will invalidate the entire file range to
> > dax_layout_busy_page_range().
> 
> Hi Alistair,
> 
> Thanks for fixing DAX related issues for virtiofs. I am wondering how are
> you testing DAX with virtiofs. AFAIK, we don't have DAX support in Rust
> virtiofsd. C version of virtiofsd used to have out of the tree patches
> for DAX. But C version got deprecated long time ago.
> 
> Do you have another implementation of virtiofsd somewhere else which
> supports DAX and allows for testing DAX related changes?

I have personally never seen a virtiofs-dax test. It sounds like you are
saying we can deprecate that support if there are no longer any users.
Or, do you expect that C-virtiofsd is alive in the ecosystem?