On Tue, Mar 04, 2025 at 05:49:51AM -0800, Christoph Hellwig wrote:

On Mon, Mar 03, 2025 at 10:03:42PM +0100, Mikulas Patocka wrote:

quoted

Swapfile does ahead of time mapping.

It does.  But it is:

 a) protected against modification by the S_SWAPFILE flag and checked
    for full allocation first
 b) something we want to get rid of because even with the above it is
    rather problematic

quoted

And I just looked at what swapfile 
does and copied the logic into dm-loop. If swapfile is not broken, how 
could dm-loop be broken?

As said above, swapfile works around the brokenness in ways that you
can't.  And just blindly copying old code without understanding it is
never a good idea.

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Would Jens Axboe agree to merge the dm-loop logic into the existing loop 
driver?

What logic?

The ahead-of-time mapping.

As said multiple times you can't do that.  The block mapping is
file system private information.

We might be able to provide an API to _request_ a stable mapping to a
file - with proper synchronization, of course.

I don't recall anyone ever trying that, it'd replace all the weird
IF_SWAPFILE() hacks and be a safe way to do these kinds of performance
optimizations.