Re: [PATCH] the dm-loop target

Kent Overstreet
2025-03-12T16:09:49Z

On Wed, Mar 12, 2025 at 07:20:11AM -0700, Christoph Hellwig wrote:
> On Wed, Mar 12, 2025 at 09:26:55AM -0400, Kent Overstreet wrote:
> > We might be able to provide an API to _request_ a stable mapping to a
> > file - with proper synchronization, of course.
>
> We already have that with the pNFS layouts.

Good point, maybe Mikulas wants to look at that if they care about
squeezing every iops...

>
> > 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.
>
> IS_SWAPFILE isn't going way, as can't allow other writers to it.
> Also asides from the that the layouts are fairly complex.
>
> The right way ahead for swap is to literally just treat it as a slightly
> special case of direct I/o that is allowed to IS_SWAPFILE files. We
> can safely do writeback to file backed folios under memory pressure,
> so we can also go through the normal file system path.

Yeah, and that gets us e.g. encrypted swap on bcachefs