Re: uvm_swap v4

[Taylor R Campbell <riastradh%NetBSD.org@localhost>]

> Date: Mon, 6 Jul 2026 11:33:56 +0200
> From: Reinoud Zandijk <reinoud%NetBSD.org@localhost>
> 
> hereby an adjusted version that re-applies after the latest changes in
> uvm_swap.c made recently.
> 
> @@ -1405,48 +1415,60 @@ swstrategy(struct buf *bp)
>  	/*
> -	 * B_RAW here implies user i/o on /dev/drum, for which we need
> -	 * to handle encryption/decryption here.
> -	 * for swap in/out, it's handled by the caller.
> +	 * if the swap is encrypted, we encrypt before we write. in the
> +	 * callback we ensure its always decrypted (again).
>  	 */
> -	if ((bp->b_flags & B_RAW) != 0 &&
> -	    atomic_load_relaxed(&uvm_swap_encrypt)) {
> -		struct sw_physio_decrypt_context *ctx;
> +	if (atomic_load_relaxed(&uvm_swap_encrypt)) {
> ...
> +		/* get iobuf for our decryption nestio setup */
> +		nbp = getiobuf(bp->b_vp, !uvm_lwp_is_pagedaemon(curlwp));
> ...
> @@ -2083,13 +2105,6 @@ uvm_swap_io(struct vm_page **pps, int st
>  	kva = uvm_pagermapin(pps, npages, mapinflags);
>  
>  	/*
> -	 * encrypt writes in place if requested
> -	 */
> -	if (write) {
> -		uvm_swap_encrypt_pages(startslot, (void *)kva, npages);
> -	}

Do I understand correctly that this introduces a new allocation, not
just when _reading_ from swap, but also when _writing_ to swap because
we are under memory pressure and unable to allocate?

That strikes me as extremely risky, and not worth the value of what
otherwise appears to be a minor refactoring.  I think this change
needs a clearer justification.