Re: git: b7761f1f0830 - main - x86/busdma: Limit reserved pages if low nsegs

On Sat, 22 Oct 2022, Colin Percival wrote:

> The branch main has been updated by cperciva:
>
> URL: https://cgit.FreeBSD.org/src/commit/?id=b7761f1f0830fc4b4bd7a1f9f364cfb39b7f4288
>
> commit b7761f1f0830fc4b4bd7a1f9f364cfb39b7f4288
> Author:     Colin Percival <cperciva@FreeBSD.org>
> AuthorDate: 2022-10-21 18:13:36 +0000
> Commit:     Colin Percival <cperciva@FreeBSD.org>
> CommitDate: 2022-10-22 05:47:33 +0000
>
>    x86/busdma: Limit reserved pages if low nsegs
>
>    When bus_dmamap_create is called, if bouncing might be required we
>    reserve enough pages for a maximum-length request, subject to the
>    MAX_BPAGES constraint (32 MB on amd64; 32 MB or 2 MB on i386
>    depending on the amount of RAM).
>
>    Since pages used for bouncing are typically non-consecutive, each
>    bounced page will typically constitute a busdma segment; as such, we
>    are unlikely to ever successfully use more pages than the nsegments
>    limit.  Limit the number of pages reserved to nsegments.

This is an interesting observation.  LinuxKPI desperately needs more
consecutive pages but can only ever use one segment.

We do have observed cases of bouncing failing due to the LinuxKPI
constraints and what busdma provided.

I wonder if this is going to make it even harder to by any chance have
consecutive pages?


I've been trying for a good year to find someone to look into that.
There's also other maths which are highly questionable in the bounce
parts (c&p and altered between archs).  In case anyone is interested
in reviewing some bits please get in contact with me.



>    On FreeBSD/Firecracker, this reduces bounce page memory consumption
>    from 32 MB to 512 kB, making VMs with 128 MB of RAM usable.
>
>    Reviewed by:    imp, mav
>    Sponsored by:   https://www.patreon.com/cperciva
>    Differential Revision:  https://reviews.freebsd.org/D37082
> ---
> sys/x86/x86/busdma_bounce.c | 1 +
> 1 file changed, 1 insertion(+)
>
> diff --git a/sys/x86/x86/busdma_bounce.c b/sys/x86/x86/busdma_bounce.c
> index faed4b7353cc..cf919c01ca64 100644
> --- a/sys/x86/x86/busdma_bounce.c
> +++ b/sys/x86/x86/busdma_bounce.c
> @@ -325,6 +325,7 @@ bounce_bus_dmamap_create(bus_dma_tag_t dmat, int flags, bus_dmamap_t *mapp)
> 		if ((dmat->bounce_flags & BUS_DMA_MIN_ALLOC_COMP) == 0 ||
> 		    (bz->map_count > 0 && bz->total_bpages < maxpages)) {
> 			pages = MAX(atop(dmat->common.maxsize), 1);
> +			pages = MIN(dmat->common.nsegments, pages);
> 			pages = MIN(maxpages - bz->total_bpages, pages);
> 			pages = MAX(pages, 1);
> 			if (alloc_bounce_pages(dmat, pages) < pages)
>

-- 
Bjoern A. Zeeb                                                     r15:7