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x86/busdma: Limit reserved pages if low nsegs

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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.

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
This commit is contained in:
Colin Percival 2022-10-21 11:13:36 -07:00
parent f585d13dd6
commit b7761f1f08
1 changed files with 1 additions and 0 deletions

1
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)