The branch main has been updated by cperciva: URL: https://cgit.FreeBSD.org/src/commit/?id=b7761f1f0830fc4b4bd7a1f9f364cfb39b7f4288
commit b7761f1f0830fc4b4bd7a1f9f364cfb39b7f4288 Author: Colin Percival <[email protected]> AuthorDate: 2022-10-21 18:13:36 +0000 Commit: Colin Percival <[email protected]> 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. 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)
