On 7/29/26 12:55, Christoph Hellwig wrote:
On Wed, Jul 29, 2026 at 12:45:47PM +0100, Pavel Begunkov wrote:
It was exposed in early version as I was passing a [{dma,len}, ...]
array, but we moved from that. Maybe I should put the minimum
segment size in the map structure, and (possibly over) split using
that for now? Keith had this chunk in his patches:
+ int offset = offset_in_page(bio->bi_iter.bi_bvec_done);
+
+ nsegs = ALIGN(bio->bi_iter.bi_size + offset, PAGE_SIZE) >>
+ PAGE_SHIFT;
+ if (bio->bi_iter.bi_size > max_bytes) {
+ bytes = max_bytes;
+ nsegs = (bytes + offset) >> PAGE_SHIFT;
+ } else if (nsegs > lim->max_segments) {
+ nsegs = lim->max_segments;
+ bytes = PAGE_SIZE * nsegs - offset;
+ } else {
+ *segs = nsegs;
+ return NULL;
+ }
This seems very pessimistic, especially for the case of the registration
only having a single segment, which I'd expect to be fairly common due
to P2P bar mappings, huge pages or IOMMU coalescing. So at very least
Agree that PAGE_SIZE is too pessimistic, but I'd calculate it from
the sg table on registration and use here. Not ideal but should cover
most common cases like huge pages.
we'd want to special case that, but in an idea world the caller would
be required to provide a useful nr_segments for the I/O.
Not sure we can do it in the straightforward way from the abstraction
perspective, mapping are hidden from io_uring, and there could be
multi-device files with differently shaped IOMMU mappings at some
point. We'll surely need readv/writev support later, so maybe we can
improve while doing that as it'd mean passing a {off,len} of some
sort from io_uring.
--
Pavel Begunkov
