On Fri, Oct 11, 2024 at 11:52:47AM +0800, Ming Lei wrote:
> Actually iov_iter_extract_pages() requires that there isn't gap in the
> extracted pages, so 'offset' only exists in the 1st page, then these
> pages can be mapped to one virtual(contiguous) address.
>
> All iov_iter_bvec() users only want to extract virt-contiguous pages from
> iov_iter_extract_pages() instead physical-contiguous pages.
>
> Change iov_iter_extract_bvec_pages() to extract virt-contiguous pages via
> bvec helper.
>
> This way can fill much more pages one time, instead of (often)one page from
> iov_iter_extract_pages() each time.
>
> The change is reasonable & safe since oher kind of iterators(UBUF, KVEC, ...)
> do return non physically-contiguous pages.
I had to read through the code to understand what this means. Here is
what I have written based on my understanding:
The iov_iter_extract_pages interface allows to return physically
discontiguous pages, as long as all but the first and last page
in the array are page aligned and page size. Rewrite
iov_iter_extract_bvec_pages to take advantage of that instead of only
returning ranges of physically contiguous pages.
> Fixes: a7e689dd1c06 ("block: Convert bio_iov_iter_get_pages to use iov_iter_extract_pages")
As far as I can tell the current behavior is highly suboptimal, but not
actually buggy, does this really warrant a fixes tag?
> +#define for_each_bvec_max(bvl, bio_vec, iter, start, nr_bvecs) \
> + for (iter = (start); \
> + (iter).bi_size && iter.bi_idx < nr_bvecs && \
> + ((bvl = bvec_iter_bvec((bio_vec), (iter))), 1); \
> + bvec_iter_advance_single((bio_vec), &(iter), (bvl).bv_len))
> +
And this helper just makes the code harder to read, and makes people
wonder what this undocumented _max interface does. What about the
below variant of your patch that open codes it, and throws in a few
comments to explain the logic?
diff --git a/lib/iov_iter.c b/lib/iov_iter.c
index 1abb32c0da50bc..7768f1b2006d81 100644
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -1677,8 +1677,8 @@ static ssize_t iov_iter_extract_xarray_pages(struct iov_iter *i,
}
/*
- * Extract a list of contiguous pages from an ITER_BVEC iterator. This does
- * not get references on the pages, nor does it get a pin on them.
+ * Extract a list of virtually contiguous pages from an ITER_BVEC iterator.
+ * This does not get references on the pages, nor does it get a pin on them.
*/
static ssize_t iov_iter_extract_bvec_pages(struct iov_iter *i,
struct page ***pages, size_t maxsize,
@@ -1686,35 +1686,57 @@ static ssize_t iov_iter_extract_bvec_pages(struct iov_iter *i,
iov_iter_extraction_t extraction_flags,
size_t *offset0)
{
- struct page **p, *page;
- size_t skip = i->iov_offset, offset, size;
- int k;
+ size_t skip = i->iov_offset, size = 0;
+ struct bvec_iter bi;
+ int k = 0;
- for (;;) {
- if (i->nr_segs == 0)
- return 0;
- size = min(maxsize, i->bvec->bv_len - skip);
- if (size)
- break;
+ if (i->nr_segs == 0)
+ return 0;
+
+ if (i->iov_offset == i->bvec->bv_len) {
i->iov_offset = 0;
i->nr_segs--;
i->bvec++;
skip = 0;
}
+ bi.bi_size = maxsize + skip;
+ bi.bi_bvec_done = skip;
+
+ maxpages = want_pages_array(pages, maxsize, skip, maxpages);
+
+ while (bi.bi_size && bi.bi_idx < i->nr_segs) {
+ struct bio_vec bv = bvec_iter_bvec(i->bvec, bi);
+
+ /*
+ * The iov_iter_extract_pages interface only allows an offset
+ * into the first page. Break out of the loop if we see an
+ * offset into subsequent pages, the caller will have to call
+ * iov_iter_extract_pages again for the reminder.
+ */
+ if (k) {
+ if (bv.bv_offset)
+ break;
+ } else {
+ *offset0 = bv.bv_offset;
+ }
- skip += i->bvec->bv_offset;
- page = i->bvec->bv_page + skip / PAGE_SIZE;
- offset = skip % PAGE_SIZE;
- *offset0 = offset;
+ (*pages)[k++] = bv.bv_page;
+ size += bv.bv_len;
- maxpages = want_pages_array(pages, size, offset, maxpages);
- if (!maxpages)
- return -ENOMEM;
- p = *pages;
- for (k = 0; k < maxpages; k++)
- p[k] = page + k;
+ if (k >= maxpages)
+ break;
+
+ /*
+ * Similarly, we are done when the end of the bvec doesn't align
+ * to a page boundary as that would create a hole in the
+ * returned space. The caller will handle this with another call
+ * to iov_iter_extract_pages.
+ */
+ if (bv.bv_offset + bv.bv_len != PAGE_SIZE)
+ break;
+ bvec_iter_advance_single(i->bvec, &bi, bv.bv_len);
+ }
- size = min_t(size_t, size, maxpages * PAGE_SIZE - offset);
iov_iter_advance(i, size);
return size;
}
