On Wed, 2017-01-25 at 08:32 -0500, Jeff Layton wrote:
> Currently, iov_iter_get_pages_alloc will only ever operate on the first
> vector that iterate_all_kinds hands back. Most of the callers however
> would like to have as long a set of pages as possible, to allow for
> fewer, but larger I/Os.
>
> When the previous vector ends on a page boundary and the current one
> begins on one, we can continue to add more pages.
>
> Change the function to first scan the iov_iter to see how long an
> array of pages we could create from the current position up to the
> maxsize passed in. Then, allocate an array that large and start
> filling in that many pages.
>
> The main impetus for this patch is to rip out a swath of code in ceph
> that tries to do this but that doesn't handle ITER_BVEC correctly.
>
> NFS also uses this function and this also allows the client to do larger
> I/Os when userland passes down an array of page-aligned iovecs in an
> O_DIRECT request. This should also make splice writes into an O_DIRECT
> file on NFS use larger I/Os, since that's now done by passing down an
> ITER_BVEC representing the data to be written.
>
> I believe the other callers (lustre and 9p) may also benefit from this
> change, but I don't have a great way to verify that.
>
> Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx>
> ---
> lib/iov_iter.c | 142 +++++++++++++++++++++++++++++++++++++++++++++------------
> 1 file changed, 113 insertions(+), 29 deletions(-)
>
> diff --git a/lib/iov_iter.c b/lib/iov_iter.c
> index e68604ae3ced..c87ba154371a 100644
> --- a/lib/iov_iter.c
> +++ b/lib/iov_iter.c
> @@ -883,6 +883,59 @@ unsigned long iov_iter_gap_alignment(const struct iov_iter *i)
> }
> EXPORT_SYMBOL(iov_iter_gap_alignment);
>
> +/**
> + * iov_iter_pvec_size - find length of page aligned iovecs in iov_iter
> + * @i: iov_iter to in which to find the size
> + * @maxsize: maximum size to return
> + *
> + * Some filesystems can stitch together multiple iovecs into a single page
> + * vector when both the previous tail and current base are page aligned. This
> + * function determines how much of the remaining iov_iter can be stuffed into
> + * a single pagevec, up to the provided maxsize value.
> + */
> +static size_t iov_iter_pvec_size(const struct iov_iter *i, size_t maxsize)
> +{
> + size_t size = min(iov_iter_count(i), maxsize);
> + size_t pv_size = 0;
> + bool contig = false, first = true;
> +
> + if (!size)
> + return 0;
> +
> + /* Pipes are naturally aligned for this */
> + if (unlikely(i->type & ITER_PIPE))
> + return size;
> +
> + /*
> + * An iov can be page vectored when the current base and previous
> + * tail are both page aligned. Note that we don't require that the
> + * initial base in the first iovec also be page aligned.
> + */
> + iterate_all_kinds(i, size, v,
> + ({
> + if (first || (contig && PAGE_ALIGNED(v.iov_base))) {
> + pv_size += v.iov_len;
> + first = false;
> + contig = PAGE_ALIGNED(v.iov_base + v.iov_len);
> + }; 0;
> + }),
> + ({
> + if (first || (contig && v.bv_offset == 0)) {
> + pv_size += v.bv_len;
> + first = false;
> + contig = PAGE_ALIGNED(v.bv_offset + v.bv_len);
> + }
> + }),
> + ({
> + if (first || (contig && PAGE_ALIGNED(v.iov_base))) {
> + pv_size += v.iov_len;
> + first = false;
> + contig = PAGE_ALIGNED(v.iov_base + v.iov_len);
> + }
> + }))
> + return pv_size;
> +}
> +
> static inline size_t __pipe_get_pages(struct iov_iter *i,
> size_t maxsize,
> struct page **pages,
> @@ -1006,47 +1059,78 @@ static ssize_t pipe_get_pages_alloc(struct iov_iter *i,
> }
>
> ssize_t iov_iter_get_pages_alloc(struct iov_iter *i,
> - struct page ***pages, size_t maxsize,
> - size_t *start)
> + struct page ***ppages, size_t maxsize,
> + size_t *pstart)
> {
> - struct page **p;
> -
> - if (maxsize > i->count)
> - maxsize = i->count;
> + struct page **p, **pc;
> + size_t start = 0;
> + ssize_t len = 0;
> + int npages, res = 0;
> + bool first = true;
>
> if (unlikely(i->type & ITER_PIPE))
> - return pipe_get_pages_alloc(i, pages, maxsize, start);
> + return pipe_get_pages_alloc(i, ppages, maxsize, pstart);
> +
> + maxsize = iov_iter_pvec_size(i, maxsize);
> + npages = DIV_ROUND_UP(maxsize, PAGE_SIZE);
Bah, the above is wrong when the offset into the first page is non-
zero. I'll fix -- this probably also points out the need for some tests
for this. I'll see if I can cook something up for xfstests.
> + p = get_pages_array(npages);
> + if (!p)
> + return -ENOMEM;
> +
> + pc = p;
> iterate_all_kinds(i, maxsize, v, ({
> unsigned long addr = (unsigned long)v.iov_base;
> - size_t len = v.iov_len + (*start = addr & (PAGE_SIZE - 1));
> + size_t slen = v.iov_len;
> int n;
> - int res;
>
> - addr &= ~(PAGE_SIZE - 1);
> - n = DIV_ROUND_UP(len, PAGE_SIZE);
> - p = get_pages_array(n);
> - if (!p)
> - return -ENOMEM;
> - res = get_user_pages_fast(addr, n, (i->type & WRITE) != WRITE, p);
> - if (unlikely(res < 0)) {
> - kvfree(p);
> - return res;
> + if (first) {
> + start = addr & (PAGE_SIZE - 1);
> + slen += start;
> + first = false;
> }
> - *pages = p;
> - return (res == n ? len : res * PAGE_SIZE) - *start;
> +
> + n = DIV_ROUND_UP(slen, PAGE_SIZE);
> + if (pc + n > p + npages) {
> + /* Did something change the iov array?!? */
> + res = -EFAULT;
> + goto out;
> + }
> + addr &= ~(PAGE_SIZE - 1);
> + res = get_user_pages_fast(addr, n,
> + (i->type & WRITE) != WRITE, pc);
> + if (unlikely(res < 0))
> + goto out;
> + len += (res == n ? slen : res * PAGE_SIZE) - start;
> + pc += res;
> 0;}),({
> - /* can't be more than PAGE_SIZE */
> - *start = v.bv_offset;
> - *pages = p = get_pages_array(1);
> - if (!p)
> - return -ENOMEM;
> - get_page(*p = v.bv_page);
> - return v.bv_len;
> + /* bio_vecs are limited to a single page each */
> + if (first) {
> + start = v.bv_offset;
> + first = false;
> + }
> + get_page(*pc = v.bv_page);
> + len += v.bv_len;
> + ++pc;
> + BUG_ON(pc > p + npages);
> }),({
> - return -EFAULT;
> + /* FIXME: should we handle this case? */
> + res = -EFAULT;
> + goto out;
> })
> )
> - return 0;
> +out:
> + if (unlikely(res < 0)) {
> + struct page **i;
> +
> + for (i = p; i < pc; i++)
> + put_page(*i);
> + kvfree(p);
> + return res;
> + }
> +
> + *ppages = p;
> + *pstart = start;
> + return len;
> }
> EXPORT_SYMBOL(iov_iter_get_pages_alloc);
>
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