Re: [PATCH] iov_iter: don't require contiguous pages in iov_iter_extract_bvec_pages

Jens Axboe
2024-11-01T18:00:59Z

On 11/1/24 11:05 AM, Eric Dumazet wrote:
>
> On 10/24/24 7:00 AM, Christoph Hellwig wrote:
>> From: Ming Lei <ming.lei@redhat.com>
>>
>> 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.
>>
>> Signed-off-by: Ming Lei <ming.lei@redhat.com>
>> [hch: minor cleanups, new commit log]
>> Signed-off-by: Christoph Hellwig <hch@lst.de>
>> ---
>> lib/iov_iter.c | 67 +++++++++++++++++++++++++++++++++-----------------
>> 1 file changed, 45 insertions(+), 22 deletions(-)
>>
>> diff --git a/lib/iov_iter.c b/lib/iov_iter.c
>> index 1abb32c0da50..9fc06f5fb748 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 +1686,58 @@ 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;
>> +
>> + /*
>> + * 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;
>> }
>
>
> This is causing major network regression in UDP sendfile, found by syzbot.
>
> I will release the syzbot report and this fix :
>
> diff --git a/lib/iov_iter.c b/lib/iov_iter.c
> index 65ec660c2960..e19aab1fccca 100644
> --- a/lib/iov_iter.c
> +++ b/lib/iov_iter.c
> @@ -1728,6 +1728,10 @@ static ssize_t iov_iter_extract_bvec_pages(struct iov_iter *i,
> (*pages)[k++] = bv.bv_page;
> size += bv.bv_len;
>
> + if (size > maxsize) {
> + size = maxsize;
> + break;
> + }
> if (k >= maxpages)
> break;

Thanks Eric, I've applied your patch.

--
Jens Axboe