Re: [PATCH v2 2/2] fs: generic_file_buffered_read() now uses find_get_pages_contig

Andrew Morton
2020-06-18T01:06:02Z

On Tue, 9 Jun 2020 21:36:42 -0400 Kent Overstreet <kent.overstreet@gmail.com> wrote:

> Convert generic_file_buffered_read() to get pages to read from in
> batches, and then copy data to userspace from many pages at once - in
> particular, we now don't touch any cachelines that might be contended
> while we're in the loop to copy data to userspace.
>
> This is is a performance improvement on workloads that do buffered reads
> with large blocksizes, and a very large performance improvement if that
> file is also being accessed concurrently by different threads.
>
> On smaller reads (512 bytes), there's a very small performance
> improvement (1%, within the margin of error).
>

checkpatch goes fairly crazy over this one, mostly legitimate.

> @@ -2255,6 +2194,79 @@ generic_file_buffered_read_no_cached_page(struct kiocb *iocb,
> return generic_file_buffered_read_readpage(filp, mapping, page);
> }
>
> +static int generic_file_buffered_read_get_pages(struct kiocb *iocb,
> + struct iov_iter *iter,
> + struct page **pages,
> + unsigned nr)
> +{
> + struct file *filp = iocb->ki_filp;
> + struct address_space *mapping = filp->f_mapping;
> + struct file_ra_state *ra = &filp->f_ra;
> + pgoff_t index = iocb->ki_pos >> PAGE_SHIFT;
> + pgoff_t last_index = (iocb->ki_pos + iter->count + PAGE_SIZE-1) >> PAGE_SHIFT;
> + int i, j, ret, err = 0;
> +
> + nr = min_t(unsigned long, last_index - index, nr);
> +find_page:
> + if (fatal_signal_pending(current))
> + return -EINTR;
> +
> + ret = find_get_pages_contig(mapping, index, nr, pages);
> + if (ret)
> + goto got_pages;
> +
> + if (iocb->ki_flags & IOCB_NOWAIT)
> + return -EAGAIN;
> +
> + page_cache_sync_readahead(mapping, ra, filp, index, last_index - index);
> +
> + ret = find_get_pages_contig(mapping, index, nr, pages);
> + if (ret)
> + goto got_pages;
> +
> + pages[0] = generic_file_buffered_read_no_cached_page(iocb, iter);
> + err = PTR_ERR_OR_ZERO(pages[0]);
> + ret = !IS_ERR_OR_NULL(pages[0]);

what?

> +got_pages:
> + for (i = 0; i < ret; i++) {

Comparing i with ret here just hurts my brain. Two lines ago ret was a
boolean, now it's a scalar.

> + struct page *page = pages[i];
> + pgoff_t pg_index = index +i;
> + loff_t pg_pos = max(iocb->ki_pos,
> + (loff_t) pg_index << PAGE_SHIFT);

hm. I guess we can't use max_t here because we need to cast the
pgoff_t before the << to avoid overflows on 32-bit. Perhaps this could
be cleaned up by using additional suitably typed and named locals.

> + loff_t pg_count = iocb->ki_pos + iter->count - pg_pos;
> +
> + if (PageReadahead(page))
> + page_cache_async_readahead(mapping, ra, filp, page,
> + pg_index, last_index - pg_index);
> +
> + if (!PageUptodate(page)) {
> + if (iocb->ki_flags & IOCB_NOWAIT) {
> + for (j = i; j < ret; j++)
> + put_page(pages[j]);
> + ret = i;
> + err = -EAGAIN;
> + break;
> + }
> +
> + page = generic_file_buffered_read_pagenotuptodate(filp,
> + iter, page, pg_pos, pg_count);
> + if (IS_ERR_OR_NULL(page)) {
> + for (j = i + 1; j < ret; j++)
> + put_page(pages[j]);
> + ret = i;
> + err = PTR_ERR_OR_ZERO(page);
> + break;
> + }
> + }
> + }
> +
> + if (likely(ret))
> + return ret;
> + if (err)
> + return err;
> + goto find_page;
> +}
> +
> /**
> * generic_file_buffered_read - generic file read routine
> * @iocb: the iocb to read
> @@ -2275,86 +2287,108 @@ static ssize_t generic_file_buffered_read(struct kiocb *iocb,
> struct iov_iter *iter, ssize_t written)
> {
> struct file *filp = iocb->ki_filp;
> + struct file_ra_state *ra = &filp->f_ra;
> struct address_space *mapping = filp->f_mapping;
> struct inode *inode = mapping->host;
> - struct file_ra_state *ra = &filp->f_ra;
> size_t orig_count = iov_iter_count(iter);
> - pgoff_t last_index;
> - int error = 0;
> + struct page *page_array[8], **pages;
> + unsigned nr_pages = ARRAY_SIZE(page_array);
> + unsigned read_nr_pages = ((iocb->ki_pos + iter->count + PAGE_SIZE-1) >> PAGE_SHIFT) -
> + (iocb->ki_pos >> PAGE_SHIFT);
> + int i, pg_nr, error = 0;
> + bool writably_mapped;
> + loff_t isize, end_offset;
>
> if (unlikely(iocb->ki_pos >= inode->i_sb->s_maxbytes))
> return 0;
> iov_iter_truncate(iter, inode->i_sb->s_maxbytes);
>
> - last_index = (iocb->ki_pos + iter->count + PAGE_SIZE-1) >> PAGE_SHIFT;
> -
> - for (;;) {
> - pgoff_t index = iocb->ki_pos >> PAGE_SHIFT;
> - struct page *page;
> + if (read_nr_pages > nr_pages &&
> + (pages = kmalloc_array(read_nr_pages, sizeof(void *), GFP_KERNEL)))

I agree with checkpatch!

> + nr_pages = read_nr_pages;
> + else
> + pages = page_array;
>
> + do {
> cond_resched();
>
> ...
>

Please, can we make all this code nice to read?