Re: [PATCH v3 04/18] mm/filemap: Use THPs in generic_file_buffered_read

[Zi Yan <ziy@nvidia.com>]

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On 9 Nov 2020, at 22:36, Matthew Wilcox (Oracle) wrote:

> Add filemap_get_read_batch() which returns the THPs which represent a
> contiguous array of bytes in the file.  It also stops when encountering
> a page marked as Readahead or !Uptodate (but does return that page)
> so it can be handled appropriately by filemap_get_pages().  That lets us
> remove the loop in filemap_get_pages() and check only the last page.
>
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> Reviewed-by: Kent Overstreet <kent.overstreet@gmail.com>
> Reviewed-by: Christoph Hellwig <hch@lst.de>
> ---
>  mm/filemap.c | 122 +++++++++++++++++++++++++++++++++++----------------
>  1 file changed, 85 insertions(+), 37 deletions(-)
>
> diff --git a/mm/filemap.c b/mm/filemap.c
> index bd02820601f8..1de586eb377e 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -2176,6 +2176,51 @@ static int lock_page_for_iocb(struct kiocb *iocb, struct page *page)
>  		return lock_page_killable(page);
>  }
>
> +/*
> + * filemap_get_read_batch - Get a batch of pages for read
> + *
> + * Get a batch of pages which represent a contiguous range of bytes
> + * in the file.  No tail pages will be returned.  If @index is in the
> + * middle of a THP, the entire THP will be returned.  The last page in
> + * the batch may have Readahead set or be not Uptodate so that the
> + * caller can take the appropriate action.
> + */
> +static void filemap_get_read_batch(struct address_space *mapping,
> +		pgoff_t index, pgoff_t max, struct pagevec *pvec)
> +{
> +	XA_STATE(xas, &mapping->i_pages, index);
> +	struct page *head;
> +
> +	rcu_read_lock();
> +	for (head = xas_load(&xas); head; head = xas_next(&xas)) {
> +		if (xas_retry(&xas, head))
> +			continue;
> +		if (xas.xa_index > max || xa_is_value(head))
> +			break;
> +		if (!page_cache_get_speculative(head))
> +			goto retry;
> +
> +		/* Has the page moved or been split? */
> +		if (unlikely(head != xas_reload(&xas)))
> +			goto put_page;
> +
> +		if (!pagevec_add(pvec, head))
> +			break;
> +		if (!PageUptodate(head))
> +			break;
> +		if (PageReadahead(head))
> +			break;
> +		xas.xa_index = head->index + thp_nr_pages(head) - 1;
> +		xas.xa_offset = (xas.xa_index >> xas.xa_shift) & XA_CHUNK_MASK;
> +		continue;
> +put_page:
> +		put_page(head);
> +retry:
> +		xas_reset(&xas);
> +	}
> +	rcu_read_unlock();
> +}
> +
>  static struct page *filemap_read_page(struct kiocb *iocb, struct file *filp,
>  		struct address_space *mapping, struct page *page)
>  {
> @@ -2329,15 +2374,15 @@ static int filemap_get_pages(struct kiocb *iocb, struct iov_iter *iter,
>  	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;
> -	unsigned int nr = min_t(unsigned long, last_index - index, PAGEVEC_SIZE);
> -	int i, j, err = 0;
> +	pgoff_t last_index;
> +	int err = 0;
>
> +	last_index = DIV_ROUND_UP(iocb->ki_pos + iter->count, PAGE_SIZE);
>  find_page:
>  	if (fatal_signal_pending(current))
>  		return -EINTR;
>
> -	pvec->nr = find_get_pages_contig(mapping, index, nr, pvec->pages);
> +	filemap_get_read_batch(mapping, index, last_index, pvec);
>  	if (pvec->nr)
>  		goto got_pages;
>
> @@ -2346,29 +2391,30 @@ static int filemap_get_pages(struct kiocb *iocb, struct iov_iter *iter,
>
>  	page_cache_sync_readahead(mapping, ra, filp, index, last_index - index);
>
> -	pvec->nr = find_get_pages_contig(mapping, index, nr, pvec->pages);
> +	filemap_get_read_batch(mapping, index, last_index, pvec);
>  	if (pvec->nr)
>  		goto got_pages;
>
>  	pvec->pages[0] = filemap_create_page(iocb, iter);
>  	err = PTR_ERR_OR_ZERO(pvec->pages[0]);
> -	if (!IS_ERR_OR_NULL(pvec->pages[0]))
> -		pvec->nr = 1;
> +	if (IS_ERR_OR_NULL(pvec->pages[0]))
> +		goto err;
> +	pvec->nr = 1;
> +	return 0;
>  got_pages:
> -	for (i = 0; i < pvec->nr; i++) {
> -		struct page *page = pvec->pages[i];
> -		pgoff_t pg_index = index + i;
> +	{
> +		struct page *page = pvec->pages[pvec->nr - 1];
> +		pgoff_t pg_index = page->index;
>  		loff_t pg_pos = max(iocb->ki_pos,
>  				    (loff_t) pg_index << PAGE_SHIFT);
>  		loff_t pg_count = iocb->ki_pos + iter->count - pg_pos;
>
>  		if (PageReadahead(page)) {
>  			if (iocb->ki_flags & IOCB_NOIO) {
> -				for (j = i; j < pvec->nr; j++)
> -					put_page(pvec->pages[j]);
> -				pvec->nr = i;
> +				put_page(page);
> +				pvec->nr--;
>  				err = -EAGAIN;
> -				break;
> +				goto err;
>  			}
>  			page_cache_async_readahead(mapping, ra, filp, page,
>  					pg_index, last_index - pg_index);
> @@ -2376,26 +2422,23 @@ static int filemap_get_pages(struct kiocb *iocb, struct iov_iter *iter,
>
>  		if (!PageUptodate(page)) {
>  			if ((iocb->ki_flags & IOCB_NOWAIT) ||
> -			    ((iocb->ki_flags & IOCB_WAITQ) && i)) {
> -				for (j = i; j < pvec->nr; j++)
> -					put_page(pvec->pages[j]);
> -				pvec->nr = i;
> +			    ((iocb->ki_flags & IOCB_WAITQ) && pvec->nr > 1)) {
> +				put_page(page);
> +				pvec->nr--;
>  				err = -EAGAIN;
> -				break;
> +				goto err;
>  			}
>
>  			page = filemap_update_page(iocb, filp, iter, page,
>  					pg_pos, pg_count);
>  			if (IS_ERR_OR_NULL(page)) {
> -				for (j = i + 1; j < pvec->nr; j++)
> -					put_page(pvec->pages[j]);
> -				pvec->nr = i;
> +				pvec->nr--;
>  				err = PTR_ERR_OR_ZERO(page);
> -				break;
>  			}
>  		}
>  	}
>
> +err:
>  	if (likely(pvec->nr))
>  		return 0;
>  	if (err)
> @@ -2437,6 +2480,7 @@ ssize_t generic_file_buffered_read(struct kiocb *iocb,
>  	if (unlikely(iocb->ki_pos >= inode->i_sb->s_maxbytes))
>  		return 0;
>  	iov_iter_truncate(iter, inode->i_sb->s_maxbytes);
> +	pagevec_init(pvec);

This should be pagevec_init(&pvec);


>
>  	do {
>  		cond_resched();
> @@ -2464,13 +2508,8 @@ ssize_t generic_file_buffered_read(struct kiocb *iocb,
>  		isize = i_size_read(inode);
>  		if (unlikely(iocb->ki_pos >= isize))
>  			goto put_pages;
> -
>  		end_offset = min_t(loff_t, isize, iocb->ki_pos + iter->count);
>
> -		while ((iocb->ki_pos >> PAGE_SHIFT) + pvec.nr >
> -		       (end_offset + PAGE_SIZE - 1) >> PAGE_SHIFT)
> -			put_page(pvec.pages[--pvec.nr]);
> -
>  		/*
>  		 * Once we start copying data, we don't want to be touching any
>  		 * cachelines that might be contended:
> @@ -2484,24 +2523,32 @@ ssize_t generic_file_buffered_read(struct kiocb *iocb,
>  		if (iocb->ki_pos >> PAGE_SHIFT !=
>  		    ra->prev_pos >> PAGE_SHIFT)
>  			mark_page_accessed(pvec.pages[0]);
> -		for (i = 1; i < pagevec_count(&pvec); i++)
> -			mark_page_accessed(pvec.pages[i]);
>
>  		for (i = 0; i < pagevec_count(&pvec); i++) {
> -			unsigned int offset = iocb->ki_pos & ~PAGE_MASK;
> -			unsigned int bytes = min_t(loff_t, end_offset - iocb->ki_pos,
> -						   PAGE_SIZE - offset);
> -			unsigned int copied;
> +			struct page *page = pvec.pages[i];
> +			size_t page_size = thp_size(page);
> +			size_t offset = iocb->ki_pos & (page_size - 1);
> +			size_t bytes = min_t(loff_t, end_offset - iocb->ki_pos,
> +					     page_size - offset);
> +			size_t copied;
>
> +			if (end_offset < page_offset(page))
> +				break;
> +			if (i > 0)
> +				mark_page_accessed(page);
>  			/*
>  			 * If users can be writing to this page using arbitrary
>  			 * virtual addresses, take care about potential aliasing
>  			 * before reading the page on the kernel side.
>  			 */
> -			if (writably_mapped)
> -				flush_dcache_page(pvec.pages[i]);
> +			if (writably_mapped) {
> +				int j;
> +
> +				for (j = 0; j < thp_nr_pages(page); j++)
> +					flush_dcache_page(page + j);
> +			}
>
> -			copied = copy_page_to_iter(pvec.pages[i], offset, bytes, iter);
> +			copied = copy_page_to_iter(page, offset, bytes, iter);
>
>  			written += copied;
>  			iocb->ki_pos += copied;
> @@ -2515,6 +2562,7 @@ ssize_t generic_file_buffered_read(struct kiocb *iocb,
>  put_pages:
>  		for (i = 0; i < pagevec_count(&pvec); i++)
>  			put_page(pvec.pages[i]);
> +		pagevec_reinit(pvec);

It should be pagevec_reinit(&pvec);

Found the above two issues during compilation.

—
Best Regards,
Yan Zi

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