Re: [PATCH v1 5/5] mm/shmem: optimize file read with folio batching

[Baolin Wang]

On 5/20/26 6:15 PM, Chi Zhiling wrote:
> From: Chi Zhiling <chizhiling@xxxxxxxxxx>
>
> Optimize shmem file read by using filemap_get_folios_contig() to
> batch fetch contiguous folios from the page cache, reducing the
> overhead of repeated shmem_get_folio() calls.
>
> When the folio batch is exhausted, attempt to refill it with
> filemap_get_folios_contig(). If no folios are found (hole or swapped
> out pages), fall back to shmem_get_folio() to handle these cases
> individually.
>
> Additionally:
> - Defer folio_put() until the batch is exhausted or on exit
> - Add folio_test_uptodate() check before copying to ensure data
>    validity
>
> Signed-off-by: Chi Zhiling <chizhiling@xxxxxxxxxx>
> ---
>   mm/shmem.c | 50 ++++++++++++++++++++++++++++++++++++--------------
>   1 file changed, 36 insertions(+), 14 deletions(-)
>
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 96ea5a4c0aff..e0eacc23cccd 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -3355,11 +3355,14 @@ static ssize_t shmem_file_read_iter(struct kiocb *iocb, struct iov_iter *to)
>   	struct file *file = iocb->ki_filp;
>   	struct inode *inode = file_inode(file);
>   	struct address_space *mapping = inode->i_mapping;
> +	struct folio_batch fbatch;
>   	pgoff_t index;
>   	unsigned long offset;
>   	int error = 0;
>   	ssize_t retval = 0;
>   
> +	folio_batch_init(&fbatch);
> +
>   	for (;;) {
>   		struct folio *folio = NULL;
>   		struct page *page = NULL;
> @@ -3372,18 +3375,38 @@ static ssize_t shmem_file_read_iter(struct kiocb *iocb, struct iov_iter *to)
>   			break;
>   
>   		index = iocb->ki_pos >> PAGE_SHIFT;
> -		error = shmem_get_folio(inode, index, 0, &folio, SGP_READ);
> -		if (error) {
> -			if (error == -EINVAL)
> -				error = 0;
> -			break;
> +fetch:
> +		folio = folio_batch_next(&fbatch);
> +		if (!folio) {
> +			pgoff_t start = index;
> +			pgoff_t end = (iocb->ki_pos + to->count - 1) >> PAGE_SHIFT;
> +
> +			if (folio_batch_count(&fbatch)) {
> +				for (int i = 0; i < folio_batch_count(&fbatch); i++)
> +					folio_put(fbatch.folios[i]);
> +				folio_batch_reinit(&fbatch);
> +			}
> +
> +			filemap_get_folios_contig(inode->i_mapping, &start, end, &fbatch);
> +			if (folio_batch_count(&fbatch))
> +				goto fetch;
> +
> +			error = shmem_get_folio(inode, index, 0, &folio, SGP_READ);
> +			if (unlikely(error)) {
> +				if (error == -EINVAL)
> +					error = 0;
> +				break;
> +			}
> +			if (folio) {
> +				folio_unlock(folio);
> +				folio_batch_add(&fbatch, folio);
> +				fbatch.i++;
> +			}
>   		}
> -		if (folio) {
> -			folio_unlock(folio);
>   
> +		if (folio) {
>   			page = folio_file_page(folio, index);
>   			if (PageHWPoison(page)) {
> -				folio_put(folio);
>   				error = -EIO;
>   				break;
>   			}

I haven't tested it yet, but I'm sure this will break the 
fallback_page_copy mode.

You're fetching from the batch at folio granularity here, but in 
fallback_page_copy mode, we still copy the data at page granularity.