Re: [PATCH v8 03/10] readahead: allocate folios with mapping_min_order in readahead

Ryan Roberts
2024-07-04T14:24:19Z

On 25/06/2024 12:44, Pankaj Raghav (Samsung) wrote:
> From: Pankaj Raghav <p.raghav@samsung.com>
>
> page_cache_ra_unbounded() was allocating single pages (0 order folios)
> if there was no folio found in an index. Allocate mapping_min_order folios
> as we need to guarantee the minimum order if it is set.
> While we are at it, rework the loop in page_cache_ra_unbounded() to
> advance with the number of pages in a folio instead of just one page at
> a time.
>
> page_cache_ra_order() tries to allocate folio to the page cache with a
> higher order if the index aligns with that order. Modify it so that the
> order does not go below the mapping_min_order requirement of the page
> cache. This function will do the right thing even if the new_order passed
> is less than the mapping_min_order.
> When adding new folios to the page cache we must also ensure the index
> used is aligned to the mapping_min_order as the page cache requires the
> index to be aligned to the order of the folio.
>
> readahead_expand() is called from readahead aops to extend the range of
> the readahead so this function can assume ractl->_index to be aligned with
> min_order.
>
> Signed-off-by: Pankaj Raghav <p.raghav@samsung.com>
> Co-developed-by: Hannes Reinecke <hare@suse.de>
> Signed-off-by: Hannes Reinecke <hare@suse.de>
> ---
> mm/readahead.c | 81 +++++++++++++++++++++++++++++++++++++++-----------
> 1 file changed, 63 insertions(+), 18 deletions(-)
>
> diff --git a/mm/readahead.c b/mm/readahead.c
> index 66058ae02f2e..2acfd6447d7b 100644
> --- a/mm/readahead.c
> +++ b/mm/readahead.c
> @@ -206,9 +206,10 @@ void page_cache_ra_unbounded(struct readahead_control *ractl,
> unsigned long nr_to_read, unsigned long lookahead_size)
> {
> struct address_space *mapping = ractl->mapping;
> - unsigned long index = readahead_index(ractl);
> + unsigned long ra_folio_index, index = readahead_index(ractl);
> gfp_t gfp_mask = readahead_gfp_mask(mapping);
> - unsigned long i;
> + unsigned long mark, i = 0;
> + unsigned int min_nrpages = mapping_min_folio_nrpages(mapping);
>
> /*
> * Partway through the readahead operation, we will have added
> @@ -223,10 +224,26 @@ void page_cache_ra_unbounded(struct readahead_control *ractl,
> unsigned int nofs = memalloc_nofs_save();
>
> filemap_invalidate_lock_shared(mapping);
> + index = mapping_align_index(mapping, index);
> +
> + /*
> + * As iterator `i` is aligned to min_nrpages, round_up the
> + * difference between nr_to_read and lookahead_size to mark the
> + * index that only has lookahead or "async_region" to set the
> + * readahead flag.
> + */
> + ra_folio_index = round_up(readahead_index(ractl) + nr_to_read - lookahead_size,
> + min_nrpages);
> + mark = ra_folio_index - index;
> + if (index != readahead_index(ractl)) {
> + nr_to_read += readahead_index(ractl) - index;
> + ractl->_index = index;
> + }
> +
> /*
> * Preallocate as many pages as we will need.
> */
> - for (i = 0; i < nr_to_read; i++) {
> + while (i < nr_to_read) {
> struct folio *folio = xa_load(&mapping->i_pages, index + i);
> int ret;
>
> @@ -240,12 +257,13 @@ void page_cache_ra_unbounded(struct readahead_control *ractl,
> * not worth getting one just for that.
> */

For the case that the folio is already in the xarray, perhaps its worth
asserting that the folio is at least min_nrpages?

> read_pages(ractl);
> - ractl->_index++;
> - i = ractl->_index + ractl->_nr_pages - index - 1;
> + ractl->_index += min_nrpages;
> + i = ractl->_index + ractl->_nr_pages - index;
> continue;
> }
>
> - folio = filemap_alloc_folio(gfp_mask, 0);
> + folio = filemap_alloc_folio(gfp_mask,
> + mapping_min_folio_order(mapping));
> if (!folio)
> break;
>
> @@ -255,14 +273,15 @@ void page_cache_ra_unbounded(struct readahead_control *ractl,
> if (ret == -ENOMEM)
> break;
> read_pages(ractl);
> - ractl->_index++;
> - i = ractl->_index + ractl->_nr_pages - index - 1;
> + ractl->_index += min_nrpages;
> + i = ractl->_index + ractl->_nr_pages - index;
> continue;
> }
> - if (i == nr_to_read - lookahead_size)
> + if (i == mark)
> folio_set_readahead(folio);
> ractl->_workingset |= folio_test_workingset(folio);
> - ractl->_nr_pages++;
> + ractl->_nr_pages += min_nrpages;
> + i += min_nrpages;
> }
>
> /*
> @@ -492,13 +511,19 @@ void page_cache_ra_order(struct readahead_control *ractl,
> {
> struct address_space *mapping = ractl->mapping;
> pgoff_t index = readahead_index(ractl);
> + unsigned int min_order = mapping_min_folio_order(mapping);
> pgoff_t limit = (i_size_read(mapping->host) - 1) >> PAGE_SHIFT;
> pgoff_t mark = index + ra->size - ra->async_size;
> unsigned int nofs;
> int err = 0;
> gfp_t gfp = readahead_gfp_mask(mapping);
> + unsigned int min_ra_size = max(4, mapping_min_folio_nrpages(mapping));
>
> - if (!mapping_large_folio_support(mapping) || ra->size < 4)
> + /*
> + * Fallback when size < min_nrpages as each folio should be
> + * at least min_nrpages anyway.
> + */
> + if (!mapping_large_folio_support(mapping) || ra->size < min_ra_size)
> goto fallback;
>
> limit = min(limit, index + ra->size - 1);
> @@ -507,11 +532,20 @@ void page_cache_ra_order(struct readahead_control *ractl,
> new_order += 2;
> new_order = min(mapping_max_folio_order(mapping), new_order);
> new_order = min_t(unsigned int, new_order, ilog2(ra->size));
> + new_order = max(new_order, min_order);
> }
>
> /* See comment in page_cache_ra_unbounded() */
> nofs = memalloc_nofs_save();
> filemap_invalidate_lock_shared(mapping);
> + /*
> + * If the new_order is greater than min_order and index is
> + * already aligned to new_order, then this will be noop as index
> + * aligned to new_order should also be aligned to min_order.
> + */
> + ractl->_index = mapping_align_index(mapping, index);
> + index = readahead_index(ractl);
> +
> while (index <= limit) {
> unsigned int order = new_order;
>
> @@ -519,7 +553,7 @@ void page_cache_ra_order(struct readahead_control *ractl,
> if (index & ((1UL << order) - 1))
> order = __ffs(index);
> /* Don't allocate pages past EOF */
> - while (index + (1UL << order) - 1 > limit)
> + while (order > min_order && index + (1UL << order) - 1 > limit)
> order--;
> err = ra_alloc_folio(ractl, index, mark, order, gfp);
> if (err)
> @@ -783,8 +817,15 @@ void readahead_expand(struct readahead_control *ractl,
> struct file_ra_state *ra = ractl->ra;
> pgoff_t new_index, new_nr_pages;
> gfp_t gfp_mask = readahead_gfp_mask(mapping);
> + unsigned long min_nrpages = mapping_min_folio_nrpages(mapping);
> + unsigned int min_order = mapping_min_folio_order(mapping);
>
> new_index = new_start / PAGE_SIZE;
> + /*
> + * Readahead code should have aligned the ractl->_index to
> + * min_nrpages before calling readahead aops.
> + */
> + VM_BUG_ON(!IS_ALIGNED(ractl->_index, min_nrpages));
>
> /* Expand the leading edge downwards */
> while (ractl->_index > new_index) {
> @@ -794,9 +835,11 @@ void readahead_expand(struct readahead_control *ractl,
> if (folio && !xa_is_value(folio))
> return; /* Folio apparently present */
>
> - folio = filemap_alloc_folio(gfp_mask, 0);
> + folio = filemap_alloc_folio(gfp_mask, min_order);
> if (!folio)
> return;
> +
> + index = mapping_align_index(mapping, index);
> if (filemap_add_folio(mapping, folio, index, gfp_mask) < 0) {
> folio_put(folio);
> return;
> @@ -806,7 +849,7 @@ void readahead_expand(struct readahead_control *ractl,
> ractl->_workingset = true;
> psi_memstall_enter(&ractl->_pflags);
> }
> - ractl->_nr_pages++;
> + ractl->_nr_pages += min_nrpages;
> ractl->_index = folio->index;
> }
>
> @@ -821,9 +864,11 @@ void readahead_expand(struct readahead_control *ractl,
> if (folio && !xa_is_value(folio))
> return; /* Folio apparently present */
>
> - folio = filemap_alloc_folio(gfp_mask, 0);
> + folio = filemap_alloc_folio(gfp_mask, min_order);
> if (!folio)
> return;
> +
> + index = mapping_align_index(mapping, index);
> if (filemap_add_folio(mapping, folio, index, gfp_mask) < 0) {
> folio_put(folio);
> return;
> @@ -833,10 +878,10 @@ void readahead_expand(struct readahead_control *ractl,
> ractl->_workingset = true;
> psi_memstall_enter(&ractl->_pflags);
> }
> - ractl->_nr_pages++;
> + ractl->_nr_pages += min_nrpages;
> if (ra) {
> - ra->size++;
> - ra->async_size++;
> + ra->size += min_nrpages;
> + ra->async_size += min_nrpages;
> }
> }
> }