Re: [PATCH v7 03/11] filemap: allocate mapping_min_order folios in the page cache

Darrick J. Wong
2024-06-12T15:40:36Z

On Fri, Jun 07, 2024 at 02:58:54PM +0000, Pankaj Raghav (Samsung) wrote:
> From: Pankaj Raghav <p.raghav@samsung.com>
>
> filemap_create_folio() and do_read_cache_folio() were always allocating
> folio of order 0. __filemap_get_folio was trying to allocate higher
> order folios when fgp_flags had higher order hint set but it will default
> to order 0 folio if higher order memory allocation fails.
>
> Supporting mapping_min_order implies that we guarantee each folio in the
> page cache has at least an order of 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.
>
> Co-developed-by: Luis Chamberlain <mcgrof@kernel.org>
> Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
> Signed-off-by: Pankaj Raghav <p.raghav@samsung.com>

Seems pretty straightforward, so
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

--D

> ---
> include/linux/pagemap.h | 20 ++++++++++++++++++++
> mm/filemap.c | 26 ++++++++++++++++++--------
> 2 files changed, 38 insertions(+), 8 deletions(-)
>
> diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
> index 228275e7049f..899b8d751768 100644
> --- a/include/linux/pagemap.h
> +++ b/include/linux/pagemap.h
> @@ -439,6 +439,26 @@ unsigned int mapping_min_folio_order(const struct address_space *mapping)
> return (mapping->flags & AS_FOLIO_ORDER_MIN_MASK) >> AS_FOLIO_ORDER_MIN;
> }
>
> +static inline unsigned long mapping_min_folio_nrpages(struct address_space *mapping)
> +{
> + return 1UL << mapping_min_folio_order(mapping);
> +}
> +
> +/**
> + * mapping_align_start_index() - Align starting index based on the min
> + * folio order of the page cache.
> + * @mapping: The address_space.
> + *
> + * Ensure the index used is aligned to the minimum folio order when adding
> + * new folios to the page cache by rounding down to the nearest minimum
> + * folio number of pages.
> + */
> +static inline pgoff_t mapping_align_start_index(struct address_space *mapping,
> + pgoff_t index)
> +{
> + return round_down(index, mapping_min_folio_nrpages(mapping));
> +}
> +
> /*
> * Large folio support currently depends on THP. These dependencies are
> * being worked on but are not yet fixed.
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 46c7a6f59788..8bb0d2bc93c5 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -859,6 +859,8 @@ noinline int __filemap_add_folio(struct address_space *mapping,
>
> VM_BUG_ON_FOLIO(!folio_test_locked(folio), folio);
> VM_BUG_ON_FOLIO(folio_test_swapbacked(folio), folio);
> + VM_BUG_ON_FOLIO(folio_order(folio) < mapping_min_folio_order(mapping),
> + folio);
> mapping_set_update(&xas, mapping);
>
> VM_BUG_ON_FOLIO(index & (folio_nr_pages(folio) - 1), folio);
> @@ -1919,8 +1921,10 @@ struct folio *__filemap_get_folio(struct address_space *mapping, pgoff_t index,
> folio_wait_stable(folio);
> no_page:
> if (!folio && (fgp_flags & FGP_CREAT)) {
> - unsigned order = FGF_GET_ORDER(fgp_flags);
> + unsigned int min_order = mapping_min_folio_order(mapping);
> + unsigned int order = max(min_order, FGF_GET_ORDER(fgp_flags));
> int err;
> + index = mapping_align_start_index(mapping, index);
>
> if ((fgp_flags & FGP_WRITE) && mapping_can_writeback(mapping))
> gfp |= __GFP_WRITE;
> @@ -1943,7 +1947,7 @@ struct folio *__filemap_get_folio(struct address_space *mapping, pgoff_t index,
> gfp_t alloc_gfp = gfp;
>
> err = -ENOMEM;
> - if (order > 0)
> + if (order > min_order)
> alloc_gfp |= __GFP_NORETRY | __GFP_NOWARN;
> folio = filemap_alloc_folio(alloc_gfp, order);
> if (!folio)
> @@ -1958,7 +1962,7 @@ struct folio *__filemap_get_folio(struct address_space *mapping, pgoff_t index,
> break;
> folio_put(folio);
> folio = NULL;
> - } while (order-- > 0);
> + } while (order-- > min_order);
>
> if (err == -EEXIST)
> goto repeat;
> @@ -2447,13 +2451,16 @@ static int filemap_update_page(struct kiocb *iocb,
> }
>
> static int filemap_create_folio(struct file *file,
> - struct address_space *mapping, pgoff_t index,
> + struct address_space *mapping, loff_t pos,
> struct folio_batch *fbatch)
> {
> struct folio *folio;
> int error;
> + unsigned int min_order = mapping_min_folio_order(mapping);
> + pgoff_t index;
>
> - folio = filemap_alloc_folio(mapping_gfp_mask(mapping), 0);
> + folio = filemap_alloc_folio(mapping_gfp_mask(mapping),
> + min_order);
> if (!folio)
> return -ENOMEM;
>
> @@ -2471,6 +2478,8 @@ static int filemap_create_folio(struct file *file,
> * well to keep locking rules simple.
> */
> filemap_invalidate_lock_shared(mapping);
> + /* index in PAGE units but aligned to min_order number of pages. */
> + index = (pos >> (PAGE_SHIFT + min_order)) << min_order;
> error = filemap_add_folio(mapping, folio, index,
> mapping_gfp_constraint(mapping, GFP_KERNEL));
> if (error == -EEXIST)
> @@ -2531,8 +2540,7 @@ static int filemap_get_pages(struct kiocb *iocb, size_t count,
> if (!folio_batch_count(fbatch)) {
> if (iocb->ki_flags & (IOCB_NOWAIT | IOCB_WAITQ))
> return -EAGAIN;
> - err = filemap_create_folio(filp, mapping,
> - iocb->ki_pos >> PAGE_SHIFT, fbatch);
> + err = filemap_create_folio(filp, mapping, iocb->ki_pos, fbatch);
> if (err == AOP_TRUNCATED_PAGE)
> goto retry;
> return err;
> @@ -3748,9 +3756,11 @@ static struct folio *do_read_cache_folio(struct address_space *mapping,
> repeat:
> folio = filemap_get_folio(mapping, index);
> if (IS_ERR(folio)) {
> - folio = filemap_alloc_folio(gfp, 0);
> + folio = filemap_alloc_folio(gfp,
> + mapping_min_folio_order(mapping));
> if (!folio)
> return ERR_PTR(-ENOMEM);
> + index = mapping_align_start_index(mapping, index);
> err = filemap_add_folio(mapping, folio, index, gfp);
> if (unlikely(err)) {
> folio_put(folio);
> --
> 2.44.1
>
>