Re: [PATCH v2 5/7] filemap: Allow __filemap_get_folio to allocate large folios

Darrick J. Wong
2023-06-04T18:09:31Z

On Fri, Jun 02, 2023 at 11:24:42PM +0100, Matthew Wilcox (Oracle) wrote:
> Allow callers of __filemap_get_folio() to specify a preferred folio
> order in the FGP flags. This is only honoured in the FGP_CREATE path;
> if there is already a folio in the page cache that covers the index,
> we will return it, no matter what its order is. No create-around is
> attempted; we will only create folios which start at the specified index.
> Unmodified callers will continue to allocate order 0 folios.
>
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> ---
> include/linux/pagemap.h | 23 ++++++++++++++++++++++
> mm/filemap.c | 42 ++++++++++++++++++++++++++++-------------
> mm/readahead.c | 13 -------------
> 3 files changed, 52 insertions(+), 26 deletions(-)
>
> diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
> index 7ab57a2bb576..667ce668f438 100644
> --- a/include/linux/pagemap.h
> +++ b/include/linux/pagemap.h
> @@ -466,6 +466,19 @@ static inline void *detach_page_private(struct page *page)
> return folio_detach_private(page_folio(page));
> }
>
> +/*
> + * There are some parts of the kernel which assume that PMD entries
> + * are exactly HPAGE_PMD_ORDER. Those should be fixed, but until then,
> + * limit the maximum allocation order to PMD size. I'm not aware of any
> + * assumptions about maximum order if THP are disabled, but 8 seems like
> + * a good order (that's 1MB if you're using 4kB pages)
> + */
> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> +#define MAX_PAGECACHE_ORDER HPAGE_PMD_ORDER
> +#else
> +#define MAX_PAGECACHE_ORDER 8
> +#endif
> +
> #ifdef CONFIG_NUMA
> struct folio *filemap_alloc_folio(gfp_t gfp, unsigned int order);
> #else
> @@ -531,9 +544,19 @@ typedef unsigned int __bitwise fgp_t;
> #define FGP_NOWAIT ((__force fgp_t)0x00000020)
> #define FGP_FOR_MMAP ((__force fgp_t)0x00000040)
> #define FGP_STABLE ((__force fgp_t)0x00000080)
> +#define FGP_GET_ORDER(fgp) (((__force unsigned)fgp) >> 26) /* top 6 bits */
>
> #define FGP_WRITEBEGIN (FGP_LOCK | FGP_WRITE | FGP_CREAT | FGP_STABLE)
>
> +static inline fgp_t fgp_set_order(size_t size)
> +{
> + unsigned int shift = ilog2(size);
> +
> + if (shift <= PAGE_SHIFT)
> + return 0;
> + return (__force fgp_t)((shift - PAGE_SHIFT) << 26);
> +}
> +
> void *filemap_get_entry(struct address_space *mapping, pgoff_t index);
> struct folio *__filemap_get_folio(struct address_space *mapping, pgoff_t index,
> fgp_t fgp_flags, gfp_t gfp);
> diff --git a/mm/filemap.c b/mm/filemap.c
> index eb89a815f2f8..10ea9321c36e 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -1937,7 +1937,9 @@ 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 = FGP_GET_ORDER(fgp_flags);
> int err;
> +
> if ((fgp_flags & FGP_WRITE) && mapping_can_writeback(mapping))
> gfp |= __GFP_WRITE;
> if (fgp_flags & FGP_NOFS)
> @@ -1946,26 +1948,40 @@ struct folio *__filemap_get_folio(struct address_space *mapping, pgoff_t index,
> gfp &= ~GFP_KERNEL;
> gfp |= GFP_NOWAIT | __GFP_NOWARN;
> }
> -
> - folio = filemap_alloc_folio(gfp, 0);
> - if (!folio)
> - return ERR_PTR(-ENOMEM);
> -
> if (WARN_ON_ONCE(!(fgp_flags & (FGP_LOCK | FGP_FOR_MMAP))))
> fgp_flags |= FGP_LOCK;
>
> - /* Init accessed so avoid atomic mark_page_accessed later */
> - if (fgp_flags & FGP_ACCESSED)
> - __folio_set_referenced(folio);
> + if (!mapping_large_folio_support(mapping))
> + order = 0;
> + if (order > MAX_PAGECACHE_ORDER)
> + order = MAX_PAGECACHE_ORDER;
> + /* If we're not aligned, allocate a smaller folio */
> + if (index & ((1UL << order) - 1))
> + order = __ffs(index);
>
> - err = filemap_add_folio(mapping, folio, index, gfp);
> - if (unlikely(err)) {
> + do {
> + err = -ENOMEM;
> + if (order == 1)
> + order = 0;

Doesn't this interrupt the scale-down progression 2M -> 1M -> 512K ->
256K -> 128K -> 64K -> 32K -> 16K -> 4k? What if I want 8k folios?

--D

> + folio = filemap_alloc_folio(gfp, order);
> + if (!folio)
> + continue;
> +
> + /* Init accessed so avoid atomic mark_page_accessed later */
> + if (fgp_flags & FGP_ACCESSED)
> + __folio_set_referenced(folio);
> +
> + err = filemap_add_folio(mapping, folio, index, gfp);
> + if (!err)
> + break;
> folio_put(folio);
> folio = NULL;
> - if (err == -EEXIST)
> - goto repeat;
> - }
> + } while (order-- > 0);
>
> + if (err == -EEXIST)
> + goto repeat;
> + if (err)
> + return ERR_PTR(err);
> /*
> * filemap_add_folio locks the page, and for mmap
> * we expect an unlocked page.
> diff --git a/mm/readahead.c b/mm/readahead.c
> index 47afbca1d122..59a071badb90 100644
> --- a/mm/readahead.c
> +++ b/mm/readahead.c
> @@ -462,19 +462,6 @@ static int try_context_readahead(struct address_space *mapping,
> return 1;
> }
>
> -/*
> - * There are some parts of the kernel which assume that PMD entries
> - * are exactly HPAGE_PMD_ORDER. Those should be fixed, but until then,
> - * limit the maximum allocation order to PMD size. I'm not aware of any
> - * assumptions about maximum order if THP are disabled, but 8 seems like
> - * a good order (that's 1MB if you're using 4kB pages)
> - */
> -#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> -#define MAX_PAGECACHE_ORDER HPAGE_PMD_ORDER
> -#else
> -#define MAX_PAGECACHE_ORDER 8
> -#endif
> -
> static inline int ra_alloc_folio(struct readahead_control *ractl, pgoff_t index,
> pgoff_t mark, unsigned int order, gfp_t gfp)
> {
> --
> 2.39.2
>