On Mon, Jul 10, 2023 at 02:02:51PM +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.
Why not just add an end_index parameter to filemap_get_folio()?
>
> Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
> Reviewed-by: Christoph Hellwig <hch@xxxxxx>
> ---
> include/linux/pagemap.h | 34 ++++++++++++++++++++++++++++++
> mm/filemap.c | 46 +++++++++++++++++++++++++++++------------
> mm/readahead.c | 13 ------------
> 3 files changed, 67 insertions(+), 26 deletions(-)
>
> diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
> index 911201fc41fc..d87840acbfb2 100644
> --- a/include/linux/pagemap.h
> +++ b/include/linux/pagemap.h
> @@ -470,6 +470,19 @@ 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
> @@ -535,9 +548,30 @@ typedef unsigned int __bitwise fgf_t;
> #define FGP_NOWAIT ((__force fgf_t)0x00000020)
> #define FGP_FOR_MMAP ((__force fgf_t)0x00000040)
> #define FGP_STABLE ((__force fgf_t)0x00000080)
> +#define FGF_GET_ORDER(fgf) (((__force unsigned)fgf) >> 26) /* top 6 bits */
>
> #define FGP_WRITEBEGIN (FGP_LOCK | FGP_WRITE | FGP_CREAT | FGP_STABLE)
>
> +/**
> + * fgf_set_order - Encode a length in the fgf_t flags.
> + * @size: The suggested size of the folio to create.
> + *
> + * The caller of __filemap_get_folio() can use this to suggest a preferred
> + * size for the folio that is created. If there is already a folio at
> + * the index, it will be returned, no matter what its size. If a folio
> + * is freshly created, it may be of a different size than requested
> + * due to alignment constraints, memory pressure, or the presence of
> + * other folios at nearby indices.
> + */
> +static inline fgf_t fgf_set_order(size_t size)
> +{
> + unsigned int shift = ilog2(size);
> +
> + if (shift <= PAGE_SHIFT)
> + return 0;
> + return (__force fgf_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,
> fgf_t fgp_flags, gfp_t gfp);
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 8a669fecfd1c..baafbf324c9f 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -1905,7 +1905,9 @@ 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 = FGF_GET_ORDER(fgp_flags);
> int err;
> +
> if ((fgp_flags & FGP_WRITE) && mapping_can_writeback(mapping))
> gfp |= __GFP_WRITE;
> if (fgp_flags & FGP_NOFS)
> @@ -1914,26 +1916,44 @@ 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 {
> + gfp_t alloc_gfp = gfp;
> +
> + err = -ENOMEM;
> + if (order == 1)
> + order = 0;
> + if (order > 0)
> + alloc_gfp |= __GFP_NORETRY | __GFP_NOWARN;
> + folio = filemap_alloc_folio(alloc_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 a9c999aa19af..e815c114de21 100644
> --- a/mm/readahead.c
> +++ b/mm/readahead.c
> @@ -461,19 +461,6 @@ 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
>
