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SubjectRe: [PATCH v2 5/9] iomap: Support arbitrarily many blocks per page
On Fri, Sep 11, 2020 at 12:47:03AM +0100, Matthew Wilcox (Oracle) wrote:
> Size the uptodate array dynamically to support larger pages in the
> page cache. With a 64kB page, we're only saving 8 bytes per page today,
> but with a 2MB maximum page size, we'd have to allocate more than 4kB
> per page. Add a few debugging assertions.
>
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> Reviewed-by: Dave Chinner <dchinner@redhat.com>

Looks ok,
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>

--D

> ---
> fs/iomap/buffered-io.c | 22 +++++++++++++++++-----
> 1 file changed, 17 insertions(+), 5 deletions(-)
>
> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> index 7fc0e02d27b0..9670c096b83e 100644
> --- a/fs/iomap/buffered-io.c
> +++ b/fs/iomap/buffered-io.c
> @@ -22,18 +22,25 @@
> #include "../internal.h"
>
> /*
> - * Structure allocated for each page when block size < PAGE_SIZE to track
> - * sub-page uptodate status and I/O completions.
> + * Structure allocated for each page or THP when block size < page size
> + * to track sub-page uptodate status and I/O completions.
> */
> struct iomap_page {
> atomic_t read_count;
> atomic_t write_count;
> spinlock_t uptodate_lock;
> - DECLARE_BITMAP(uptodate, PAGE_SIZE / 512);
> + unsigned long uptodate[];
> };
>
> static inline struct iomap_page *to_iomap_page(struct page *page)
> {
> + /*
> + * per-block data is stored in the head page. Callers should
> + * not be dealing with tail pages (and if they are, they can
> + * call thp_head() first.
> + */
> + VM_BUG_ON_PGFLAGS(PageTail(page), page);
> +
> if (page_has_private(page))
> return (struct iomap_page *)page_private(page);
> return NULL;
> @@ -45,11 +52,13 @@ static struct iomap_page *
> iomap_page_create(struct inode *inode, struct page *page)
> {
> struct iomap_page *iop = to_iomap_page(page);
> + unsigned int nr_blocks = i_blocks_per_page(inode, page);
>
> - if (iop || i_blocks_per_page(inode, page) <= 1)
> + if (iop || nr_blocks <= 1)
> return iop;
>
> - iop = kzalloc(sizeof(*iop), GFP_NOFS | __GFP_NOFAIL);
> + iop = kzalloc(struct_size(iop, uptodate, BITS_TO_LONGS(nr_blocks)),
> + GFP_NOFS | __GFP_NOFAIL);
> spin_lock_init(&iop->uptodate_lock);
> attach_page_private(page, iop);
> return iop;
> @@ -59,11 +68,14 @@ static void
> iomap_page_release(struct page *page)
> {
> struct iomap_page *iop = detach_page_private(page);
> + unsigned int nr_blocks = i_blocks_per_page(page->mapping->host, page);
>
> if (!iop)
> return;
> WARN_ON_ONCE(atomic_read(&iop->read_count));
> WARN_ON_ONCE(atomic_read(&iop->write_count));
> + WARN_ON_ONCE(bitmap_full(iop->uptodate, nr_blocks) !=
> + PageUptodate(page));
> kfree(iop);
> }
>
> --
> 2.28.0
>

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