On Wed, Feb 26, 2025 at 07:51:33AM -0800, Christoph Hellwig wrote:
> Lift handling of shmem and slab backed buffers into xfs_buf_alloc_pages
> and rename the result to xfs_buf_alloc_backing_mem. This shares more
> code and ensures uncached buffers can also use slab, which slightly
> reduces the memory usage of growfs on 512 byte sector size file systems,
> but more importantly means the allocation invariants are the same for
> cached and uncached buffers. Document these new invariants with a big
> fat comment mostly stolen from a patch by Dave Chinner.
>
> Signed-off-by: Christoph Hellwig <hch@xxxxxx>
That's a nice refactoring. I'd wondered about get_buf_uncached not
going for slab memory when I was writing the xmbuf code, but didn't want
to overcomplicate that patchset and then forgot about it :/
Reviewed-by: "Darrick J. Wong" <djwong@xxxxxxxxxx>
--D
> ---
> fs/xfs/xfs_buf.c | 55 +++++++++++++++++++++++++++++++-----------------
> 1 file changed, 36 insertions(+), 19 deletions(-)
>
> diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
> index af1389ebdd69..e8783ee23623 100644
> --- a/fs/xfs/xfs_buf.c
> +++ b/fs/xfs/xfs_buf.c
> @@ -329,19 +329,49 @@ xfs_buf_alloc_kmem(
> return 0;
> }
>
> +/*
> + * Allocate backing memory for a buffer.
> + *
> + * For tmpfs-backed buffers used by in-memory btrees this directly maps the
> + * tmpfs page cache folios.
> + *
> + * For real file system buffers there are two different kinds backing memory:
> + *
> + * The first type backs the buffer by a kmalloc allocation. This is done for
> + * less than PAGE_SIZE allocations to avoid wasting memory.
> + *
> + * The second type of buffer is the multi-page buffer. These are always made
> + * up of single pages so that they can be fed to vmap_ram() to return a
> + * contiguous memory region we can access the data through, or mark it as
> + * XBF_UNMAPPED and access the data directly through individual page_address()
> + * calls.
> + */
> static int
> -xfs_buf_alloc_pages(
> +xfs_buf_alloc_backing_mem(
> struct xfs_buf *bp,
> xfs_buf_flags_t flags)
> {
> + size_t size = BBTOB(bp->b_length);
> gfp_t gfp_mask = GFP_KERNEL | __GFP_NOLOCKDEP | __GFP_NOWARN;
> long filled = 0;
>
> + if (xfs_buftarg_is_mem(bp->b_target))
> + return xmbuf_map_page(bp);
> +
> + /*
> + * For buffers that fit entirely within a single page, first attempt to
> + * allocate the memory from the heap to minimise memory usage. If we
> + * can't get heap memory for these small buffers, we fall back to using
> + * the page allocator.
> + */
> + if (size < PAGE_SIZE && xfs_buf_alloc_kmem(new_bp, flags) == 0)
> + return 0;
> +
> if (flags & XBF_READ_AHEAD)
> gfp_mask |= __GFP_NORETRY;
>
> /* Make sure that we have a page list */
> - bp->b_page_count = DIV_ROUND_UP(BBTOB(bp->b_length), PAGE_SIZE);
> + bp->b_page_count = DIV_ROUND_UP(size, PAGE_SIZE);
> if (bp->b_page_count <= XB_PAGES) {
> bp->b_pages = bp->b_page_array;
> } else {
> @@ -622,18 +652,7 @@ xfs_buf_find_insert(
> if (error)
> goto out_drop_pag;
>
> - if (xfs_buftarg_is_mem(new_bp->b_target)) {
> - error = xmbuf_map_page(new_bp);
> - } else if (BBTOB(new_bp->b_length) >= PAGE_SIZE ||
> - xfs_buf_alloc_kmem(new_bp, flags) < 0) {
> - /*
> - * For buffers that fit entirely within a single page, first
> - * attempt to allocate the memory from the heap to minimise
> - * memory usage. If we can't get heap memory for these small
> - * buffers, we fall back to using the page allocator.
> - */
> - error = xfs_buf_alloc_pages(new_bp, flags);
> - }
> + error = xfs_buf_alloc_backing_mem(new_bp, flags);
> if (error)
> goto out_free_buf;
>
> @@ -995,14 +1014,12 @@ xfs_buf_get_uncached(
> if (error)
> return error;
>
> - if (xfs_buftarg_is_mem(bp->b_target))
> - error = xmbuf_map_page(bp);
> - else
> - error = xfs_buf_alloc_pages(bp, flags);
> + error = xfs_buf_alloc_backing_mem(bp, flags);
> if (error)
> goto fail_free_buf;
>
> - error = _xfs_buf_map_pages(bp, 0);
> + if (!bp->b_addr)
> + error = _xfs_buf_map_pages(bp, 0);
> if (unlikely(error)) {
> xfs_warn(target->bt_mount,
> "%s: failed to map pages", __func__);
> --
> 2.45.2
>
>
