Re: [RFC 0/3] Add madvise(..., MADV_WILLWRITE)

Jan Kara
2013-08-07T13:40:58Z

On Mon 05-08-13 12:43:58, Andy Lutomirski wrote:
> My application fallocates and mmaps (shared, writable) a lot (several
> GB) of data at startup. Those mappings are mlocked, and they live on
> ext4. The first write to any given page is slow because
> ext4_da_get_block_prep can block. This means that, to get decent
> performance, I need to write something to all of these pages at
> startup. This, in turn, causes a giant IO storm as several GB of
> zeros get pointlessly written to disk.
>
> This series is an attempt to add madvise(..., MADV_WILLWRITE) to
> signal to the kernel that I will eventually write to the referenced
> pages. It should cause any expensive operations that happen on the
> first write to happen immediately, but it should not result in
> dirtying the pages.
>
> madvice(addr, len, MADV_WILLWRITE) returns the number of bytes that
> the operation succeeded on or a negative error code if there was an
> actual failure. A return value of zero signifies that the kernel
> doesn't know how to "willwrite" the range and that userspace should
> implement a fallback.
>
> For now, it only works on shared writable ext4 mappings. Eventually
> it should support other filesystems as well as private pages (it
> should COW the pages but not cause swap IO) and anonymous pages (it
> should COW the zero page if applicable).
>
> The implementation leaves much to be desired. In particular, it
> generates dirty buffer heads on a clean page, and this scares me.
>
> Thoughts?
One question before I look at the patches: Why don't you use fallocate()
in your application? The functionality you require seems to be pretty
similar to it - writing to an already allocated block is usually quick.

Honza

> Andy Lutomirski (3):
> mm: Add MADV_WILLWRITE to indicate that a range will be written to
> fs: Add block_willwrite
> ext4: Implement willwrite for the delalloc case
>
> fs/buffer.c | 57 ++++++++++++++++++++++++++++++++++
> fs/ext4/ext4.h | 2 ++
> fs/ext4/file.c | 1 +
> fs/ext4/inode.c | 22 +++++++++++++
> include/linux/buffer_head.h | 3 ++
> include/linux/mm.h | 12 +++++++
> include/uapi/asm-generic/mman-common.h | 3 ++
> mm/madvise.c | 28 +++++++++++++++--
> 8 files changed, 126 insertions(+), 2 deletions(-)
>
> --
> 1.8.3.1
>
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--
Jan Kara <jack@suse.cz>
SUSE Labs, CR