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

[Jan Kara <jack@suse.cz>]

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

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