On Mon 31-07-17 11:29:46, Jeff Layton wrote:
> From: Jeff Layton <jlayton@xxxxxxxxxx>
>
> Marcelo added this i_size based optimization with a patch in 2004
> (commit 765dad09b4ac in the linux-history tree):
>
> commit 765dad09b4ac101a32d87af2bb793c3060497d3c
> Author: Marcelo Tosatti <marcelo.tosatti@xxxxxxxxxxxx>
> Date: Tue Sep 7 17:51:17 2004 -0700
>
> small wait_on_page_writeback_range() optimization
>
> filemap_fdatawait() calls wait_on_page_writeback_range() with -1
> as "end" parameter. This is not needed since we know the EOF
> from the inode. Use that instead.
>
> There may be races here, particularly with clustered or network
> filesystems. Block devices always have an i_size of 0 as well, which
> makes using this with a blockdev inode sort of pointless.
Well, you are not quite right here. You are correct that
file_inode(file)->i_size is 0, however file->f_mapping->host->i_size is the
device size and that's what will be used for filemap_fdatawait(). So that
function works fine for block devices.
> It also seems like a bit of a layering violation since we're operating
> on an address_space here, not an inode.
>
> Finally, it's also questionable whether this optimization really helps
> on workloads that we care about. Should we be optimizing for writeback
> vs. truncate races in a codepath where we expect to wait anyway? It
> doesn't seem worth the risk.
>
> Remove this optimization from the filemap_fdatawait codepaths. This
> means that filemap_fdatawait becomes a trivial wrapper around
> filemap_fdatawait_range.
Agreed for all the other reasons so feel free to add:
Reviewed-by: Jan Kara <jack@xxxxxxx>
Honza
>
> Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx>
> ---
> include/linux/fs.h | 9 +++++++--
> mm/filemap.c | 30 +-----------------------------
> 2 files changed, 8 insertions(+), 31 deletions(-)
>
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index af592ca3d509..656e04c6983e 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -2538,10 +2538,15 @@ extern int invalidate_inode_pages2_range(struct address_space *mapping,
> extern int write_inode_now(struct inode *, int);
> extern int filemap_fdatawrite(struct address_space *);
> extern int filemap_flush(struct address_space *);
> -extern int filemap_fdatawait(struct address_space *);
> -extern int filemap_fdatawait_keep_errors(struct address_space *mapping);
> extern int filemap_fdatawait_range(struct address_space *, loff_t lstart,
> loff_t lend);
> +extern int filemap_fdatawait_keep_errors(struct address_space *mapping);
> +
> +static inline int filemap_fdatawait(struct address_space *mapping)
> +{
> + return filemap_fdatawait_range(mapping, 0, LLONG_MAX);
> +}
> +
> extern bool filemap_range_has_page(struct address_space *, loff_t lstart,
> loff_t lend);
> extern int __must_check file_fdatawait_range(struct file *file, loff_t lstart,
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 394bb5e96f87..85dfe3bee324 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -512,39 +512,11 @@ EXPORT_SYMBOL(file_fdatawait_range);
> */
> int filemap_fdatawait_keep_errors(struct address_space *mapping)
> {
> - loff_t i_size = i_size_read(mapping->host);
> -
> - if (i_size == 0)
> - return 0;
> -
> - __filemap_fdatawait_range(mapping, 0, i_size - 1);
> + __filemap_fdatawait_range(mapping, 0, LLONG_MAX);
> return filemap_check_and_keep_errors(mapping);
> }
> EXPORT_SYMBOL(filemap_fdatawait_keep_errors);
>
> -/**
> - * filemap_fdatawait - wait for all under-writeback pages to complete
> - * @mapping: address space structure to wait for
> - *
> - * Walk the list of under-writeback pages of the given address space
> - * and wait for all of them. Check error status of the address space
> - * and return it.
> - *
> - * Since the error status of the address space is cleared by this function,
> - * callers are responsible for checking the return value and handling and/or
> - * reporting the error.
> - */
> -int filemap_fdatawait(struct address_space *mapping)
> -{
> - loff_t i_size = i_size_read(mapping->host);
> -
> - if (i_size == 0)
> - return 0;
> -
> - return filemap_fdatawait_range(mapping, 0, i_size - 1);
> -}
> -EXPORT_SYMBOL(filemap_fdatawait);
> -
> static bool mapping_needs_writeback(struct address_space *mapping)
> {
> return (!dax_mapping(mapping) && mapping->nrpages) ||
> --
> 2.13.3
>
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
