linux-kernel - Re: [PATCH] mm: remove optimizations based on i

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Date:   Tue, 1 Aug 2017 11:04:04 +0200
From:   Jan Kara <jack@...e.cz>
To:     Jeff Layton <jlayton@...nel.org>
Cc:     Jan Kara <jack@...e.cz>, Andrew Morton <akpm@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, Marcelo Tosatti <mtosatti@...hat.com>
Subject: Re: [PATCH] mm: remove optimizations based on i_size in mapping
 writeback waits

On Mon 31-07-17 11:29:46, Jeff Layton wrote:
> From: Jeff Layton <jlayton@...hat.com>
> 
> 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@...lades.com>
>     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@...e.cz>

								Honza

> 
> Signed-off-by: Jeff Layton <jlayton@...hat.com>
> ---
>  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@...e.com>
SUSE Labs, CR