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Message-ID: <20170731133243.GB27589@quack2.suse.cz>
Date: Mon, 31 Jul 2017 15:32:43 +0200
From: Jan Kara <jack@...e.cz>
To: Jeff Layton <jlayton@...hat.com>
Cc: Jan Kara <jack@...e.cz>, Steven Whitehouse <swhiteho@...hat.com>,
Marcelo Tosatti <mtosatti@...hat.com>,
Alexander Viro <viro@...iv.linux.org.uk>,
"J . Bruce Fields" <bfields@...ldses.org>,
Andrew Morton <akpm@...ux-foundation.org>,
linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
linux-mm@...ck.org, Matthew Wilcox <willy@...radead.org>,
Bob Peterson <rpeterso@...hat.com>, cluster-devel@...hat.com
Subject: Re: [PATCH v2 2/4] mm: add file_fdatawait_range and
file_write_and_wait
On Mon 31-07-17 09:00:37, Jeff Layton wrote:
> On Mon, 2017-07-31 at 14:07 +0200, Jan Kara wrote:
> > On Mon 31-07-17 07:44:16, Jeff Layton wrote:
> > > On Mon, 2017-07-31 at 12:32 +0100, Steven Whitehouse wrote:
> > > > On 31/07/17 12:27, Jeff Layton wrote:
> > > > > On Thu, 2017-07-27 at 08:48 -0400, Jeff Layton wrote:
> > > > > > On Thu, 2017-07-27 at 10:49 +0200, Jan Kara wrote:
> > > > > > > On Wed 26-07-17 13:55:36, Jeff Layton wrote:
> > > > > > > > +int file_write_and_wait(struct file *file)
> > > > > > > > +{
> > > > > > > > + int err = 0, err2;
> > > > > > > > + struct address_space *mapping = file->f_mapping;
> > > > > > > > +
> > > > > > > > + if ((!dax_mapping(mapping) && mapping->nrpages) ||
> > > > > > > > + (dax_mapping(mapping) && mapping->nrexceptional)) {
> > > > > > > > + err = filemap_fdatawrite(mapping);
> > > > > > > > + /* See comment of filemap_write_and_wait() */
> > > > > > > > + if (err != -EIO) {
> > > > > > > > + loff_t i_size = i_size_read(mapping->host);
> > > > > > > > +
> > > > > > > > + if (i_size != 0)
> > > > > > > > + __filemap_fdatawait_range(mapping, 0,
> > > > > > > > + i_size - 1);
> > > > > > > > + }
> > > > > > > > + }
> > > > > > >
> > > > > > > Err, what's the i_size check doing here? I'd just pass ~0 as the end of the
> > > > > > > range and ignore i_size. It is much easier than trying to wrap your head
> > > > > > > around possible races with file operations modifying i_size.
> > > > > > >
> > > > > > > Honza
> > > > > >
> > > > > > I'm basically emulating _exactly_ what filemap_write_and_wait does here,
> > > > > > as I'm leery of making subtle behavior changes in the actual writeback
> > > > > > behavior. For example:
> > > > > >
> > > > > > -----------------8<----------------
> > > > > > static inline int __filemap_fdatawrite(struct address_space *mapping,
> > > > > > int sync_mode)
> > > > > > {
> > > > > > return __filemap_fdatawrite_range(mapping, 0, LLONG_MAX, sync_mode);
> > > > > > }
> > > > > >
> > > > > > int filemap_fdatawrite(struct address_space *mapping)
> > > > > > {
> > > > > > return __filemap_fdatawrite(mapping, WB_SYNC_ALL);
> > > > > > }
> > > > > > EXPORT_SYMBOL(filemap_fdatawrite);
> > > > > > -----------------8<----------------
> > > > > >
> > > > > > ...which then sets up the wbc with the right ranges and sync mode and
> > > > > > kicks off writepages. But then, it does the i_size_read to figure out
> > > > > > what range it should wait on (with the shortcut for the size == 0 case).
> > > > > >
> > > > > > My assumption was that it was intentionally designed that way, but I'm
> > > > > > guessing from your comments that it wasn't? If so, then we can turn
> > > > > > file_write_and_wait a static inline wrapper around
> > > > > > file_write_and_wait_range.
> > > > >
> > > > > FWIW, I did a bit of archaeology in the linux-history tree and found
> > > > > this patch from Marcelo in 2004. Is this optimization still helpful? If
> > > > > not, then that does simplify the code a bit.
> > > > >
> > > > > -------------------8<--------------------
> > > > >
> > > > > [PATCH] 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.
> > > > >
> > > > > Signed-off-by: Marcelo Tosatti <marcelo.tosatti@...lades.com>
> > > > > Signed-off-by: Andrew Morton <akpm@...l.org>
> > > > > Signed-off-by: Linus Torvalds <torvalds@...l.org>
> > > > > ---
> > > > > mm/filemap.c | 8 +++++++-
> > > > > 1 file changed, 7 insertions(+), 1 deletion(-)
> > > > >
> > > > > diff --git a/mm/filemap.c b/mm/filemap.c
> > > > > index 78e18b7639b6..55fb7b4141e4 100644
> > > > > --- a/mm/filemap.c
> > > > > +++ b/mm/filemap.c
> > > > > @@ -287,7 +287,13 @@ EXPORT_SYMBOL(sync_page_range);
> > > > > */
> > > > > int filemap_fdatawait(struct address_space *mapping)
> > > > > {
> > > > > - return wait_on_page_writeback_range(mapping, 0, -1);
> > > > > + loff_t i_size = i_size_read(mapping->host);
> > > > > +
> > > > > + if (i_size == 0)
> > > > > + return 0;
> > > > > +
> > > > > + return wait_on_page_writeback_range(mapping, 0,
> > > > > + (i_size - 1) >> PAGE_CACHE_SHIFT);
> > > > > }
> > > > > EXPORT_SYMBOL(filemap_fdatawait);
> > > > >
> > > >
> > > > Does this ever get called in cases where we would not hold fs locks? In
> > > > that case we definitely don't want to be relying on i_size,
> > > >
> > > > Steve.
> > > >
> > >
> > > Yes. We can initiate and wait on writeback from any context where you
> > > can sleep, really.
> > >
> > > We're just waiting on whole file writeback here, so I don't think
> > > there's anything wrong. As long as the i_size was valid at some point in
> > > time prior to waiting then you're ok.
> > >
> > > The question I have is more whether this optimization is still useful.
> > >
> > > What we do now is just walk the radix tree and wait_on_page_writeback
> > > for each page. Do we gain anything by avoiding ranges beyond the current
> > > EOF with the pagecache infrastructure of 2017?
> >
> > FWIW I'm not aware of any significant benefit of using i_size in
> > filemap_fdatawait() - we iterate to the end of the radix tree node anyway
> > since pagevec_lookup_tag() does not support range searches anyway (I'm
> > working on fixing that however even after that the benefit would be still
> > rather marginal).
> >
> > What Marcello might have meant even back in 2004 was that if we are in the
> > middle of truncate, i_size is already reduced but page cache not truncated
> > yet, then filemap_fdatawait() does not have to wait for writeback of
> > truncated pages. That might be a noticeable benefit even today if such race
> > happens however I'm not sure it's worth optimizing for and surprises
> > arising from randomly snapshotting i_size (which especially for clustered
> > filesystems may be out of date) IMHO overweight the possible advantage.
> >
> > Honza
>
> Thanks for clarifying.
>
> Given that file_write_and_wait is a new helper function anyway, I'll
> just make it a wrapper around file_write_and_wait_range. Since it might
Agreed.
> be racy, should remove this optimization from the "legacy"
> filemap_fdatawait / filemap_fdatawait_keep_errors calls?
I'm for it.
Honza
--
Jan Kara <jack@...e.com>
SUSE Labs, CR
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