linux-ext4 - Re: [POC][PATCH] xfs: reduce ilock contention on buffered randrw workload

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Date:   Mon, 8 Apr 2019 16:11:14 +0200
From:   Jan Kara <jack@...e.cz>
To:     Amir Goldstein <amir73il@...il.com>
Cc:     Dave Chinner <david@...morbit.com>,
        "Darrick J . Wong" <darrick.wong@...cle.com>,
        Christoph Hellwig <hch@....de>,
        Matthew Wilcox <willy@...radead.org>,
        linux-xfs <linux-xfs@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Ext4 <linux-ext4@...r.kernel.org>,
        Lukas Czerner <lczerner@...hat.com>,
        Theodore Tso <tytso@....edu>, Jan Kara <jack@...e.cz>
Subject: Re: [POC][PATCH] xfs: reduce ilock contention on buffered randrw
 workload

On Mon 08-04-19 12:02:34, Amir Goldstein wrote:
> On Mon, Apr 8, 2019 at 2:27 AM Dave Chinner <david@...morbit.com> wrote:
> >
> > On Fri, Apr 05, 2019 at 05:02:33PM +0300, Amir Goldstein wrote:
> > > On Fri, Apr 5, 2019 at 12:17 AM Dave Chinner <david@...morbit.com> wrote:
> > > >
> > > > On Thu, Apr 04, 2019 at 07:57:37PM +0300, Amir Goldstein wrote:
> > > > > This patch improves performance of mixed random rw workload
> > > > > on xfs without relaxing the atomic buffered read/write guaranty
> > > > > that xfs has always provided.
> > > > >
> > > > > We achieve that by calling generic_file_read_iter() twice.
> > > > > Once with a discard iterator to warm up page cache before taking
> > > > > the shared ilock and once again under shared ilock.
> > > >
> > > > This will race with thing like truncate, hole punching, etc that
> > > > serialise IO and invalidate the page cache for data integrity
> > > > reasons under the IOLOCK. These rely on there being no IO to the
> > > > inode in progress at all to work correctly, which this patch
> > > > violates. IOWs, while this is fast, it is not safe and so not a
> > > > viable approach to solving the problem.
> > > >
> > >
> > > This statement leaves me wondering, if ext4 does not takes
> > > i_rwsem on generic_file_read_iter(), how does ext4 (or any other
> > > fs for that matter) guaranty buffered read synchronization with
> > > truncate, hole punching etc?
> > > The answer in ext4 case is i_mmap_sem, which is read locked
> > > in the page fault handler.
> >
> > Nope, the  i_mmap_sem is for serialisation of /page faults/ against
> > truncate, holepunching, etc. Completely irrelevant to the read()
> > path.
> >
> 
> I'm at lost here. Why are page faults completely irrelevant to read()
> path? Aren't full pages supposed to be faulted in on read() after
> truncate_pagecache_range()?

During read(2), pages are not "faulted in". Just look at
what generic_file_buffered_read() does. It uses completely separate code to
add page to page cache, trigger readahead, and possibly call ->readpage() to
fill the page with data. "fault" path (handled by filemap_fault()) applies
only to accesses from userspace to mmaps.

> And aren't partial pages supposed to be partially zeroed and uptodate
> after truncate_pagecache_range()?

truncate_pagecache_range() does take care of page zeroing, that is correct
(if those pages are in page cache in the first place). I'm not sure how
this relates to the above discussion though.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR