Re: [RFC v2 0/9] Replacing the readpages a_op

From: Jan Kara <jack@suse.cz>
To: Matthew Wilcox <willy@infradead.org>
Cc: Jan Kara <jack@suse.cz>,
	linux-xfs@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	linux-mm@kvack.org, Jeff Layton <jlayton@kernel.org>,
	Christoph Hellwig <hch@infradead.org>, Chris Mason <clm@fb.com>
Subject: Re: [RFC v2 0/9] Replacing the readpages a_op
Date: Thu, 23 Jan 2020 11:31:21 +0100	[thread overview]
Message-ID: <20200123103121.GB5728@quack2.suse.cz> (raw)
In-Reply-To: <20200122094414.GC12845@quack2.suse.cz>

On Wed 22-01-20 10:44:14, Jan Kara wrote:
> On Tue 21-01-20 13:48:45, Matthew Wilcox wrote:
> > On Tue, Jan 21, 2020 at 12:36:27PM +0100, Jan Kara wrote:
> > > > v2: Chris asked me to show what this would look like if we just have
> > > > the implementation look up the pages in the page cache, and I managed
> > > > to figure out some things I'd done wrong last time.  It's even simpler
> > > > than v1 (net 104 lines deleted).
> > > 
> > > I have an unfinished patch series laying around that pulls the ->readpage
> > > / ->readpages API in somewhat different direction so I'd like to discuss
> > > whether it's possible to solve my problem using your API. The problem I
> > > have is that currently some operations such as hole punching can race with
> > > ->readpage / ->readpages like:
> > > 
> > > CPU0						CPU1
> > > fallocate(fd, FALLOC_FL_PUNCH_HOLE, off, len)
> > >   filemap_write_and_wait_range()
> > >   down_write(inode->i_rwsem);
> > >   truncate_pagecache_range();
> > > 						readahead(fd, off, len)
> > > 						  creates pages in page cache
> > > 						  looks up block mapping
> > >   removes blocks from inode and frees them
> > > 						  issues bio
> > > 						    - reads stale data -
> > > 						      potential security
> > > 						      issue
> > > 
> > > Now how I wanted to address this is that I'd change the API convention for
> > > ->readpage() so that we call it with the page unlocked and the function
> > > would lock the page, check it's still OK, and do what it needs. And this
> > > will allow ->readpage() and also ->readpages() to grab lock
> > > (EXT4_I(inode)->i_mmap_sem in case of ext4) to synchronize with hole punching
> > > while we are adding pages to page cache and mapping underlying blocks.
> > > 
> > > Now your API makes even ->readpages() (actually ->readahead) called with
> > > pages locked so that makes this approach problematic because of lock
> > > inversions. So I'd prefer if we could keep the situation that ->readpages /
> > > ->readahead gets called without any pages in page cache locked...
> > 
> > I'm not a huge fan of that approach because it increases the number of
> > atomic ops (right now, we __SetPageLocked on the page before adding it
> > to i_pages).
> 
> Yeah, good point. The per-page cost of locking may be noticeable.

Thinking about this a bit more, we should be using ->readpages() to fill
most of the data. And for ->readpages() there would be no additional
overhead. Just for ->readpage() which should be rarely needed. We just need
to come up with a good solution for filesystems that have ->readpage but
not ->readpages.

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