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From: Matthew Wilcox <willy@infradead.org>
To: Dominique Martinet <asmadeus@codewreck.org>
Cc: linux-fsdevel@vger.kernel.org, linux-cifs@vger.kernel.org,
	Richard Weinberger <richard@nod.at>,
	ecryptfs@vger.kernel.org, linux-um@lists.infradead.org,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	linux-mtd@lists.infradead.org,
	v9fs-developer@lists.sourceforge.net, ceph-devel@vger.kernel.org,
	linux-afs@lists.infradead.org
Subject: Re: [V9fs-developer] [PATCH 02/13] 9p: Tell the VFS that readpage was synchronous
Date: Fri, 18 Sep 2020 12:19:16 +0100	[thread overview]
Message-ID: <20200918111916.GA32101@casper.infradead.org> (raw)
In-Reply-To: <20200918055919.GA30929@nautica>

On Fri, Sep 18, 2020 at 07:59:19AM +0200, Dominique Martinet wrote:
> Matthew Wilcox (Oracle) wrote on Thu, Sep 17, 2020:
> > diff --git a/fs/9p/vfs_addr.c b/fs/9p/vfs_addr.c
> > index cce9ace651a2..506ca0ba2ec7 100644
> > --- a/fs/9p/vfs_addr.c
> > +++ b/fs/9p/vfs_addr.c
> > @@ -280,6 +280,10 @@ static int v9fs_write_begin(struct file *filp, struct address_space *mapping,
> >  		goto out;
> >  
> >  	retval = v9fs_fid_readpage(v9inode->writeback_fid, page);
> > +	if (retval == AOP_UPDATED_PAGE) {
> > +		retval = 0;
> > +		goto out;
> > +	}
> 
> FWIW this is a change of behaviour; for some reason the code used to
> loop back to grab_cache_page_write_begin() and bail out on
> PageUptodate() I suppose; some sort of race check?
> The whole pattern is a bit weird to me and 9p has no guarantee on
> concurrent writes to a file with cache enabled (except that it will
> corrupt something), so this part is fine with me.
> 
> What I'm curious about is the page used to be both unlocked and put, but
> now isn't either and the return value hasn't changed for the caller to
> make a difference on write_begin / I don't see any code change in the
> vfs  to handle that.
> What did I miss?

The page cache is kind of subtle.  The grab_cache_page_write_begin()
will return a Locked page with an increased refcount.  If it's Uptodate,
that's exactly what we want, and we return it.  If we have to read the
page, readpage used to unlock the page before returning, and rather than
re-lock it, we would drop the reference to the page and look it up again.
It's possible that after dropping the lock on that page that the page
was replaced in the page cache and so we'd get a different page.

Anyway, now (unless fscache is involved), v9fs_fid_readpage will return
the page without unlocking it.  So we don't need to do the dance of
dropping the lock, putting the refcount and looking the page back up
again.  We can just return the page.  The VFS doesn't need a special
return code because nothing has changed from the VFS's point of view --
it asked you to get a page and you got the page.

  reply	other threads:[~2020-09-18 11:19 UTC|newest]

Thread overview: 30+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-09-17 15:10 [PATCH 00/13] Allow readpage to return a locked page Matthew Wilcox (Oracle)
2020-09-17 15:10 ` [PATCH 01/13] mm: Add AOP_UPDATED_PAGE return value Matthew Wilcox (Oracle)
2020-09-17 22:03   ` Matthew Wilcox
2020-09-17 15:10 ` [PATCH 02/13] 9p: Tell the VFS that readpage was synchronous Matthew Wilcox (Oracle)
2020-09-18  5:59   ` [V9fs-developer] " Dominique Martinet
2020-09-18 11:19     ` Matthew Wilcox [this message]
2020-09-18 12:30   ` Dominique Martinet
2020-09-17 15:10 ` [PATCH 03/13] afs: " Matthew Wilcox (Oracle)
2020-09-17 15:10 ` [PATCH 04/13] ceph: " Matthew Wilcox (Oracle)
2020-09-17 16:49   ` Jeff Layton
2020-09-17 15:10 ` [PATCH 05/13] cifs: " Matthew Wilcox (Oracle)
2020-09-17 15:10 ` [PATCH 06/13] cramfs: " Matthew Wilcox (Oracle)
2020-09-17 15:10 ` [PATCH 07/13] ecryptfs: " Matthew Wilcox (Oracle)
2020-09-17 15:10 ` [PATCH 08/13] fuse: " Matthew Wilcox (Oracle)
2020-09-17 15:10 ` [PATCH 09/13] hostfs: " Matthew Wilcox (Oracle)
2020-09-17 15:10 ` [PATCH 10/13] jffs2: " Matthew Wilcox (Oracle)
2020-09-17 15:10 ` [PATCH 11/13] ubifs: " Matthew Wilcox (Oracle)
2020-09-17 20:46   ` Richard Weinberger
2020-09-17 15:10 ` [PATCH 12/13] udf: " Matthew Wilcox (Oracle)
2020-09-24  9:00   ` Jan Kara
2020-09-17 15:10 ` [PATCH 13/13] vboxsf: " Matthew Wilcox (Oracle)
2020-09-17 22:56 ` [PATCH 14/13] iomap: Inline iomap_iop_set_range_uptodate into its one caller Matthew Wilcox (Oracle)
2020-09-17 22:56   ` [PATCH 15/13] iomap: Inline iomap_read_finish " Matthew Wilcox (Oracle)
2020-09-19  6:31     ` Christoph Hellwig
2020-09-17 22:56   ` [PATCH 16/13] iomap: Make readpage synchronous Matthew Wilcox (Oracle)
2020-09-19  6:39     ` Christoph Hellwig
2020-09-19  6:43       ` Christoph Hellwig
2020-09-19 17:03         ` Matthew Wilcox
2020-09-19 17:10       ` Matthew Wilcox
2020-09-19  6:31   ` [PATCH 14/13] iomap: Inline iomap_iop_set_range_uptodate into its one caller Christoph Hellwig

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