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From: Dominique Martinet <asmadeus@codewreck.org>
To: "Matthew Wilcox (Oracle)" <willy@infradead.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 14:30:25 +0200	[thread overview]
Message-ID: <20200918123025.GA735@nautica> (raw)
In-Reply-To: <20200918111916.GA32101@casper.infradead.org> <20200917151050.5363-3-willy@infradead.org>

Matthew Wilcox (Oracle) wrote on Thu, Sep 17, 2020:
> The 9p readpage implementation was already synchronous, so use
> AOP_UPDATED_PAGE to avoid cycling the page lock.
> 
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>

Acked-by: Dominique Martinet <asmadeus@codewreck.org>

(I assume it'll be merged together with the rest)

> > 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.

Thanks for the explanation, I didn't realize the page already is
gotten/locked at the PageUptodate goto out.

> 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.

Yes, looks good to me.

Cheers,
-- 
Dominique

  parent reply	other threads:[~2020-09-18 12:30 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
2020-09-18 12:30   ` Dominique Martinet [this message]
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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