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From: Matthew Wilcox <willy@infradead.org>
To: David Howells <dhowells@redhat.com>
Cc: Jeff Layton <jlayton@kernel.org>,
	linux-cachefs@redhat.com, idryomov@gmail.com, pfmeec@rit.edu,
	ceph-devel@vger.kernel.org, Andrew W Elble <aweits@rit.edu>
Subject: Re: [PATCH] netfs: fix test for whether we can skip read when writing beyond EOF
Date: Mon, 14 Jun 2021 12:58:51 +0100	[thread overview]
Message-ID: <YMdEe+d2SA5MBrT7@casper.infradead.org> (raw)
In-Reply-To: <348581.1623671134@warthog.procyon.org.uk>

On Mon, Jun 14, 2021 at 12:45:34PM +0100, David Howells wrote:
> Jeff Layton <jlayton@kernel.org> wrote:
> 
> > > Why not:
> > > 
> > > 	if (page_offset(page) >= i_size)
> > > 
> > 
> > That doesn't handle THP's correctly. It's just a PAGE_SIZE shift.
> 
> I asked Willy about that one and he said it will.  Now, granted, the code
> doesn't seem to do that, but possibly he has a patch for it?

a THP has its index in units of PAGE_SIZE.  If you have an order-4
page at file offset 32 * PAGE_SIZE, it will have page->index set to 32.
So shifting by PAGE_SIZE is correct.  This contrasts with the insanity
of hugetlbfs which has its index in units of the hpage_size.

The only thing is that you have to pass around the head page.  tail->index
is meaningless.  But you should always be passing around the head page
unless there's a really good reason not to (eg vmf->page where we really
need to know which subpage the fault was in).

  reply	other threads:[~2021-06-14 11:59 UTC|newest]

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-06-13 23:33 [PATCH] netfs: fix test for whether we can skip read when writing beyond EOF Jeff Layton
2021-06-14  0:08 ` Matthew Wilcox
2021-06-14  9:50 ` David Howells
2021-06-14 10:04 ` David Howells
2021-06-14 11:35   ` Jeff Layton
2021-06-14 11:45   ` David Howells
2021-06-14 11:58     ` Matthew Wilcox [this message]
2021-06-14 11:51   ` Matthew Wilcox
2021-06-14 10:19 ` David Howells
2021-06-14 10:23 ` David Howells
2021-06-14 12:25 ` David Howells
2021-06-14 12:31   ` Jeff Layton
2021-06-14 12:54   ` Matthew Wilcox

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