linux-kernel - Re: [PATCH 01/19] mm: Make default vm_ops provide ->page

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Message-ID: <20120327220819.GK5020@quack.suse.cz>
Date:	Wed, 28 Mar 2012 00:08:19 +0200
From:	Jan Kara <jack@...e.cz>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Jan Kara <jack@...e.cz>, LKML <linux-kernel@...r.kernel.org>,
	linux-fsdevel@...r.kernel.org, Al Viro <viro@...IV.linux.org.uk>,
	Christoph Hellwig <hch@...radead.org>, dchinner@...hat.com,
	sandeen@...hat.com, Kamal Mostafa <kamal@...onical.com>
Subject: Re: [PATCH 01/19] mm: Make default vm_ops provide ->page_mkwrite
 handler

On Tue 27-03-12 14:38:15, Andrew Morton wrote:
> On Tue, 27 Mar 2012 09:55:27 +0200
> Jan Kara <jack@...e.cz> wrote:
> > On Fri 23-03-12 15:45:02, Andrew Morton wrote:
> > > On Mon,  5 Mar 2012 17:00:59 +0100
> > > Jan Kara <jack@...e.cz> wrote:
> > > 
> > > > --- a/mm/filemap.c
> > > > +++ b/mm/filemap.c
> > > > @@ -1759,8 +1759,28 @@ page_not_uptodate:
> > > >  }
> > > >  EXPORT_SYMBOL(filemap_fault);
> > > >  
> > > > +int filemap_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf)
> > > > +{
> > > > +	struct page *page = vmf->page;
> > > > +	struct inode *inode = vma->vm_file->f_path.dentry->d_inode;
> > > > +	int ret = VM_FAULT_LOCKED;
> > > > +
> > > > +	file_update_time(vma->vm_file);
> > > > +	lock_page(page);
> > > > +	if ((page->mapping != inode->i_mapping) ||
> > > > +	    (page_offset(page) > i_size_read(inode))) {
> > > 
> > > Would benefit from a comment explaining how the page can come to be
> > > outside i_size, and why we fail in that case.
> 
> This?
> 
> > > I don't think i_mutex is held here, so this test is rather meaningless
> > > and racy anyway?
> >   i_size test is racy if that's what you mean by "this test". Just I did
> > the test this way because it's like this in other places and I figured
> > truncate_pagecache() can take relatively long time so the test has some
> > effect. But if you think it's not worth it, I can remove it.
> 
> It bugs me when we copy-n-paste code without remembering why we had it
> there in the first place :( iirc, mmapped pages outside i_size can and
> do happen in some race situations, and are benign.
  Yeah. Certainly there can be pages beyond i_size because we first set
file size and then go and remove pages beyond new i_size one by one when we
do truncate. We must be careful not to create any new pages beyond i_size
but that's what filemap_fault() takes care of. So I think i_size check in
->page_mkwrite() isn't strictly needed.

> But it's several
> years since I thought about it and all the details have evaporated and
> it would take a lot of work to reinstantiate it all.  argh.
> 
> Also, it's off-by-one, isn't it?  Should be page_offset(page) >= i_size?
  Yes, it is. I'll just remove it.

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