Re: [PATCH 01/19] mm: Make default vm_ops provide ->page_mkwrite handler

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 27 Mar 2012 09:55:27 +0200
Jan Kara <jack@suse.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@suse.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.  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?