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Re: [PATCH 1 of 2] Implement generic block_page_mkwrite() functionality

To: David Chinner <dgc@xxxxxxx>
Subject: Re: [PATCH 1 of 2] Implement generic block_page_mkwrite() functionality
From: Chris Mason <chris.mason@xxxxxxxxxx>
Date: Wed, 7 Feb 2007 10:52:45 -0500
Cc: Hugh Dickins <hugh@xxxxxxxxxxx>, xfs@xxxxxxxxxxx, linux-fsdevel@xxxxxxxxxxxxxxx, linux-mm@xxxxxxxxx
In-reply-to: <20070207144415.GN44411608@xxxxxxxxxxxxxxxxx>
References: <20070207124922.GK44411608@xxxxxxxxxxxxxxxxx> <Pine.LNX.4.64.0702071256530.25060@xxxxxxxxxxxxxxxxxxxxxx> <20070207144415.GN44411608@xxxxxxxxxxxxxxxxx>
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On Thu, Feb 08, 2007 at 01:44:15AM +1100, David Chinner wrote:
> On Wed, Feb 07, 2007 at 01:00:28PM +0000, Hugh Dickins wrote:
> > On Wed, 7 Feb 2007, David Chinner wrote:
> > 
> > > On Christoph's suggestion, take the guts of the proposed
> > > xfs_vm_page_mkwrite function and implement it as a generic
> > > core function as it used no specific XFS code at all.
> > > 
> > > This allows any filesystem to easily hook the ->page_mkwrite()
> > > VM callout to allow them to set up pages dirtied by mmap
> > > writes correctly for later writeout.
> > > 
> > > Signed-Off-By: Dave Chinner <dgc@xxxxxxx>
> > 
> > I'm worried about concurrent truncation.  Isn't it the case that
> > i_mutex is held when prepare_write and commit_write are normally
> > called?  But not here when page_mkwrite is called.
> 
> I'm not holding i_mutex. I assumed that it was probably safe to do
> because we are likely to be reading the page off disk just before we
> call mkwrite and that has to be synchronised with truncate in some
> manner....

In general, commit_write is allowed to update i_size, and prepare/commit
are called with i_mutex.  block_prepare_write and block_commit_write
both look safe to me for calling with only the page lock held.  It more
or less translates to: call get_block in a sane fashion and zero out the
parts of the page past eof.

But, if someone copies the code and puts their own fancy
prepare/commit_write in there, they will get in trouble in a hurry...

> 
> So, do I need to grab the i_mutex here? Is that safe to do that in
> the middle of a page fault? If we do race with a truncate and the
> page is now beyond EOF, what am I supposed to return?

Should it check to make sure the page is still in the address space
after locking it?

-chris


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