Re: [patch] mm: close page_mkwrite races

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

On Tue, 31 Mar 2009 12:55:16 -0700 (PDT)
Sage Weil <sage@newdream.net> wrote:

> On Mon, 30 Mar 2009, Nick Piggin wrote:
> > [Fixed linux-mm address. Please reply here]
> > 
> > Hi,
> > 
> > I'd like opinions on this patch (applies on top of the previous
> > page_mkwrite fixes in -mm). I was not going to ask to merge it
> > immediately however it appears that fsblock is not the only one who
> > needs it...
> > --
> > 
> > I want to have the page be protected by page lock between page_mkwrite
> > notification to the filesystem, and the actual setting of the page
> > dirty. Do this by allowing the filesystem to return a locked page from
> > page_mkwrite, and have the page fault code keep it held until after it
> > calls set_page_dirty.
> > 
> > I need this in fsblock because I am working to ensure filesystem metadata
> > can be correctly allocated and refcounted. This means that page cleaning
> > should not require memory allocation.

wot?  "page cleaning" involves writeout.  How can we avoid doing
allocations there?

> > Without this patch, then for example we could get a concurrent writeout
> > after the page_mkwrite (which allocates page metadata required to clean
> > it), but before the set_page_dirty. The writeout will clean the page and
> > notice that the metadata is now unused and may be deallocated (because
> > it appears clean as set_page_dirty hasn't been called yet). So at this
> > point the page may be dirtied via the pte without enough metadata to be
> > able to write it back.
> > 
> > Sage needs this race closed for ceph, and Trond maybe for NFS.
> 
> I ran a few tests and this fixes the problem for me (although fyi the 
> patch didn't apply cleanly on top of your previously posted page_mkwrite 
> prototype change patch, due to some differences in block_page_mkwrite).

What is "the problem"?  Can we get "the problem"'s description included
in the changelog?

The patch is fairly ugly, somewhat costly and makes things (even) more
complex.    Sigh.

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