Re: [PATCH][RFC] truncate vs add_to_page_cache race

[Nick Piggin]

Nick Piggin wrote:
> Nick Piggin wrote:
>
> > Andrew Morton wrote:
> >
> > > Nick Piggin <nickpiggin@xxxxxxxxxxxx> wrote:
> >
> >
> >
> > > > OK, I made a debug patch to printk and schedule_timeout in this
> > > > race window so I can easily truncate the file. When this happens,
> > > > it turns out that the readpage thinks it is reading a hole and
> > > > fills the page with zeros -> invalid result?
> > >
> > >
> > >
> > >
> > > A zero-filled pagecache page outside i_size is OK.
> >
> > Yes. But in this case the zero filled page actually gets
> > read by read(2).
> >
> > In any case, I think my patch won't close the race completely.
>
> This following patch should be right.
>
> It causes the zeros to not get copied back unless i_size
> gets extended again.

However, it causes the fast path reading off the end of a file
to always go into ->readpage and copy the non-existant page of
zeros. This could be fixed no problem, but I'll shut up and let
others comment in case I'm making a fool of myself :)
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