Re: infinite loop? with mmap, nfs, pwrite, O_DIRECT

[Andrew Morton]

Trond Myklebust <trond.myklebust@xxxxxxxxxx> wrote:
>
> On Mon, 2005-11-21 at 16:09 -0800, Andrew Morton wrote:
> > Trond Myklebust <trond.myklebust@xxxxxxxxxx> wrote:
> > >
> > > The only difference I can see between the two paths is the call to
> > >  unmap_mapping_range(). What effect would that have?
> > 
> > It shoots down any mapped pagecache over the affected file region.  Because
> > the direct-io write is about to make that pagecache out-of-date.  If the
> > application tries to use that data again it'll get a major fault and will
> > re-read the file contents.
> 
> I assume then, that this couldn't be the cause of the
> invalidate_inode_pages() failing to complete?

It sounds unlikely.  This hang is associated with crossing the 2G boundary
isn't it?

I don't think we've seen a sysrq-T trace from the hang?

> Unless there is some
> method to prevent applications from faulting in the page while we're
> inside generic_file_direct_IO(), then the same race would be able to
> occur there.

Yes, there are still windows.

Another thing the unmap_mapping_range() does is to push pte-dirty bits into
the software-dirty flags, so the modified data does get written.  If we
didn't do this, a page which was dirtied via mmap before the direct-io
write would get written back _after_ the direct-io write, arguably causing
corruption.
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