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From: Andrew Morton <akpm@osdl.org>
To: suparna@in.ibm.com
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Subject: Re: [PATCH linux-2.6.0-test10-mm1] filemap_fdatawait.patch
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Suparna Bhattacharya <suparna@in.ibm.com> wrote:
>
> If PG_writeback is not set, it moves on to process the next page -- without
>  falling through to wait_on_page_writeback, isn't that so ? 
>  Another thread could have pulled the page off io_pages, put it on locked
>  pages, and not set PG_writeback as yet.

OK, so a (well commented!) lock_page() in filemap_fdatawait() would seem to
plug that.  I recall being worried that this might be livelockable against
ongoing read activity, but that doesn't sound right - such pages won't be
on locked_pages().