Re: page->mapping == 0

[Daniel Phillips <news-innominate.list.linux.kernel@innominate.de>]

Paul Mackerras wrote:
> I am not any kind of expert on the page cache but it seems to me that
> maybe after locking the page we should check if it is still the page
> we want (i.e. page->mapping and page->index are still correct), and go
> back and look up the page again if not.  Presumably there will be
> quite a few places besides do_generic_file_read where that check would
> be needed also.

I'm getting at least some kind of clue about the page cache, though I
don't claim to be an expert.  You've pointed out how the operation of
becoming a reader on a page can fail because the page disappears while
you're waiting.  That's an error, right?  It's the result of a user
space race, which in turn is the result of parallel processes doing
nonatomic file operations with no synchronization.  It sounds like
bailing out of the file_read if the page turns out to be unmapped after
the lock is the right thing to do, at least for a quick fix.

  http://lxr.linux.no/source/mm/filemap.c?v=2.4.0-test9#L1029

There actually aren't that many places where lock_page is called:

  http://lxr.linux.no/ident?v=2.4.0-test9;i=lock_page

Maybe lock_page needs to return an error code.

--
Daniel
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