__add_to_page_cache() - why alias check?

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Hi,

  I'm looking at 2.4.0-test5, but the code has been present in earlier
kernels.

  __add_to_page_cache(), in mm/filemap.c, is re-checking the page cache
after adding a new entry;

        __add_to_page_cache(...)
        {
                ....
                alias = __find_page_nolock(mapping, offset, *hash);
                if (alias != page)
                        BUG();
        }

  Obviously, this is good for debugging, but will it still be there when
2.4 becomes stable? There isn't a comment suggesting its removal.

  BTW: As new pages appear to always be added to the head of a chain in
the page cache, and searches always start at a head, the alias check isn't
much of a debug check; it should find the page just added. Given this,
the above check isn't much of a performance hit, either.

Mark

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