On Wed, 2 Nov 2005, Blaisorblade wrote:
> While looking into this, I probably found another problem, a race with
> install_page(), which doesn't use the seqlock-style check we use for
> everything else (aka do_no_page) but simply assumes a page is valid if its
> index is below the current file size.
>
> This is clearly "truncate" specific, and is already racy. Suppose I truncate a
> file and reduce its size, and then re-extend it, the page which I previously
> fetched from the cache is invalid. The current install_page code generates
> corruption.
No, it should be fine as is (unless perhaps some barrier is needed).
The check
size = (i_size_read(inode) + PAGE_CACHE_SIZE - 1) >> PAGE_CACHE_SHIFT;
if (!page->mapping || page->index >= size)
goto err_unlock;
handles the case that worries you: page->mapping will be NULL.
do_no_page has to do the more complicated truncate_count business because
it deals with all kinds of ->nopage, some of which leave page->mapping NULL:
so it's unable to distinguish one where the driver left it NULL from one
where truncation has suddenly made it NULL.
Hugh
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