Hugh Dickins wrote: > Checking page counts in a GB file prior to sealing does not appeal at > all: we'd be lucky ever to find them all accounted for.
Here is a refinement of that idea: during a seal operation, iterate over all the pages in the file and check their refcounts. On any page that has an unexpected extra reference, allocate a new page, copy the data over to the new page, and then replace the page having the extra reference with the newly-allocated page in the file. That way you still get zero-copy on pages that don't have extra references, and you don't have to fail the seal operation if some of the pages are still being referenced by something else. The downside of course is the extra memory usage and memcpy overhead if something is holding extra references to the pages. So whether this is a good approach depends on: *) Whether extra page references would happen frequently or infrequently under various kernel configurations and usage scenarios. I don't know enough about the mm system to answer this myself. *) Whether or not the extra memory usage and memcpy overhead could be considered a DoS attack vector by someone who has found a way to add extra references to the pages intentionally. Tony Battersby -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
