On Tue, 18 Sep 2012 13:24:21 -0300 Rafael Aquini <aquini@xxxxxxxxxx> wrote: > On Mon, Sep 17, 2012 at 03:15:43PM -0700, Andrew Morton wrote: > > > +/* return code to identify when a ballooned page has been migrated */ > > > +#define BALLOON_MIGRATION_RETURN 0xba1100 > > > > I didn't really spend enough time to work out why this was done this > > way, but I know a hack when I see one! > > > Yes, I'm afraid it's a hack, but, unfortunately, it's a necessary one (IMHO). > > This 'distinct' return code is used to flag a sucessful balloon page migration > at the following unmap_and_move() snippet (patch 2). > If by any reason we fail to identify a sucessfull balloon page migration, we > will cause a page leak, as the old 'page' won't be properly released. > ..... > rc = __unmap_and_move(page, newpage, force, offlining, mode); > + > + if (unlikely(rc == BALLOON_MIGRATION_RETURN)) { > + /* > + * A ballooned page has been migrated already. > + * Now, it's the time to remove the old page from the isolated > + * pageset list and handle it back to Buddy, wrap-up counters > + * and return. > + */ > ...... > > By reaching that point in code, we cannot rely on testing page->mapping flags > anymore for both 'page' and 'newpage' because: > a) migration has already finished and 'page'->mapping is wiped out; > b) balloon might have started to deflate, and 'newpage' might be released > already; > > If the return code approach is unnaceptable, we might defer the 'page'->mapping > wipe-out step to that point in code for the balloon page case. > That, however, tends to be a little bit heavier, IMHO, as it will require us to > acquire the page lock once more to proceed the mapping wipe out, thus > potentially introducing overhead by lock contention (specially when several > parallel compaction threads are scanning pages for isolation) I think the return code approach _is_ acceptable, but the implementation could be improved. As it stands, a naive caller could be expecting either 0 (success) or a negative errno. A large positive return value could trigger havoc. We can defend against such programming mistakes with code commentary, but a better approach would be to enumerate the return values. Along the lines of /* * Return values from addresss_space_operations.migratepage(). Returns a * negative errno on failure. */ #define MIGRATEPAGE_SUCCESS 0 #define MIGRATEPAGE_BALLOON_THINGY 1 /* nice comment goes here */ and convert all callers to explicitly check for MIGRATEPAGE_SUCCESS, not literal zero. We should be particularly careful to look for codesites which are unprepared for positive return values, such as ret = migratepage(); if (ret < 0) return ret; ... return ret; /* success!! */ If we wanted to be really vigilant about this, we could do #define MIGRATEPAGE_SUCCESS 1 #define MIGRATEPAGE_BALLOON_THINGY 2 so any naive code which tests for literal zero will nicely explode early in testing. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx"> email@xxxxxxxxx </a>