On Mon 14-12-15 14:04:56, Kirill A. Shutemov wrote: > On Wed, Dec 09, 2015 at 02:02:05PM +0100, Michal Hocko wrote: > > Hi Kirill, > > [ sorry for late reply, just back from vacation. ] > > > while looking at the issue reported by Minchan [1] I have noticed that > > there is nothing to prevent from "isolating" a tail page from LRU because > > isolate_lru_page checks PageLRU which is > > PAGEFLAG(LRU, lru, PF_HEAD) > > so it is checked on the head page rather than the given page directly > > but the rest of the operation is done on the given (tail) page. > > Looks like most (all?) callers already exclude PTE-mapped THP already one > way or another. I can see e.g. do_move_page_to_node_array not doing a similar thing. It isolates and then migrates potentially a tail page. I haven't looked closer whether there is other hand break on the way though. The point I was trying to make is that this is really _subtle_. We are changing something else than we operate later on. > Probably, VM_BUG_ON_PAGE(PageTail(page), page) in isolate_lru_page() would > be appropriate. > > > This is really subtle because this expects that every caller of this > > function checks for the tail page otherwise we would clobber statistics > > and who knows what else (I haven't checked that in detail) as the page > > cannot be on the LRU list and the operation makes sense only on the head > > page. > > > > Would it make more sense to make PageLRU PF_ANY? That would return > > false for PageLRU on any tail page and so it would be ignored by > > isolate_lru_page. > > I don't think this is right way to go. What we put on LRU is compound > page, not 4k subpages. PageLRU() should return true if the compound page > is on LRU regardless if you ask for head or tail page. Hmm, but then we should operate on the head page because that is what PageLRU operated on, no? > False-negatives PageLRU() can be as bad as bug Minchan reported, but > perhaps more silent. -- Michal Hocko SUSE Labs -- 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>