On 26.01.22 17:53, Yang Shi wrote: > On Wed, Jan 26, 2022 at 3:57 AM David Hildenbrand <david@xxxxxxxxxx> wrote: >> >> On 26.01.22 12:48, Jann Horn wrote: >>> On Wed, Jan 26, 2022 at 12:38 PM David Hildenbrand <david@xxxxxxxxxx> wrote: >>>> On 26.01.22 12:29, Jann Horn wrote: >>>>> On Wed, Jan 26, 2022 at 11:51 AM David Hildenbrand <david@xxxxxxxxxx> wrote: >>>>>> On 20.01.22 21:28, Yang Shi wrote: >>>>>>> The syzbot reported the below BUG: >>>>>>> >>>>>>> kernel BUG at include/linux/page-flags.h:785! >>> [...] >>>>>>> RIP: 0010:PageDoubleMap include/linux/page-flags.h:785 [inline] >>>>>>> RIP: 0010:__page_mapcount+0x2d2/0x350 mm/util.c:744 >>> [...] >>>>>> Does this point at the bigger issue that reading the mapcount without >>>>>> having the page locked is completely unstable? >>>>> >>>>> (See also https://lore.kernel.org/all/CAG48ez0M=iwJu=Q8yUQHD-+eZDg6ZF8QCF86Sb=CN1petP=Y0Q@xxxxxxxxxxxxxx/ >>>>> for context.) >>>> >>>> Thanks for the pointer. >>>> >>>>> >>>>> I'm not sure what you mean by "unstable". Do you mean "the result is >>>>> not guaranteed to still be valid when the call returns", "the result >>>>> might not have ever been valid", or "the call might crash because the >>>>> page's state as a compound page is unstable"? >>>> >>>> A little bit of everything :) >>> [...] >>>>> In case you mean "the result might not have ever been valid": >>>>> Yes, even with this patch applied, in theory concurrent THP splits >>>>> could cause us to count some page mappings twice. Arguably that's not >>>>> entirely correct. >>>> >>>> Yes, the snapshot is not atomic and, thereby, unreliable. That what I >>>> mostly meant as "unstable". >>>> >>>>> >>>>> In case you mean "the call might crash because the page's state as a >>>>> compound page could concurrently change": >>>> >>>> I think that's just a side-product of the snapshot not being "correct", >>>> right? >>> >>> I guess you could see it that way? The way I look at it is that >>> page_mapcount() is designed to return a number that's at least as high >>> as the number of mappings (rarely higher due to races), and using >>> page_mapcount() on an unlocked page is legitimate if you're fine with >>> the rare double-counting of references. In my view, the problem here >>> is: >>> >>> There are different types of references to "struct page" - some of >>> them allow you to call page_mapcount(), some don't. And in particular, >>> get_page() doesn't give you a reference that can be used with >>> page_mapcount(), but locking a (real, non-migration) PTE pointing to >>> the page does give you such a reference. >> >> I assume the point is that as long as the page cannot be unmapped >> because you block it from getting unmapped (PT lock), the compound page >> cannot get split. As long as the page cannot get unmapped from that page >> table you should have at least a mapcount of 1. > > If you mean holding ptl could prevent THP from splitting, then it is > not true since you may be in the middle of THP split just exactly like > the race condition solved by this patch. While you hold the PT lock and discover a mapped page, unmap_page() cannot continue and unmap the page. That's what I meant "as long as the page cannot be unmapped". What doesn't work is if you hold the PT lock and discover a migration entry, because then you're already past unmap_page(). That's the issue you're fixing. > > Just page lock or elevated page refcount could serialize against THP > split AFAIK. > >> >> But yeah, using the mapcount of a page that is not even mapped >> (migration entry) is clearly wrong. >> >> To summarize: reading the mapcount on an unlocked page will easily >> return a wrong result and the result should not be relied upon. reading >> the mapcount of a migration entry is dangerous and certainly wrong. > > Depends on your usecase. Some just want to get a snapshot, just like > smaps, they don't care. Right, but as discussed, even the snapshot might be slightly wrong. That might be just fine for smaps (and I would have enjoyed a comment in the code stating that :) ). -- Thanks, David / dhildenb
