Quoting Lionel Landwerlin (2019-07-26 14:38:40) > On 17/07/2019 21:09, Tvrtko Ursulin wrote: > > > > On 17/07/2019 15:06, Chris Wilson wrote: > >> Quoting Tvrtko Ursulin (2019-07-17 14:46:15) > >>> > >>> On 17/07/2019 14:35, Chris Wilson wrote: > >>>> Quoting Tvrtko Ursulin (2019-07-17 14:23:55) > >>>>> > >>>>> On 17/07/2019 14:17, Chris Wilson wrote: > >>>>>> Quoting Tvrtko Ursulin (2019-07-17 14:09:00) > >>>>>>> > >>>>>>> On 16/07/2019 16:37, Chris Wilson wrote: > >>>>>>>> Quoting Tvrtko Ursulin (2019-07-16 16:25:22) > >>>>>>>>> > >>>>>>>>> On 16/07/2019 13:49, Chris Wilson wrote: > >>>>>>>>>> Following a try_to_unmap() we may want to remove the userptr > >>>>>>>>>> and so call > >>>>>>>>>> put_pages(). However, try_to_unmap() acquires the page lock > >>>>>>>>>> and so we > >>>>>>>>>> must avoid recursively locking the pages ourselves -- which > >>>>>>>>>> means that > >>>>>>>>>> we cannot safely acquire the lock around set_page_dirty(). > >>>>>>>>>> Since we > >>>>>>>>>> can't be sure of the lock, we have to risk skip dirtying the > >>>>>>>>>> page, or > >>>>>>>>>> else risk calling set_page_dirty() without a lock and so risk fs > >>>>>>>>>> corruption. > >>>>>>>>> > >>>>>>>>> So if trylock randomly fail we get data corruption in whatever > >>>>>>>>> data set > >>>>>>>>> application is working on, which is what the original patch > >>>>>>>>> was trying > >>>>>>>>> to avoid? Are we able to detect the backing store type so at > >>>>>>>>> least we > >>>>>>>>> don't risk skipping set_page_dirty with anonymous/shmemfs? > >>>>>>>> > >>>>>>>> page->mapping??? > >>>>>>> > >>>>>>> Would page->mapping work? What is it telling us? > >>>>>> > >>>>>> It basically tells us if there is a fs around; anything that is > >>>>>> the most > >>>>>> basic of malloc (even tmpfs/shmemfs has page->mapping). > >>>>> > >>>>> Normal malloc so anonymous pages? Or you meant everything _apart_ > >>>>> from > >>>>> the most basic malloc? > >>>> > >>>> Aye missed the not. > >>>> > >>>>>>>> We still have the issue that if there is a mapping we should be > >>>>>>>> taking > >>>>>>>> the lock, and we may have both a mapping and be inside > >>>>>>>> try_to_unmap(). > >>>>>>> > >>>>>>> Is this a problem? On a path with mappings we trylock and so > >>>>>>> solve the > >>>>>>> set_dirty_locked and recursive deadlock issues, and with no > >>>>>>> mappings > >>>>>>> with always dirty the page and avoid data corruption. > >>>>>> > >>>>>> The problem as I see it is !page->mapping are likely an > >>>>>> insignificant > >>>>>> minority of userptr; as I think even memfd are essentially > >>>>>> shmemfs (or > >>>>>> hugetlbfs) and so have mappings. > >>>>> > >>>>> Better then nothing, no? If easy to do.. > >>>> > >>>> Actually, I erring on the opposite side. Peeking at mm/ internals does > >>>> not bode confidence and feels indefensible. I'd much rather throw my > >>>> hands up and say "this is the best we can do with the API provided, > >>>> please tell us what we should have done." To which the answer is > >>>> probably to not have used gup in the first place :| > >>> > >>> """ > >>> /* > >>> * set_page_dirty() is racy if the caller has no reference against > >>> * page->mapping->host, and if the page is unlocked. This is > >>> because another > >>> * CPU could truncate the page off the mapping and then free the > >>> mapping. > >>> * > >>> * Usually, the page _is_ locked, or the caller is a user-space > >>> process which > >>> * holds a reference on the inode by having an open file. > >>> * > >>> * In other cases, the page should be locked before running > >>> set_page_dirty(). > >>> */ > >>> int set_page_dirty_lock(struct page *page) > >>> """ > >>> > >>> Could we hold a reference to page->mapping->host while having pages > >>> and then would be okay to call plain set_page_dirty? > >> > >> We would then be hitting the warnings in ext4 for unlocked pages again. > > > > Ah true.. > > > >> Essentially the argument is whether or not that warn is valid, to > >> which I > >> think requires inner knowledge of vfs + ext4. To hold a reference on the > >> host would require us tracking page->mapping (reasonable since we > >> already hooked into mmu and so will get an invalidate + fresh gup on > >> any changes), plus iterating over all to acquire the extra reference if > >> applicable -- and I have no idea what the side-effects of that would be. > >> Could well be positive side-effects. Just feels like wandering even > >> further off the beaten path without a map. Good news hmm is just around > >> the corner (which will probably prohibit this use-case) :| > > > > ... can we reach out to someone more knowledgeable in mm matters to > > recommend us what to do? > > > > Regards, > > > > Tvrtko > > > Just a reminder to not let this slip. > We run into userptr bugs in CI quite regularly. Remind away. Revert or trylock, there doesn't seem to be a good answer. -Chris
