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. 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) :| -Chris
