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? Regards, Tvrtko
