On Tue, Jun 19, 2018 at 10:29:49AM +0200, Jan Kara wrote: > And for record, the problem with page cache pages is not only that > try_to_unmap() may unmap them. It is also that page_mkclean() can > write-protect them. And once PTEs are write-protected filesystems may end > up doing bad things if DMA then modifies the page contents (DIF/DIX > failures, data corruption, oopses). As such I don't think that solutions > based on page reference count have a big chance of dealing with the > problem. > > And your page flag approach would also need to take page_mkclean() into > account. And there the issue is that until the flag is cleared (i.e., we > are sure there are no writers using references from GUP) you cannot > writeback the page safely which does not work well with your idea of > clearing the flag only once the page is evicted from page cache (hint, page > cache page cannot get evicted until it is written back). > > So as sad as it is, I don't see an easy solution here. Pages which are "got" don't need to be on the LRU list. They'll be marked dirty when they're put, so we can use page->lru for fun things like a "got" refcount. If we use bit 1 of page->lru for PageGot, we've got 30/62 bits in the first word and a full 64 bits in the second word.
