On 10/24/22 16:35, Vlastimil Babka wrote: > On 10/3/22 19:00, Matthew Wilcox wrote: >> On Sun, Oct 02, 2022 at 02:48:02PM +0900, Hyeonggon Yoo wrote: >>> Just one more thing, rcu_leak_callback too. RCU seem to use it >>> internally to catch double call_rcu(). >>> >>> And some suggestions: >>> - what about adding runtime WARN() on slab init code to catch >>> unexpected arch/toolchain issues? >>> - instead of 4, we may use macro definition? like (PAGE_MAPPING_FLAGS + 1)? >> >> I think the real problem here is that isolate_movable_page() is >> insufficiently paranoid. Looking at the gyrations that GUP and the >> page cache do to convince themselves that the page they got really is >> the page they wanted, there are a few missing pieces (eg checking that >> you actually got a refcount on _this_ page and not some random other >> page you were temporarily part of a compound page with). >> >> This patch does three things: >> >> - Turns one of the comments into English. There are some others >> which I'm still scratching my head over. >> - Uses a folio to help distinguish which operations are being done >> to the head vs the specific page (this is somewhat an abuse of the >> folio concept, but it's acceptable) >> - Add the aforementioned check that we're actually operating on the >> page that we think we want to be. >> - Add a check that the folio isn't secretly a slab. >> >> We could put the slab check in PageMapping and call it after taking >> the folio lock, but that seems pointless. It's the acquisition of >> the refcount which stabilises the slab flag, not holding the lock. >> > > I would like to have a working safe version in -next, even if we are able > simplify it later thanks to frozen refcounts. I've made a formal patch of > yours, but I'm still convinced the slab check needs to be more paranoid so > it can't observe a false positive __folio_test_movable() while missing the > folio_test_slab(), hence I added the barriers as in my previous attempt [1]. > Does that work for you and can I add your S-o-b? Tentatively the series is here for anyone interested, will send it for proper review after the S-o-b is clarified. https://git.kernel.org/pub/scm/linux/kernel/git/vbabka/slab.git/log/?h=slab/for-6.2/fit_rcu_head > [1] https://lore.kernel.org/all/aec59f53-0e53-1736-5932-25407125d4d4@xxxxxxx/
