On Tue, 13 Jun 2023 at 11:00, Xiaoming Ding (丁晓明) <Xiaoming.Ding@xxxxxxxxxxxx> wrote: > > So do we have a conclution about this patch? or need more time to > study the possible risks Please avoid top posting. As already discussed here [1], RLIMIT_MEMLOCK checks have to be implemented if we switch to FOLL_LONGTERM. [1] https://lists.trustedfirmware.org/archives/list/op-tee@xxxxxxxxxxxxxxxxxxxxxxxxx/message/UEOMNYLDFHDFQNLODGCJVFDOQBR723EQ/ -Sumit > > On Tue, 2023-05-23 at 08:25 +0100, Lorenzo Stoakes wrote: > > External email : Please do not click links or open attachments until > > you have verified the sender or the content. > > > > > > On Mon, May 22, 2023 at 06:54:29PM -0700, John Hubbard wrote: > > > On 5/18/23 06:56, David Hildenbrand wrote: > > > > On 18.05.23 08:08, Sumit Garg wrote: > > > > > On Thu, 18 May 2023 at 09:51, Christoph Hellwig < > > > > > hch@xxxxxxxxxxxxx> wrote: > > > > > > > > > > > > On Wed, May 17, 2023 at 08:23:33PM +0200, David Hildenbrand > > > > > > wrote: > > > > > > > In general: if user space controls it -> possibly forever > > > > > > > -> long-term. Even > > > > > > > if in most cases it's a short delay: there is no trusting > > > > > > > on user space. > > > > > > > > > > > > > > For example, iouring fixed buffers keep pages pinned until > > > > > > > user space > > > > > > > decides to unregistered the buffers -> long-term. > > > > > > > > > > > > > > Short-term is, for example, something like O_DIRECT where > > > > > > > we pin -> DMA -> > > > > > > > unpin in essentially one operation. > > > > > > > > > > > > Btw, one thing that's been on my mind is that I think we got > > > > > > the > > > > > > polarity on FOLL_LONGTERM wrong. Instead of opting into the > > > > > > long term > > > > > > behavior it really should be the default, with a > > > > > > FOLL_EPHEMERAL flag > > > > > > to opt out of it. And every users of this flag is required > > > > > > to have > > > > > > a comment explaining the life time rules for the pin.. > > > > I couldn't agree more, based on my recent forays into GUP the > > interface > > continues to strike me as odd:- > > > > - FOLL_GET is a wing and a prayer that nothing that > > [folio|page]_maybe_dma_pinned() prevents happens in the brief > > period the > > page is pinned/manipulated. So agree completely with David's > > concept of > > unexporting that and perhaps carefully considering our use of > > it. Obviously the comments around functions like gup_remote() make > > clear > > that 'this page not be what you think it is' but I wonder whether > > many > > callers of GUP _truly_ take that on board. > > > > - FOLL_LONGTERM is entirely optional for PUP and you can just go > > ahead and > > fragment page blocks to your heart's content. Of course this would > > be an > > abuse, but abuses happen. > > > > - With the recent change to PUP/FOLL_LONGTERM disallowing dirty > > tracked > > file-backed mappings we're now really relying on this flag > > indicating a > > _long term_ pin semantically. By defaulting to this being switched > > on, we > > avoid cases of callers who might end up treating the won't > > reclaim/etc. aspect of PUP as all they care about while ignoring > > the > > MIGRATE_MOVABLE aspect. > > > > > > > > I see maybe 10 or 20 call sites today. So it is definitely feasible > > > to add > > > documentation at each, explaining the why it wants a long term pin. > > > > > > > Yeah, my efforts at e.g. dropping vmas has been eye-opening in > > actually > > quite how often a refactoring like this often ends up being more > > straightforward than you might imagine. > > > > > > > > > > > > It does look like a better approach to me given the very nature > > > > > of > > > > > user space pages. > > > > > > > > Yeah, there is a lot of historical baggage. For example, FOLL_GET > > > > should be inaccessible to kernel modules completely at one point, > > > > to be only used by selected core-mm pieces. > > > > > > Yes. When I first mass-converted call sites from gup to pup, I just > > > preserved FOLL_GET behavior in order to keep from changing too much > > > at > > > once. But I agree that that it would be nice to make FOLL_GET an > > > mm internal-only flag like FOLL_PIN. > > > > Very glad you did that work! And totally understandable as to you > > being > > conservative with that, but I think we're at a point where there's > > more > > acceptance of incremental improvements to GUP as a whole. > > > > I have another patch series saved up for _yet more_ changes on this. > > But > > mindful of churn I am trying to space them out... until Jason nudges > > me of > > course :) > > > > > > > > > > > > > Maybe we should even disallow passing in FOLL_LONGTERM as a flag > > > > and only provide functions like pin_user_pages() vs. > > > > pin_user_pages_longterm(). Then, discussions about conditional > > > > flag-setting are no more :) > > > > > > > > ... or even use pin_user_pages_shortterm() vs. pin_user_pages() > > > > ... to make the default be longterm. > > > > > > > > > > Yes, it is true that having most gup flags be internal to mm does > > > tend > > > to avoid some bugs. But it's also a lot of churn. I'm still on the > > > fence > > > as to whether it's really a good move to do this for FOLL_LONGTERM > > > or > > > not. But it's really easy to push me off of fences. :) > > > > *nudge* ;) > > > > > > > > thanks, > > > -- > > > John Hubbard > > > NVIDIA > > > > > > > Looking at non-fast, non-FOLL_LONGTERM PUP callers (forgive me if I > > missed any):- > > > > - pin_user_pages_remote() in process_vm_rw_single_vec() for the > > process_vm_access functionality. > > > > - pin_user_pages_remote() in user_event_enabler_write() in > > kernel/trace/trace_events_user.c. > > > > - pin_user_pages_unlocked() in ivtv_udma_setup() in > > drivers/media/pci/ivtv/ivtv-udma.c and ivtv_yuv_prep_user_dma() in > > ivtv-yuv.c. > > > > And none that actually directly invoke PUP without FOLL_LOGNTERM... > > That > > suggests that we could simply disallow non-FOLL_LONGTERM non-fast PUP > > calls > > altogether and move to pin_user_pages_longterm() [I'm happy to write > > a > > patch series doing this]. > > > > The ivtv callers look like they really actually want FOLL_LONGTERM > > unless > > I'm missing something so we should probably change that too? > > > > I haven't surveyed the fast versions, but I think defaulting to > > FOLL_LONGTERM on them also makes sense. >
