On 12/21/22 17:10, Peter Xu wrote: > On Wed, Dec 21, 2022 at 01:39:39PM -0800, Mike Kravetz wrote: > > On 12/21/22 15:21, James Houghton wrote: > > > On Wed, Dec 21, 2022 at 2:23 PM Peter Xu <peterx@xxxxxxxxxx> wrote: > > > > > > > > James, > > > > > > > > On Wed, Nov 16, 2022 at 03:30:00PM -0800, James Houghton wrote: > > > > > On Wed, Nov 16, 2022 at 2:28 PM Peter Xu <peterx@xxxxxxxxxx> wrote: > > > > > > > > > > > > On Fri, Oct 21, 2022 at 04:36:49PM +0000, James Houghton wrote: > > > > > > > Userspace must provide this new feature when it calls UFFDIO_API to > > > > > > > enable HGM. Userspace can check if the feature exists in > > > > > > > uffdio_api.features, and if it does not exist, the kernel does not > > > > > > > support and therefore did not enable HGM. > > > > > > > > > > > > > > Signed-off-by: James Houghton <jthoughton@xxxxxxxxxx> > > > > > > > > > > > > It's still slightly a pity that this can only be enabled by an uffd context > > > > > > plus a minor fault, so generic hugetlb users cannot directly leverage this. > > > > > > > > > > The idea here is that, for applications that can conceivably benefit > > > > > from HGM, we have a mechanism for enabling it for that application. So > > > > > this patch creates that mechanism for userfaultfd/UFFDIO_CONTINUE. I > > > > > prefer this approach over something more general like MADV_ENABLE_HGM > > > > > or something. > > > > > > > > Sorry to get back to this very late - I know this has been discussed since > > > > the very early stage of the feature, but is there any reasoning behind? > > > > > > > > When I start to think seriously on applying this to process snapshot with > > > > uffd-wp I found that the minor mode trick won't easily play - normally > > > > that's a case where all the pages were there mapped huge, but when the app > > > > wants UFFDIO_WRITEPROTECT it may want to remap the huge pages into smaller > > > > pages, probably some size that the user can specify. It'll be non-trivial > > > > to enable HGM during that phase using MINOR mode because in that case the > > > > pages are all mapped. > > > > > > > > For the long term, I am just still worried the current interface is still > > > > not as flexible. > > > > > > Thanks for bringing this up, Peter. I think the main reason was: > > > having separate UFFD_FEATUREs clearly indicates to userspace what is > > > and is not supported. > > > > IIRC, I think we wanted to initially limit the usage to the very > > specific use case (live migration). The idea is that we could then > > expand usage as more use cases came to light. > > > > Another good thing is that userfaultfd has versioning built into the > > API. Thus a user can determine if HGM is enabled in their running > > kernel. > > I don't worry much on this one, afaiu if we have any way to enable hgm then > the user can just try enabling it on a test vma, just like when an app > wants to detect whether a new madvise() is present on the current host OS. > > Besides, I'm wondering whether something like /sys/kernel/vm/hugepages/hgm > would work too. > > > > > > For UFFDIO_WRITEPROTECT, a user could remap huge pages into smaller > > > pages by issuing a high-granularity UFFDIO_WRITEPROTECT. That isn't > > > allowed as of this patch series, but it could be allowed in the > > > future. To add support in the same way as this series, we would add > > > another feature, say UFFD_FEATURE_WP_HUGETLBFS_HGM. I agree that > > > having to add another feature isn't great; is this what you're > > > concerned about? > > > > > > Considering MADV_ENABLE_HUGETLB... > > > 1. If a user provides this, then the contract becomes: "the kernel may > > > allow UFFDIO_CONTINUE and UFFDIO_WRITEPROTECT for HugeTLB at > > > high-granularities, provided the support exists", but it becomes > > > unclear to userspace to know what's supported and what isn't. > > > 2. We would then need to keep track if a user explicitly enabled it, > > > or if it got enabled automatically in response to memory poison, for > > > example. Not a big problem, just a complication. (Otherwise, if HGM > > > got enabled for poison, suddenly userspace would be allowed to do > > > things it wasn't allowed to do before.) > > We could alternatively have two flags for each vma: (a) hgm_advised and (b) > hgm_enabled. (a) always sets (b) but not vice versa. We can limit poison > to set (b) only. For this patchset, it can be all about (a). > > > > 3. This API makes sense for enabling HGM for something outside of > > > userfaultfd, like MADV_DONTNEED. > > > > I think #3 is key here. Once we start applying HGM to things outside > > userfaultfd, then more thought will be required on APIs. The API is > > somewhat limited by design until the basic functionality is in place. > > Mike, could you elaborate what's the major concern of having hgm used > outside uffd and live migration use cases? > > I feel like I miss something here. I can understand we want to limit the > usage only when the user specifies using hgm because we want to keep the > old behavior intact. However if we want another way to enable hgm it'll > still need one knob anyway even outside uffd, and I thought that'll service > the same purpose, or maybe not? I am not opposed to using hgm outside the use cases targeted by this series. It seems that when we were previously discussing the API we spent a bunch of time going around in circles trying to get the API correct. That is expected as it is more difficult to take all users/uses/abuses of the API into account. Since the initial use case was fairly limited, it seemed like a good idea to limit the API to userfaultfd. In this way we could focus on the underlying code/implementation and then expand as needed. Of course, with an eye on anything that may be a limiting factor in the future. I was not aware of the uffd-wp use case, and am more than happy to discuss expanding the API. -- Mike Kravetz
