On Tue, 16 May 2023 15:34:40 -0700 Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote: > On 05/15/23 10:04, Mike Kravetz wrote: > > On 05/12/23 16:29, Mike Kravetz wrote: > > > On 05/12/23 14:26, James Houghton wrote: > > > > On Fri, May 12, 2023 at 12:20 AM Junxiao Chang <junxiao.chang@xxxxxxxxx> wrote: > > > > > > > > This alone doesn't fix mapcounting for PTE-mapped HugeTLB pages. You > > > > need something like [1]. I can resend it if that's what we should be > > > > doing, but this mapcounting scheme doesn't work when the page structs > > > > have been freed. > > > > > > > > It seems like it was a mistake to include support for hugetlb memfds in udmabuf. > > > > > > IIUC, it was added with commit 16c243e99d33 udmabuf: Add support for mapping > > > hugepages (v4). Looks like it was never sent to linux-mm? That is unfortunate > > > as hugetlb vmemmap freeing went in at about the same time. And, as you have > > > noted udmabuf will not work if hugetlb vmemmap freeing is enabled. > > > > > > Sigh! > > > > > > Trying to think of a way forward. > > > -- > > > Mike Kravetz > > > > > > > > > > > [1]: https://lore.kernel.org/linux-mm/20230306230004.1387007-2-jthoughton@xxxxxxxxxx/ > > > > > > > > - James > > > > Adding people and list on Cc: involved with commit 16c243e99d33. > > > > There are several issues with trying to map tail pages of hugetllb pages > > not taken into account with udmabuf. James spent quite a bit of time trying > > to understand and address all the issues with the HGM code. While using > > the scheme proposed by James, may be an approach to the mapcount issue there > > are also other issues that need attention. For example, I do not see how > > the fault code checks the state of the hugetlb page (such as poison) as none > > of that state is carried in tail pages. > > > > The more I think about it, the more I think udmabuf should treat hugetlb > > pages as hugetlb pages. They should be mapped at the appropriate level > > in the page table. Of course, this would impose new restrictions on the > > API (mmap and ioctl) that may break existing users. I have no idea how > > extensively udmabuf is being used with hugetlb mappings. > > Verified that using udmabug on a hugetlb mapping with vmemmap optimization will > BUG as: BUGs aren't good. Can we please find a way to push this along? Have we heard anything from any udmabuf people?
