| Message ID | 20250327092922.536-1-link@vivo.com (mailing list archive) |
|---|---|
| Headers | show |
| Series | Deep talk about folio vmap | expand |
On Thu, Mar 27, 2025 at 05:28:27PM +0800, Huan Yang wrote: > Bingbu reported an issue in [1] that udmabuf vmap failed and in [2], we > discussed the scenario of folio vmap due to the misuse of vmap_pfn > in udmabuf. > > We reached the conclusion that vmap_pfn prohibits the use of page-based > PFNs: > Christoph Hellwig : 'No, vmap_pfn is entirely for memory not backed by > pages or folios, i.e. PCIe BARs and similar memory. This must not be > mixed with proper folio backed memory.' > > But udmabuf still need consider HVO based folio's vmap, and need fix > vmap issue. This RFC code want to show the two point that I mentioned > in [2], and more deep talk it: > > Point1. simple copy vmap_pfn code, don't bother common vmap_pfn, use by > itself and remove pfn_valid check. > > Point2. implement folio array based vmap(vmap_folios), which can given a > range of each folio(offset, nr_pages), so can suit HVO folio's vmap. > > Patch 1-2 implement point1, and add a test simple set in udmabuf driver. > Patch 3-5 implement point2, also can test it. > > Kasireddy also show that 'another option is to just limit udmabuf's vmap() > to only shmem folios'(This I guess folio_test_hugetlb_vmemmap_optimized > can help.) > > But I prefer point2 to solution this issue, and IMO, folio based vmap still > need. > > Compare to page based vmap(or pfn based), we need split each large folio > into single page struct, this need more large array struct and more longer > iter. If each tail page struct not exist(like HVO), can only use pfn vmap, > but there are no common api to do this. > > In [2], we talked that udmabuf can use hugetlb as the memory > provider, and can give a range use. So if HVO used in hugetlb, each folio's > tail page may freed, so we can't use page based vmap, only can use pfn > based, which show in point1. > > Further more, Folio based vmap only need record each folio(and offset, > nr_pages if range need). For 20MB vmap, page based need 5120 pages(40KB), > 2MB folios only need 10 folio(80Byte). > > Matthew show that Vishal also offered a folio based vmap - vmap_file[3]. > This RFC patch want a range based folio, not only a full folio's map(like > file's folio), to resolve some problem like HVO's range folio vmap. Hmmm, I should've been more communicative, sorry about that. V1 was poorly implemented, and I've had a V2 sitting around that does Exactly what you want. I'll send V2 to the mailing list and you can take a look at it; preferrably you integrate that into this patchset instead (it would make both the udma and vmalloc code much neater). > Please give me more suggestion. > > Test case: > //enable/disable HVO > 1. echo [1|0] > /proc/sys/vm/hugetlb_optimize_vmemmap > //prepare HUGETLB > 2. echo 10 > /sys/kernel/mm/hugepages/hugepages-2048kB/nr_hugepages > 3. ./udmabuf_vmap > 4. check output, and dmesg if any warn. > > [1] https://lore.kernel.org/all/9172a601-c360-0d5b-ba1b-33deba430455@linux.intel.com/ > [2] https://lore.kernel.org/lkml/20250312061513.1126496-1-link@vivo.com/ > [3] https://lore.kernel.org/linux-mm/20250131001806.92349-1-vishal.moola@gmail.com/ > > Huan Yang (6): > udmabuf: try fix udmabuf vmap > udmabuf: try udmabuf vmap test > mm/vmalloc: try add vmap folios range > udmabuf: use vmap_range_folios > udmabuf: vmap test suit for pages and pfns compare > udmabuf: remove no need code > > drivers/dma-buf/udmabuf.c | 29 +++++++++----------- > include/linux/vmalloc.h | 57 +++++++++++++++++++++++++++++++++++++++ > mm/vmalloc.c | 47 ++++++++++++++++++++++++++++++++ > 3 files changed, 117 insertions(+), 16 deletions(-) > > -- > 2.48.1 >