I see the number of entries in the list often exceeds list_limit
currently hardcoded to 1024 for full HD scanout resource (==
1920*1080*4 bytes). Can we include a change to increase it to something
like 4096 or higher in this patch?

On Fri, Jun 04, 2021 at 01:59:39PM -0700, Vivek Kasireddy wrote:
> If the VMM's (Qemu) memory backend is backed up by memfd + Hugepages
> (hugetlbfs and not THP), we have to first find the hugepage(s) where
> the Guest allocations are located and then extract the regular 4k
> sized subpages from them.
> 
> v2: Ensure that the subpage and hugepage offsets are calculated correctly
> when the range of subpage allocations cuts across multiple hugepages.
> 
> v3: Instead of repeatedly looking up the hugepage for each subpage,
> only do it when the subpage allocation crosses over into a different
> hugepage. (suggested by Gerd and DW)
> 
> Cc: Gerd Hoffmann <kra...@redhat.com>
> Signed-off-by: Vivek Kasireddy <vivek.kasire...@intel.com>
> Signed-off-by: Dongwon Kim <dongwon....@intel.com>
> ---
>  drivers/dma-buf/udmabuf.c | 51 +++++++++++++++++++++++++++++++++------
>  1 file changed, 44 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/dma-buf/udmabuf.c b/drivers/dma-buf/udmabuf.c
> index db732f71e59a..2e02bbfe30fd 100644
> --- a/drivers/dma-buf/udmabuf.c
> +++ b/drivers/dma-buf/udmabuf.c
> @@ -11,6 +11,7 @@
>  #include <linux/shmem_fs.h>
>  #include <linux/slab.h>
>  #include <linux/udmabuf.h>
> +#include <linux/hugetlb.h>
>  
>  static const u32    list_limit = 1024;  /* udmabuf_create_list->count limit 
> */
>  static const size_t size_limit_mb = 64; /* total dmabuf size, in megabytes  
> */
> @@ -163,7 +164,9 @@ static long udmabuf_create(struct miscdevice *device,
>       struct udmabuf *ubuf;
>       struct dma_buf *buf;
>       pgoff_t pgoff, pgcnt, pgidx, pgbuf = 0, pglimit;
> -     struct page *page;
> +     struct page *page, *hpage = NULL;
> +     pgoff_t subpgoff, maxsubpgs;
> +     struct hstate *hpstate;
>       int seals, ret = -EINVAL;
>       u32 i, flags;
>  
> @@ -194,7 +197,8 @@ static long udmabuf_create(struct miscdevice *device,
>               memfd = fget(list[i].memfd);
>               if (!memfd)
>                       goto err;
> -             if (!shmem_mapping(file_inode(memfd)->i_mapping))
> +             if (!shmem_mapping(file_inode(memfd)->i_mapping) &&
> +                 !is_file_hugepages(memfd))
>                       goto err;
>               seals = memfd_fcntl(memfd, F_GET_SEALS, 0);
>               if (seals == -EINVAL)
> @@ -205,17 +209,50 @@ static long udmabuf_create(struct miscdevice *device,
>                       goto err;
>               pgoff = list[i].offset >> PAGE_SHIFT;
>               pgcnt = list[i].size   >> PAGE_SHIFT;
> +             if (is_file_hugepages(memfd)) {
> +                     hpstate = hstate_file(memfd);
> +                     pgoff = list[i].offset >> huge_page_shift(hpstate);
> +                     subpgoff = (list[i].offset &
> +                                 ~huge_page_mask(hpstate)) >> PAGE_SHIFT;
> +                     maxsubpgs = huge_page_size(hpstate) >> PAGE_SHIFT;
> +             }
>               for (pgidx = 0; pgidx < pgcnt; pgidx++) {
> -                     page = shmem_read_mapping_page(
> -                             file_inode(memfd)->i_mapping, pgoff + pgidx);
> -                     if (IS_ERR(page)) {
> -                             ret = PTR_ERR(page);
> -                             goto err;
> +                     if (is_file_hugepages(memfd)) {
> +                             if (!hpage) {
> +                                     hpage = find_get_page_flags(
> +                                             file_inode(memfd)->i_mapping,
> +                                             pgoff, FGP_ACCESSED);
> +                                     if (IS_ERR(hpage)) {
> +                                             ret = PTR_ERR(hpage);
> +                                             goto err;
> +                                     }
> +                             }
> +                             page = hpage + subpgoff;
> +                             get_page(page);
> +                             subpgoff++;
> +                             if (subpgoff == maxsubpgs) {
> +                                     put_page(hpage);
> +                                     hpage = NULL;
> +                                     subpgoff = 0;
> +                                     pgoff++;
> +                             }
> +                     } else {
> +                             page = shmem_read_mapping_page(
> +                                     file_inode(memfd)->i_mapping,
> +                                     pgoff + pgidx);
> +                             if (IS_ERR(page)) {
> +                                     ret = PTR_ERR(page);
> +                                     goto err;
> +                             }
>                       }
>                       ubuf->pages[pgbuf++] = page;
>               }
>               fput(memfd);
>               memfd = NULL;
> +             if (hpage) {
> +                     put_page(hpage);
> +                     hpage = NULL;
> +             }
>       }
>  
>       exp_info.ops  = &udmabuf_ops;
> -- 
> 2.30.2
> 

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