[Devel] [PATCH VZ9] fs/fuse kio: Refactor pcs_mr to avoid large contiguous memory allocation

[Alexey Kuznetsov]

Ack

On Fri, Aug 1, 2025 at 4:56 PM Liu Kui <kui.liu at virtuozzo.com> wrote:
>
> Fix page allocation failure in kcalloc by converting the large 1-dimensional
> array into a 2-dimensional array of order-0 (4KB) pages. This eliminates the
> need for non-order-0 allocations, improving allocation reliability under memory
> pressure. The 2D array ensures virtual contiguity for the caller maintaining
> functional equivalence.
>
> Related to #VSTOR-112413
> https://virtuozzo.atlassian.net/browse/VSTOR-112413
>
> Signed-off-by: Liu Kui <kui.liu at virtuozzo.com>
> ---
>  fs/fuse/kio/pcs/pcs_mr.c | 55 +++++++++++++++++++++++++---------------
>  fs/fuse/kio/pcs/pcs_mr.h | 21 ++++++++++-----
>  2 files changed, 50 insertions(+), 26 deletions(-)
>
> diff --git a/fs/fuse/kio/pcs/pcs_mr.c b/fs/fuse/kio/pcs/pcs_mr.c
> index cbd3b440dd1b..8e1dbc1121ce 100644
> --- a/fs/fuse/kio/pcs/pcs_mr.c
> +++ b/fs/fuse/kio/pcs/pcs_mr.c
> @@ -13,11 +13,18 @@
>  void pcs_umem_release(struct pcs_umem *umem)
>  {
>         struct mm_struct *mm_s = umem->mm;
> +       int i, npages = umem->npages;
>
> -       unpin_user_pages(umem->pages, umem->npages);
> +       for (i = 0; npages; i++) {
> +               int to_free = min_t(int, PCS_PAGES_PER_CHUNK, npages);
> +
> +               unpin_user_pages(umem->page_chunk[i].pages, to_free);
> +               kfree(umem->page_chunk[i].pages);
> +               npages -= to_free;
> +       }
>         atomic64_sub(umem->npages, &mm_s->pinned_vm);
>         mmdrop(mm_s);
> -       kfree(umem->pages);
> +       kfree(umem->page_chunk);
>         kfree(umem);
>  }
>
> @@ -27,14 +34,13 @@ void pcs_umem_release(struct pcs_umem *umem)
>  struct pcs_umem *pcs_umem_get(u64 start, u64 len)
>  {
>         struct pcs_umem *umem = NULL;
> -       struct page **pages;
> -       int npages;
> -       u64 fp_va;
>         struct mm_struct *mm_s;
> -       int got, ret;
> +       u64 fp_va;
> +       int npages, nchunks, i, ret;
>
>         fp_va = start & PAGE_MASK;
>         npages = PAGE_ALIGN(start + len - fp_va) >> PAGE_SHIFT;
> +       nchunks = (npages >> PCS_PAGE_CHUNK_SHIFT) + 1;
>
>         umem = kzalloc(sizeof(*umem), GFP_KERNEL);
>         if (!umem)
> @@ -48,25 +54,34 @@ struct pcs_umem *pcs_umem_get(u64 start, u64 len)
>         mmap_read_lock(mm_s);
>
>         umem->fp_addr = fp_va;
> -       umem->pages = kcalloc(npages, sizeof(struct page *), GFP_KERNEL);
> -       if (!umem->pages) {
> +       umem->page_chunk = kcalloc(nchunks, sizeof(struct pcs_page_chunk *), GFP_KERNEL);
> +       if (!umem->page_chunk) {
>                 ret = -ENOMEM;
>                 goto out_err;
>         }
>
> -       got = 0;
> -       while (npages) {
> -               pages = &umem->pages[got];
> -               ret = pin_user_pages(fp_va, npages, FOLL_WRITE | FOLL_LONGTERM, pages, NULL);
> -               if (ret < 0)
> -                       goto out_err;
> +       for (i = 0; npages; i++) {
> +               int n = min_t(int, npages, PCS_PAGES_PER_CHUNK);
> +               struct page **pages = kcalloc(n, sizeof(struct page *), GFP_KERNEL);
>
> -               WARN_ON(ret == 0);
> -               umem->npages += ret;
> -               atomic64_add(ret, &mm_s->pinned_vm);
> -               fp_va += ret * PAGE_SIZE;
> -               npages -= ret;
> -               got += ret;
> +               if (!pages) {
> +                       ret = -ENOMEM;
> +                       goto out_err;
> +               }
> +               umem->page_chunk[i].pages = pages;
> +
> +               while (n) {
> +                       ret = pin_user_pages(fp_va, n, FOLL_WRITE | FOLL_LONGTERM, pages, NULL);
> +                       if (ret < 0)
> +                               goto out_err;
> +
> +                       atomic64_add(ret, &mm_s->pinned_vm);
> +                       umem->npages += ret;
> +                       fp_va += ret * PAGE_SIZE;
> +                       pages += ret;
> +                       n -= ret;
> +                       npages -= ret;
> +               }
>         }
>         mmap_read_unlock(mm_s);
>
> diff --git a/fs/fuse/kio/pcs/pcs_mr.h b/fs/fuse/kio/pcs/pcs_mr.h
> index dae9931d9967..64f237f57dec 100644
> --- a/fs/fuse/kio/pcs/pcs_mr.h
> +++ b/fs/fuse/kio/pcs/pcs_mr.h
> @@ -11,6 +11,8 @@
>  struct pcs_umem;
>
>  #define PCS_MAX_MR             0x10000
> +#define PCS_PAGE_CHUNK_SHIFT 9
> +#define PCS_PAGES_PER_CHUNK (1 << PCS_PAGE_CHUNK_SHIFT)
>
>  struct pcs_mr_set {
>         struct xarray   mr_xa;  /* array of registered MRs*/
> @@ -18,10 +20,15 @@ struct pcs_mr_set {
>         atomic_t        mr_num;         /* number of registered MRs*/
>  };
>
> +struct pcs_page_chunk
> +{
> +       struct page **pages; /* array of pinned pages */
> +};
> +
>  struct pcs_umem {
> -       u64     fp_addr;        /* First page base address */
> -       int     npages;         /* number of pinned pages */
> -       struct page     **pages;        /* array of pinned pages */
> +       struct pcs_page_chunk *page_chunk;
> +       int npages; /* number of pinned pages */
> +       u64 fp_addr; /* First page base address */
>         struct mm_struct *mm;   /* mm the memory belongs to */
>  };
>
> @@ -40,10 +47,12 @@ struct pcs_mr {
>   */
>  static inline struct page *pcs_umem_page(struct pcs_umem *umem, u64 addr)
>  {
> -       unsigned int idx = (addr - umem->fp_addr) >> PAGE_SHIFT;
> +       unsigned int page_idx = (addr - umem->fp_addr) >> PAGE_SHIFT;
> +       unsigned int chunk_idx = page_idx >> PCS_PAGE_CHUNK_SHIFT;
> +       unsigned int page_in_chunk = page_idx & (PCS_PAGES_PER_CHUNK - 1);
>
> -       if (likely(idx < umem->npages))
> -               return umem->pages[idx];
> +       if (likely(page_idx < umem->npages))
> +               return umem->page_chunk[chunk_idx].pages[page_in_chunk];
>         return NULL;
>  }
>
> --
> 2.39.5 (Apple Git-154)