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From:   =?UTF-8?B?TWljaGHFgiBNaXJvc8WCYXc=?= <emmir@google.com>
Date:   Mon, 24 Jul 2023 16:38:01 +0200
Message-ID: <CABb0KFF6M2_94Ect72zMtaRLBpOoHjHYJA-Ube3oQAh4cXSg5w@mail.gmail.com>
Subject: Re: [v2] fs/proc/task_mmu: Implement IOCTL for efficient page table scanning
To:     Muhammad Usama Anjum <usama.anjum@collabora.com>
Cc:     =?UTF-8?B?TWljaGHFgiBNaXJvc8WCYXc=?= <mirq-linux@rere.qmqm.pl>,
        Andrei Vagin <avagin@gmail.com>,
        Danylo Mocherniuk <mdanylo@google.com>,
        Alex Sierra <alex.sierra@amd.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Andrew Morton <akpm@linux-foundation.org>,
        Axel Rasmussen <axelrasmussen@google.com>,
        Christian Brauner <brauner@kernel.org>,
        Cyrill Gorcunov <gorcunov@gmail.com>,
        Dan Williams <dan.j.williams@intel.com>,
        David Hildenbrand <david@redhat.com>,
        Greg KH <gregkh@linuxfoundation.org>,
        "Gustavo A . R . Silva" <gustavoars@kernel.org>,
        "Liam R . Howlett" <Liam.Howlett@oracle.com>,
        Matthew Wilcox <willy@infradead.org>,
        Mike Rapoport <rppt@kernel.org>, Nadav Amit <namit@vmware.com>,
        Pasha Tatashin <pasha.tatashin@soleen.com>,
        Paul Gofman <pgofman@codeweavers.com>,
        Peter Xu <peterx@redhat.com>, Shuah Khan <shuah@kernel.org>,
        Suren Baghdasaryan <surenb@google.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Yang Shi <shy828301@gmail.com>,
        Yun Zhou <yun.zhou@windriver.com>,
        linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-mm@kvack.org, linux-kselftest@vger.kernel.org,
        kernel@collabora.com
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On Mon, 24 Jul 2023 at 16:04, Muhammad Usama Anjum
<usama.anjum@collabora.com> wrote:
>
> Fixed found bugs. Testing it further.
>
> - Split and backoff in case buffer full case as well
> - Fix the wrong breaking of loop if page isn't interesting, skip intead
> - Untag the address and save them into struct
> - Round off the end address to next page
>
> Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
> ---
>  fs/proc/task_mmu.c | 54 ++++++++++++++++++++++++++--------------------
>  1 file changed, 31 insertions(+), 23 deletions(-)
>
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index add21fdf3c9a..64b326d0ec6d 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -1985,18 +1989,19 @@ static int pagemap_scan_output(unsigned long
> categories,
>         unsigned long n_pages, total_pages;
>         int ret =3D 0;
>
> +       if (!p->vec_buf)
> +               return 0;
> +
>         if (!pagemap_scan_is_interesting_page(categories, p)) {
>                 *end =3D addr;
>                 return 0;
>         }
>
> -       if (!p->vec_buf)
> -               return 0;
> -
>         categories &=3D p->arg.return_mask;

This is wrong - is_interesting() check must happen before output as
the `*end =3D addr` means the range should be skipped, but return 0
requests continuing of the walk.

> @@ -2044,7 +2050,7 @@ static int pagemap_scan_thp_entry(pmd_t *pmd,
> unsigned long start,
>          * Break huge page into small pages if the WP operation
>          * need to be performed is on a portion of the huge page.
>          */
> -       if (end !=3D start + HPAGE_SIZE) {
> +       if (end !=3D start + HPAGE_SIZE || ret =3D=3D -ENOSPC) {

Why is it needed? If `end =3D=3D start + HPAGE_SIZE` then we're handling a
full hugepage anyway.

> @@ -2066,8 +2072,8 @@ static int pagemap_scan_pmd_entry(pmd_t *pmd,
> unsigned long start,
>  {
>         struct pagemap_scan_private *p =3D walk->private;
>         struct vm_area_struct *vma =3D walk->vma;
> +       unsigned long addr, categories, next;
>         pte_t *pte, *start_pte;
> -       unsigned long addr;
>         bool flush =3D false;
>         spinlock_t *ptl;
>         int ret;
> @@ -2088,12 +2094,14 @@ static int pagemap_scan_pmd_entry(pmd_t *pmd,
> unsigned long start,
>         }
>
>         for (addr =3D start; addr !=3D end; pte++, addr +=3D PAGE_SIZE) {
> -               unsigned long categories =3D p->cur_vma_category |
> -                       pagemap_page_category(vma, addr, ptep_get(pte));
> -               unsigned long next =3D addr + PAGE_SIZE;
> +               categories =3D p->cur_vma_category |
> +                            pagemap_page_category(vma, addr, ptep_get(pt=
e));
> +               next =3D addr + PAGE_SIZE;

Why moving the variable declarations out of the loop?

>
>                 ret =3D pagemap_scan_output(categories, p, addr, &next);
> -               if (next =3D=3D addr)
> +               if (ret =3D=3D 0 && next =3D=3D addr)
> +                       continue;
> +               else if (next =3D=3D addr)
>                         break;

Ah, this indeed was a bug. Nit:

if (next =3D=3D addr) { if (!ret) continue; break; }

> @@ -2204,8 +2212,6 @@ static const struct mm_walk_ops pagemap_scan_ops =
=3D {
>  static int pagemap_scan_get_args(struct pm_scan_arg *arg,
>                                  unsigned long uarg)
>  {
> -       unsigned long start, end, vec;
> -
>         if (copy_from_user(arg, (void __user *)uarg, sizeof(*arg)))
>                 return -EFAULT;
>
> @@ -2219,22 +2225,24 @@ static int pagemap_scan_get_args(struct pm_scan_a=
rg
> *arg,
>              arg->category_anyof_mask | arg->return_mask) & ~PM_SCAN_CATE=
GORIES)
>                 return -EINVAL;
>
> -       start =3D untagged_addr((unsigned long)arg->start);
> -       end =3D untagged_addr((unsigned long)arg->end);
> -       vec =3D untagged_addr((unsigned long)arg->vec);
> +       arg->start =3D untagged_addr((unsigned long)arg->start);
> +       arg->end =3D untagged_addr((unsigned long)arg->end);
> +       arg->vec =3D untagged_addr((unsigned long)arg->vec);

BTW, We should we keep the tag in args writeback().

>         /* Validate memory pointers */
> -       if (!IS_ALIGNED(start, PAGE_SIZE))
> +       if (!IS_ALIGNED(arg->start, PAGE_SIZE))
>                 return -EINVAL;
> -       if (!access_ok((void __user *)start, end - start))
> +       if (!access_ok((void __user *)arg->start, arg->end - arg->start))
>                 return -EFAULT;
> -       if (!vec && arg->vec_len)
> +       if (!arg->vec && arg->vec_len)
>                 return -EFAULT;
> -       if (vec && !access_ok((void __user *)vec,
> +       if (arg->vec && !access_ok((void __user *)arg->vec,
>                               arg->vec_len * sizeof(struct page_region)))
>                 return -EFAULT;
>
>         /* Fixup default values */
> +       arg->end =3D (arg->end & ~PAGE_MASK) ?
> +                  ((arg->end & PAGE_MASK) + PAGE_SIZE) : (arg->end);

arg->end =3D ALIGN(arg->end, PAGE_SIZE);

>         if (!arg->max_pages)
>                 arg->max_pages =3D ULONG_MAX;
>

Best Regards
Micha=C5=82 Miros=C5=82aw