linux-kernel - Re: [PATCH v27 2/6] fs/proc/task_mmu: Implement IOCTL to get and optionally clear info about PTEs

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Date:   Tue, 8 Aug 2023 12:21:10 -0700
From:   Andrei Vagin <avagin@...il.com>
To:     Muhammad Usama Anjum <usama.anjum@...labora.com>
Cc:     Peter Xu <peterx@...hat.com>, David Hildenbrand <david@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michał Mirosław <emmir@...gle.com>,
        Danylo Mocherniuk <mdanylo@...gle.com>,
        Paul Gofman <pgofman@...eweavers.com>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Mike Rapoport <rppt@...nel.org>, Nadav Amit <namit@...are.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Shuah Khan <shuah@...nel.org>,
        Christian Brauner <brauner@...nel.org>,
        Yang Shi <shy828301@...il.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        Yun Zhou <yun.zhou@...driver.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Alex Sierra <alex.sierra@....com>,
        Matthew Wilcox <willy@...radead.org>,
        Pasha Tatashin <pasha.tatashin@...een.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        "Gustavo A . R . Silva" <gustavoars@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, linux-kselftest@...r.kernel.org,
        Greg KH <gregkh@...uxfoundation.org>, kernel@...labora.com,
        Michał Mirosław <mirq-linux@...e.qmqm.pl>
Subject: Re: [PATCH v27 2/6] fs/proc/task_mmu: Implement IOCTL to get and
 optionally clear info about PTEs

On Tue, Aug 8, 2023 at 3:43 AM Muhammad Usama Anjum
<usama.anjum@...labora.com> wrote:

....

> +static int pagemap_scan_output(unsigned long categories,
> +                              struct pagemap_scan_private *p,
> +                              unsigned long addr, unsigned long *end)
> +{
> +       unsigned long n_pages, total_pages;
> +       int ret = 0;
> +
> +       if (!p->vec_buf)
> +               return 0;
> +
> +       categories &= p->arg.return_mask;
> +
> +       n_pages = (*end - addr) / PAGE_SIZE;
> +       if (check_add_overflow(p->found_pages, n_pages, &total_pages) || //TODO

Need to fix this TODO.

> +           total_pages > p->arg.max_pages) {
> +               size_t n_too_much = total_pages - p->arg.max_pages;
> +               *end -= n_too_much * PAGE_SIZE;
> +               n_pages -= n_too_much;
> +               ret = -ENOSPC;
> +       }
> +
> +       if (!pagemap_scan_push_range(categories, p, addr, *end)) {
> +               *end = addr;
> +               n_pages = 0;
> +               ret = -ENOSPC;
> +       }
> +
> +       p->found_pages += n_pages;
> +       if (ret)
> +               p->walk_end_addr = *end;
> +
> +       return ret;
> +}
> +

...

> +static long do_pagemap_scan(struct mm_struct *mm, unsigned long uarg)
> +{
> +       struct mmu_notifier_range range;
> +       struct pagemap_scan_private p;
> +       unsigned long walk_start;
> +       size_t n_ranges_out = 0;
> +       int ret;
> +
> +       memset(&p, 0, sizeof(p));
> +       ret = pagemap_scan_get_args(&p.arg, uarg);
> +       if (ret)
> +               return ret;
> +
> +       p.masks_of_interest = MASKS_OF_INTEREST(p.arg);
> +       ret = pagemap_scan_init_bounce_buffer(&p);
> +       if (ret)
> +               return ret;
> +
> +       /* Protection change for the range is going to happen. */
> +       if (p.arg.flags & PM_SCAN_WP_MATCHING) {
> +               mmu_notifier_range_init(&range, MMU_NOTIFY_PROTECTION_VMA, 0,
> +                                       mm, p.arg.start, p.arg.end);
> +               mmu_notifier_invalidate_range_start(&range);
> +       }
> +
> +       walk_start = p.arg.start;
> +       for (; walk_start < p.arg.end; walk_start = p.arg.walk_end) {
> +               int n_out;
> +
> +               if (fatal_signal_pending(current)) {
> +                       ret = -EINTR;
> +                       break;
> +               }
> +
> +               ret = mmap_read_lock_killable(mm);
> +               if (ret)
> +                       break;
> +               ret = walk_page_range(mm, walk_start, p.arg.end,
> +                                     &pagemap_scan_ops, &p);
> +               mmap_read_unlock(mm);
> +
> +               n_out = pagemap_scan_flush_buffer(&p);
> +               if (n_out < 0)
> +                       ret = n_out;
> +               else
> +                       n_ranges_out += n_out;
> +
> +               if (ret != -ENOSPC || p.arg.vec_len - 1 == 0 ||
> +                   p.found_pages == p.arg.max_pages) {
> +                       p.walk_end_addr = p.arg.end;

You should not change p.walk_end_addr If ret is ENOSPC. Pls add a test
case to check this.

> +                       break;
> +               }
> +       }
> +
> +       if (p.cur_buf.start != p.cur_buf.end) {
> +               if (copy_to_user(p.vec_out, &p.cur_buf, sizeof(p.cur_buf)))
> +                       ret = -EFAULT;
> +               else
> +                       ++n_ranges_out;
> +       }
> +
> +       /* ENOSPC signifies early stop (buffer full) from the walk. */
> +       if (!ret || ret == -ENOSPC)
> +               ret = n_ranges_out;
> +
> +       p.arg.walk_end = p.walk_end_addr ? p.walk_end_addr : walk_start;
> +       if (pagemap_scan_writeback_args(&p.arg, uarg))
> +               ret = -EFAULT;
> +
> +       if (p.arg.flags & PM_SCAN_WP_MATCHING)
> +               mmu_notifier_invalidate_range_end(&range);
> +
> +       kfree(p.vec_buf);
> +       return ret;
> +}

Thanks,
Andrei