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Date: Mon, 24 Jul 2023 16:38:01 +0200
From: Michał Mirosław <emmir@...gle.com>
To: Muhammad Usama Anjum <usama.anjum@...labora.com>
Cc: Michał Mirosław <mirq-linux@...e.qmqm.pl>,
Andrei Vagin <avagin@...il.com>,
Danylo Mocherniuk <mdanylo@...gle.com>,
Alex Sierra <alex.sierra@....com>,
Alexander Viro <viro@...iv.linux.org.uk>,
Andrew Morton <akpm@...ux-foundation.org>,
Axel Rasmussen <axelrasmussen@...gle.com>,
Christian Brauner <brauner@...nel.org>,
Cyrill Gorcunov <gorcunov@...il.com>,
Dan Williams <dan.j.williams@...el.com>,
David Hildenbrand <david@...hat.com>,
Greg KH <gregkh@...uxfoundation.org>,
"Gustavo A . R . Silva" <gustavoars@...nel.org>,
"Liam R . Howlett" <Liam.Howlett@...cle.com>,
Matthew Wilcox <willy@...radead.org>,
Mike Rapoport <rppt@...nel.org>, Nadav Amit <namit@...are.com>,
Pasha Tatashin <pasha.tatashin@...een.com>,
Paul Gofman <pgofman@...eweavers.com>,
Peter Xu <peterx@...hat.com>, Shuah Khan <shuah@...nel.org>,
Suren Baghdasaryan <surenb@...gle.com>,
Vlastimil Babka <vbabka@...e.cz>,
Yang Shi <shy828301@...il.com>,
Yun Zhou <yun.zhou@...driver.com>,
linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
linux-mm@...ck.org, linux-kselftest@...r.kernel.org,
kernel@...labora.com
Subject: Re: [v2] fs/proc/task_mmu: Implement IOCTL for efficient page table scanning
On Mon, 24 Jul 2023 at 16:04, Muhammad Usama Anjum
<usama.anjum@...labora.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@...labora.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 = 0;
>
> + if (!p->vec_buf)
> + return 0;
> +
> if (!pagemap_scan_is_interesting_page(categories, p)) {
> *end = addr;
> return 0;
> }
>
> - if (!p->vec_buf)
> - return 0;
> -
> categories &= p->arg.return_mask;
This is wrong - is_interesting() check must happen before output as
the `*end = 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 != start + HPAGE_SIZE) {
> + if (end != start + HPAGE_SIZE || ret == -ENOSPC) {
Why is it needed? If `end == 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 = walk->private;
> struct vm_area_struct *vma = walk->vma;
> + unsigned long addr, categories, next;
> pte_t *pte, *start_pte;
> - unsigned long addr;
> bool flush = false;
> spinlock_t *ptl;
> int ret;
> @@ -2088,12 +2094,14 @@ static int pagemap_scan_pmd_entry(pmd_t *pmd,
> unsigned long start,
> }
>
> for (addr = start; addr != end; pte++, addr += PAGE_SIZE) {
> - unsigned long categories = p->cur_vma_category |
> - pagemap_page_category(vma, addr, ptep_get(pte));
> - unsigned long next = addr + PAGE_SIZE;
> + categories = p->cur_vma_category |
> + pagemap_page_category(vma, addr, ptep_get(pte));
> + next = addr + PAGE_SIZE;
Why moving the variable declarations out of the loop?
>
> ret = pagemap_scan_output(categories, p, addr, &next);
> - if (next == addr)
> + if (ret == 0 && next == addr)
> + continue;
> + else if (next == addr)
> break;
Ah, this indeed was a bug. Nit:
if (next == addr) { if (!ret) continue; break; }
> @@ -2204,8 +2212,6 @@ static const struct mm_walk_ops pagemap_scan_ops = {
> 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_arg
> *arg,
> arg->category_anyof_mask | arg->return_mask) & ~PM_SCAN_CATEGORIES)
> return -EINVAL;
>
> - start = untagged_addr((unsigned long)arg->start);
> - end = untagged_addr((unsigned long)arg->end);
> - vec = untagged_addr((unsigned long)arg->vec);
> + arg->start = untagged_addr((unsigned long)arg->start);
> + arg->end = untagged_addr((unsigned long)arg->end);
> + arg->vec = 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 = (arg->end & ~PAGE_MASK) ?
> + ((arg->end & PAGE_MASK) + PAGE_SIZE) : (arg->end);
arg->end = ALIGN(arg->end, PAGE_SIZE);
> if (!arg->max_pages)
> arg->max_pages = ULONG_MAX;
>
Best Regards
Michał Mirosław
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