Re: [v2] fs/proc/task_mmu: Implement IOCTL for efficient page table scanning

[Michał Mirosław]

On Mon, 24 Jul 2023 at 17:22, Muhammad Usama Anjum
<usama.anjum@xxxxxxxxxxxxx> wrote:
>
> On 7/24/23 7:38 PM, Michał Mirosław wrote:
> > On Mon, 24 Jul 2023 at 16:04, Muhammad Usama Anjum
> > <usama.anjum@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx>
> >> ---
> >>  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
> >> @@ -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.
> If we weren't able to add the complete thp in the output buffer and we need
> to perform WP on the entire page, we should split and rollback. Otherwise
> we'll WP the entire thp and we'll lose the state on the remaining THP which
> wasn't added to output.
>
> Lets say max=100
> only 100 pages would be added to output
> we need to split and rollback otherwise other 412 pages would get WP

In this case *end will be truncated by output() to match the number of
pages that fit.

> >> @@ -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?
> Saving spaces inside loop. What are pros of declation of variable in loop?

Informing the reader that the variables have scope limited to the loop body.

[...]
> >> @@ -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().
> Sorry what?
> After this function, the start, end and vec would be used. We need to make
> sure that the address are untagged before that.

We do write back the address the walk ended at to arg->start in
userspace. This pointer I think needs the tag reconstructed so that
retrying the ioctl() will be possible.

 Best Regards
Michał Mirosław