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From:   =?UTF-8?B?TWljaGHFgiBNaXJvc8WCYXc=?= <emmir@google.com>
Date:   Mon, 24 Jul 2023 18:10:49 +0200
Message-ID: <CABb0KFHJVeEkh4f6WWK6FThCbA+NE8iYUZE68nV1YAxaHwiwog@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 17:22, Muhammad Usama Anjum
<usama.anjum@collabora.com> wrote:
>
> On 7/24/23 7:38=E2=80=AFPM, Micha=C5=82 Miros=C5=82aw wrote:
> > 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 intea=
d
> >> - 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
> >> @@ -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 handlin=
g a
> > full hugepage anyway.
> If we weren't able to add the complete thp in the output buffer and we ne=
ed
> 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 whi=
ch
> wasn't added to output.
>
> Lets say max=3D100
> 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 =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=
(pte));
> >> +               next =3D 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_sca=
n_arg
> >> *arg,
> >>              arg->category_anyof_mask | arg->return_mask) & ~PM_SCAN_C=
ATEGORIES)
> >>                 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().
> Sorry what?
> After this function, the start, end and vec would be used. We need to mak=
e
> 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
Micha=C5=82 Miros=C5=82aw