linux-kernel - Re: [PATCH v12 2/5] fs/proc/task_mmu: Implement IOCTL to get and optionally clear info about PTEs

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Date:   Fri, 7 Apr 2023 12:21:07 +0200
From:   Michał Mirosław <emmir@...gle.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>,
        Andrei Vagin <avagin@...il.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
Subject: Re: [PATCH v12 2/5] fs/proc/task_mmu: Implement IOCTL to get and
 optionally clear info about PTEs

On Fri, 7 Apr 2023 at 12:15, Muhammad Usama Anjum
<usama.anjum@...labora.com> wrote:
> On 4/7/23 3:04 PM, Michał Mirosław wrote:
> > On Fri, 7 Apr 2023 at 11:35, Muhammad Usama Anjum
> > <usama.anjum@...labora.com> wrote:
> >> On 4/7/23 12:23 PM, Michał Mirosław wrote:
> >>> On Thu, 6 Apr 2023 at 23:12, Muhammad Usama Anjum
> >>> <usama.anjum@...labora.com> wrote:
> >>>> On 4/7/23 1:12 AM, Michał Mirosław wrote:
> >>>>> On Thu, 6 Apr 2023 at 09:40, Muhammad Usama Anjum
> >>>>> <usama.anjum@...labora.com> wrote:
> >>>>> [...]
> >>>>>> --- a/fs/proc/task_mmu.c
> >>>>>> +++ b/fs/proc/task_mmu.c
> >>>>> [...]
> >>>>>> +static int pagemap_scan_pmd_entry(pmd_t *pmd, unsigned long start,
> >>>>>> +                                 unsigned long end, struct mm_walk *walk)
> >>>>>> +{
> >>> [...]
> >>>>>> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> >>>>>> +       ptl = pmd_trans_huge_lock(pmd, vma);
> >>>>>> +       if (ptl) {
> >>>>> [...]
> >>>>>> +               return ret;
> >>>>>> +       }
> >>>>>> +process_smaller_pages:
> >>>>>> +       if (pmd_trans_unstable(pmd))
> >>>>>> +               return 0;
> >>>>>
> >>>>> Why pmd_trans_unstable() is needed here and not only after split_huge_pmd()?
> >>>> I'm not entirely sure. But the idea is if THP is unstable, we should
> >>>> return. As it doesn't seem like after splitting THP can be unstable, we
> >>>> should not check it. Do you agree with the following?
> >>>
> >>> The description of pmd_trans_unstable() [1] seems to indicate that it
> >>> is needed only after split_huge_pmd().
> >>>
> >>> [1] https://elixir.bootlin.com/linux/v6.3-rc5/source/include/linux/pgtable.h#L1394
> >> Sorry, yeah pmd_trans_unstable() is need after split. But it is also needed
> >> in normal case when ptl is NULL to rule out the case if pmd is unstable
> >> before performing operation on normal pages:
> >>
> >> ptl = pmd_trans_huge_lock(pmd, vma);
> >> if (ptl) {
> >> ...
> >> }
> >> if (pmd_trans_unstable(pmd))
> >>         return 0;
> >>
> >> This file has usage examples of pmd_trans_unstable():
> >>
> >> https://elixir.bootlin.com/linux/v6.3-rc5/source/fs/proc/task_mmu.c#L634
> >> https://elixir.bootlin.com/linux/v6.3-rc5/source/fs/proc/task_mmu.c#L1195
> >> https://elixir.bootlin.com/linux/v6.3-rc5/source/fs/proc/task_mmu.c#L1543
> >> https://elixir.bootlin.com/linux/v6.3-rc5/source/fs/proc/task_mmu.c#L1887
> >>
> >> So we are good with what we have in this patch.
> >
> > Shouldn't we signal ACTION_AGAIN then in order to call .pte_hole?
> I'm not sure. I've not done research on it if we need to signal
> ACTION_AGAIN as this function pagemap_scan_pmd_entry() mimics how
> pagemap_pmd_range() handles reads to the pagemap file. pagemap_pmd_range()
> isn't doing anything if pmd is unstable. Hence we also not doing anything.

Doesn't this mean that if we scan a file-backed vma we would miss
non-present parts of the mapping in the output?

Best Regards
Michał Mirosław