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

From: "Michał Mirosław" <emmir@google.com>
To: Muhammad Usama Anjum <usama.anjum@collabora.com>
Cc: Peter Xu <peterx@redhat.com>,
	David Hildenbrand <david@redhat.com>,
	 Andrew Morton <akpm@linux-foundation.org>,
	Andrei Vagin <avagin@gmail.com>,
	 Danylo Mocherniuk <mdanylo@google.com>,
	Paul Gofman <pgofman@codeweavers.com>,
	 Cyrill Gorcunov <gorcunov@gmail.com>,
	Mike Rapoport <rppt@kernel.org>, Nadav Amit <namit@vmware.com>,
	 Alexander Viro <viro@zeniv.linux.org.uk>,
	Shuah Khan <shuah@kernel.org>,
	 Christian Brauner <brauner@kernel.org>,
	Yang Shi <shy828301@gmail.com>,  Vlastimil Babka <vbabka@suse.cz>,
	"Liam R . Howlett" <Liam.Howlett@oracle.com>,
	 Yun Zhou <yun.zhou@windriver.com>,
	Suren Baghdasaryan <surenb@google.com>,
	 Alex Sierra <alex.sierra@amd.com>,
	Matthew Wilcox <willy@infradead.org>,
	 Pasha Tatashin <pasha.tatashin@soleen.com>,
	Axel Rasmussen <axelrasmussen@google.com>,
	 "Gustavo A . R . Silva" <gustavoars@kernel.org>,
	Dan Williams <dan.j.williams@intel.com>,
	 linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	 linux-mm@kvack.org, linux-kselftest@vger.kernel.org,
	 Greg KH <gregkh@linuxfoundation.org>,
	kernel@collabora.com
Subject: Re: [PATCH v12 2/5] fs/proc/task_mmu: Implement IOCTL to get and optionally clear info about PTEs
Date: Fri, 7 Apr 2023 09:23:55 +0200	[thread overview]
Message-ID: <CABb0KFF+sKSv7jdxBbXpt5A2WO83tKb9viq-kKurXN_e1VcFhQ@mail.gmail.com> (raw)
In-Reply-To: <b737dceb-a228-7ffe-0758-421505f1a61d@collabora.com>

On Thu, 6 Apr 2023 at 23:12, Muhammad Usama Anjum
<usama.anjum@collabora.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@collabora.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

Best Regards
Michał Mirosław