From: Oleg Nesterov <oleg@redhat.com>
To: Song Liu <songliubraving@fb.com>
Cc: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Linux MM <linux-mm@kvack.org>,
Andrew Morton <akpm@linux-foundation.org>,
Matthew Wilcox <matthew.wilcox@oracle.com>,
"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
Kernel Team <Kernel-team@fb.com>,
William Kucharski <william.kucharski@oracle.com>,
"srikar@linux.vnet.ibm.com" <srikar@linux.vnet.ibm.com>
Subject: Re: [PATCH v12 5/6] khugepaged: enable collapse pmd for pte-mapped THP
Date: Fri, 9 Aug 2019 17:24:04 +0200 [thread overview]
Message-ID: <20190809152404.GA21489@redhat.com> (raw)
In-Reply-To: <770B3C29-CE8F-4228-8992-3C6E2B5487B6@fb.com>
On 08/08, Song Liu wrote:
>
> > On Aug 8, 2019, at 9:33 AM, Oleg Nesterov <oleg@redhat.com> wrote:
> >
> >> + for (i = 0, addr = haddr; i < HPAGE_PMD_NR; i++, addr += PAGE_SIZE) {
> >> + pte_t *pte = pte_offset_map(pmd, addr);
> >> + struct page *page;
> >> +
> >> + if (pte_none(*pte))
> >> + continue;
> >> +
> >> + page = vm_normal_page(vma, addr, *pte);
just noticed... shouldn't you also check pte_present() before
vm_normal_page() ?
> >> + if (!page || !PageCompound(page))
> >> + return;
> >> +
> >> + if (!hpage) {
> >> + hpage = compound_head(page);
> >
> > OK,
> >
> >> + if (hpage->mapping != vma->vm_file->f_mapping)
> >> + return;
> >
> > is it really possible? May be WARN_ON(hpage->mapping != vm_file->f_mapping)
> > makes more sense ?
>
> I haven't found code paths lead to this,
Neither me, that is why I asked. I think this should not be possible,
but again this is not my area.
> but this is technically possible.
> This pmd could contain subpages from different THPs.
Then please explain how this can happen ?
> The __replace_page()
> function in uprobes.c creates similar pmd.
No it doesn't,
> Current uprobe code won't really create this problem, because
> !PageCompound() check above is sufficient. But it won't be difficult to
> modify uprobe code to break this.
I bet it will be a) difficult and b) the very idea to do this would be wrong.
> For this code to be accurate and safe,
> I think both this check and the one below are necessary.
I didn't suggest to remove these checks.
> Also, this code
> is not on any critical path, so the overhead should be negligible.
I do not care about overhead. But I do care about a poor reader like me
who will try to understand this code.
If you too do not understand how a THP page can have a different mapping
then use VM_WARN or at least add a comment to explain that this is not
supposed to happen!
> Does this make sense?
Not to me :/
Oleg.
next prev parent reply other threads:[~2019-08-09 15:24 UTC|newest]
Thread overview: 30+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-08-07 23:37 [PATCH v12 0/6] THP aware uprobe Song Liu
2019-08-07 23:37 ` [PATCH v12 1/6] mm: move memcmp_pages() and pages_identical() Song Liu
2019-08-07 23:37 ` [PATCH v12 2/6] uprobe: use original page when all uprobes are removed Song Liu
2019-08-07 23:37 ` [PATCH v12 3/6] mm, thp: introduce FOLL_SPLIT_PMD Song Liu
2019-08-08 16:37 ` Oleg Nesterov
2019-08-08 17:16 ` Song Liu
2019-08-09 16:35 ` Oleg Nesterov
2019-08-09 16:50 ` Song Liu
2019-08-07 23:37 ` [PATCH v12 4/6] uprobe: use FOLL_SPLIT_PMD instead of FOLL_SPLIT Song Liu
2019-08-07 23:37 ` [PATCH v12 5/6] khugepaged: enable collapse pmd for pte-mapped THP Song Liu
2019-08-08 16:33 ` Oleg Nesterov
2019-08-08 17:05 ` Song Liu
2019-08-09 15:24 ` Oleg Nesterov [this message]
2019-08-09 16:30 ` Song Liu
2019-08-09 18:01 ` Song Liu
2019-08-12 12:11 ` Kirill A. Shutemov
2019-08-12 13:22 ` Oleg Nesterov
2019-08-12 14:40 ` Kirill A. Shutemov
2019-08-12 21:04 ` Song Liu
2019-08-13 14:44 ` Song Liu
2019-08-15 10:16 ` Oleg Nesterov
2019-08-15 16:27 ` Song Liu
2019-08-13 13:30 ` Oleg Nesterov
2019-08-13 14:05 ` Oleg Nesterov
2019-08-13 15:05 ` Kirill A. Shutemov
2019-08-13 16:24 ` Oleg Nesterov
2019-08-16 14:54 ` Kirill A. Shutemov
2019-08-12 13:06 ` Oleg Nesterov
2019-08-12 14:36 ` Song Liu
2019-08-07 23:37 ` [PATCH v12 6/6] uprobe: collapse THP pmd after removing all uprobes Song Liu
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20190809152404.GA21489@redhat.com \
--to=oleg@redhat.com \
--cc=Kernel-team@fb.com \
--cc=akpm@linux-foundation.org \
--cc=kirill.shutemov@linux.intel.com \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=matthew.wilcox@oracle.com \
--cc=songliubraving@fb.com \
--cc=srikar@linux.vnet.ibm.com \
--cc=william.kucharski@oracle.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).