From: Alexander Shishkin <alexander.shishkin@linux.intel.com>
To: Song Liu <songliubraving@fb.com>, linux-kernel@vger.kernel.org
Cc: kernel-team@fb.com, Song Liu <songliubraving@fb.com>,
Arnaldo Carvalho de Melo <acme@redhat.com>,
Jiri Olsa <jolsa@kernel.org>,
Peter Zijlstra <peterz@infradead.org>,
alexander.shishkin@linux.intel.com
Subject: Re: [PATCH v2] perf/core: fix mlock accounting in perf_mmap()
Date: Thu, 23 Jan 2020 11:33:47 +0200 [thread overview]
Message-ID: <8736c6bjpg.fsf@ashishki-desk.ger.corp.intel.com> (raw)
In-Reply-To: <20200122190447.1920297-1-songliubraving@fb.com>
Song Liu <songliubraving@fb.com> writes:
> sysctl_perf_event_mlock and user->locked_vm can change value
> independently, so we can't guarantee:
Looks good, I still have some suggestions below.
>
> user->locked_vm <= user_lock_limit
>
> When user->locked_vm is larger than user_lock_limit, we cannot simply
> update extra and user_extra as:
>
> extra = user_locked - user_lock_limit;
> user_extra -= extra;
>
> Otherwise, user_extra will be negative. In extreme cases, this may lead to
> negative user->locked_vm (until this perf-mmap is closed), which break
> locked_vm badly.
>
> Fix this by adjusting user_locked before calculating extra and user_extra.
The commit message is just talking about the code. We can see the code
when we scroll down to the diff. What this can be instead is:
1. Problem statement: decreasing sysctl_perf_event_mlock between two
consecutive mmap()s of a perf ring buffer may lead to an integer
underflow in locked memory accounting. This may lead to the following
undesired behavior: <an example of bad behavior as opposed to expected
behavior>.
2. Fix description: address this by adjusting the accounting logic to
take into account the possibility that the amount of already locked
memory may exceed the current limit.
> Fixes: c4b75479741c ("perf/core: Make the mlock accounting simple again")
> Signed-off-by: Song Liu <songliubraving@fb.com>
> Suggested-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
> Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
> Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
> Cc: Jiri Olsa <jolsa@kernel.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> ---
> kernel/events/core.c | 13 ++++++++++++-
> 1 file changed, 12 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 2173c23c25b4..d25f2de45996 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -5916,8 +5916,19 @@ static int perf_mmap(struct file *file, struct vm_area_struct *vma)
> */
> user_lock_limit *= num_online_cpus();
>
> - user_locked = atomic_long_read(&user->locked_vm) + user_extra;
> + user_locked = atomic_long_read(&user->locked_vm);
>
> + /*
> + * sysctl_perf_event_mlock and user->locked_vm can change value
> + * independently. so we can't guarantee:
> + * user->locked_vm <= user_lock_limit
"sysctl_perf_event_mlock may have changed, so that user->locked_vm >
user_lock_limit".
> + *
> + * Adjust user_locked to be <= user_lock_limit so we can calcualte
> + * correct extra and user_extra.
This comment is also verbalizing the C code that follows. I don't think
it's necessary.
> + */
> + user_locked = min_t(unsigned long, user_locked, user_lock_limit);
A matter of preference, but to me the "if (user_locked >=
user_lock_limit)" is easier to read.
> +
> + user_locked += user_extra;
> if (user_locked > user_lock_limit) {
> /*
> * charge locked_vm until it hits user_lock_limit;
Thanks,
--
Alex
prev parent reply other threads:[~2020-01-23 9:33 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-01-22 19:04 [PATCH v2] perf/core: fix mlock accounting in perf_mmap() Song Liu
2020-01-23 9:33 ` Alexander Shishkin [this message]
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=8736c6bjpg.fsf@ashishki-desk.ger.corp.intel.com \
--to=alexander.shishkin@linux.intel.com \
--cc=acme@redhat.com \
--cc=jolsa@kernel.org \
--cc=kernel-team@fb.com \
--cc=linux-kernel@vger.kernel.org \
--cc=peterz@infradead.org \
--cc=songliubraving@fb.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).