From: Dave Hansen <dave.hansen@intel.com>
To: Colin Cross <ccross@google.com>
Cc: "Kees Cook" <keescook@chromium.org>,
"Sumit Semwal" <sumit.semwal@linaro.org>,
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Subject: Re: [PATCH v7 3/3] mm: add a field to store names for private anonymous memory
Date: Thu, 3 Sep 2020 11:40:06 -0700 [thread overview]
Message-ID: <5d0358ab-8c47-2f5f-8e43-23b89d6a8e95@intel.com> (raw)
In-Reply-To: <CAMbhsRSjSJUPVhjV8y5cPcoo6tBDGw5rSjtryVLF816zLhdS1w@mail.gmail.com>
On 9/3/20 11:26 AM, Colin Cross wrote:
>> FWIW, I don't have any objections to this.
>>
>> Refcounting strings was what I think I had the strongest reaction to
>> back in the good old days of 2013. strdup() on split plus strcmp() on
>> merge doesn't sound afwul to me, and it is darn straightforward. The
>> biggest downside is probably kernel memory consumption. We should
>> probably just think through whether having so many duplicates changes
>> things materially.
>>
>> For instance, should/could we penalize a task's vm.max_map_count when
>> it's using this mechanism?
> Just to provide some concrete numbers, the ART process I examined
> (https://pastebin.com/YNUTvZyz) had 280 named anonymous mappings using
> a total of 6566 bytes for the names. There were only 63 unique names,
> using 1925 bytes. On my personal usage device, there are currently a
> total of 59769 named anonymous devices across all processes using
> 1224119 bytes, 5843 of them unique using 121754 bytes. The vast
> majority of the unique names are of the form "stack_and_tls:999",
> which are dynamically allocated in the userspace process' heap. There
> are only 132 names that do not contain stack_and_tls using 9540 bytes,
> repeated 49938 times using 1030651 bytes (108x). Most of those are
> constant strings, meaning the pointer is into the .rodata section of a
> file mapping that is shared between all processes.
Thanks for the data! That seems like totally reasonable memory
consumption in the normal cases.
I'm mostly concerned about the worst-case kernel memory consumption.
It's a DoS if there's a big change to the amount of kernel memory a
process can consume. For instance, each VMA is 192 bytes. If the name
limit was, say, 4k, it makes the per-VMA kernel memory consumption go up
by a factor of 20, which isn't nice. But, if it's, say 16 bytes, it's
only a 10% increase over what each VMA consumes today.
> Is fork a concern? It would have to strdup every name.
Should be pretty easy to confirm the worst case experimentally. Create
a process with ~64k VMAs, time the fork()s. Then do the same with 64k
VMAs with the longest names set on each VMA. If you can't time a
difference, you'll have a strong argument that someone with 10 of these
things in a process won't notice.
prev parent reply other threads:[~2020-09-03 18:40 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-09-01 16:14 [PATCH v7 0/3] Anonymous VMA naming patches Sumit Semwal
2020-09-01 16:14 ` [PATCH v7 1/3] mm: rearrange madvise code to allow for reuse Sumit Semwal
2020-09-01 16:14 ` [PATCH v7 2/3] mm: memory: Add access_remote_vm_locked variant Sumit Semwal
2020-09-01 16:14 ` [PATCH v7 3/3] mm: add a field to store names for private anonymous memory Sumit Semwal
2020-09-03 13:25 ` Kirill A. Shutemov
2020-09-03 13:43 ` Matthew Wilcox
2020-09-03 13:58 ` Kirill A. Shutemov
2020-09-03 15:59 ` Colin Cross
2020-09-03 17:31 ` Kees Cook
2020-09-03 17:54 ` Colin Cross
2020-09-03 18:00 ` Dave Hansen
2020-09-03 18:00 ` Kees Cook
2020-09-03 18:09 ` Dave Hansen
2020-09-03 18:26 ` Colin Cross
2020-09-03 18:40 ` Dave Hansen [this message]
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