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Date:   Sun, 28 Apr 2019 00:45:22 +0300
From:   Alexey Dobriyan <adobriyan@gmail.com>
To:     Joel Savitz <jsavitz@redhat.com>
Cc:     linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Ram Pai <linuxram@us.ibm.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Huang Ying <ying.huang@intel.com>,
        Sandeep Patil <sspatil@android.com>,
        Rafael Aquini <aquini@redhat.com>,
        linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH v2] fs/proc: add VmTaskSize field to /proc/$$/status
Message-ID: <20190427214522.GA7074@avx2>
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On Fri, Apr 26, 2019 at 03:02:08PM -0400, Joel Savitz wrote:
> In the mainline kernel, there is no quick mechanism to get the virtual
> memory size of the current process from userspace.
> 
> Despite the current state of affairs, this information is available to the
> user through several means, one being a linear search of the entire address
> space. This is an inefficient use of cpu cycles.

You can test only a few known per arch values. Linear search is a self
inflicted wound.

prctl(2) is more natural place and will also be arch neutral.

> A component of the libhugetlb kernel test does exactly this, and as
> systems' address spaces increase beyond 32-bits, this method becomes
> exceedingly tedious.

> For example, on a ppc64le system with a 47-bit address space, the linear
> search causes the test to hang for some unknown amount of time. I
> couldn't give you an exact number because I just ran it for about 10-20
> minutes and went to go do something else, probably to get coffee or
> something, and when I came back, I just killed the test and patched it
> to use this new mechanism. I re-ran my new version of the test using a
> kernel with this patch, and of course it passed through the previously
> bottlenecking codepath nearly instantaneously.
> 
> This patched enabled me to upgrade an O(n) codepath to O(1) in an
> architecture-independent manner.

> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -74,7 +74,10 @@ void task_mem(struct seq_file *m, struct mm_struct *mm)
>  	seq_put_decimal_ull_width(m,
>  		    " kB\nVmPTE:\t", mm_pgtables_bytes(mm) >> 10, 8);
>  	SEQ_PUT_DEC(" kB\nVmSwap:\t", swap);
> -	seq_puts(m, " kB\n");
> +	SEQ_PUT_DEC(" kB\nVmSwap:\t", swap);
> +	seq_put_decimal_ull_width(m,
> +		    " kB\nVmTaskSize:\t", TASK_SIZE >> 10, 8);
> +	seq_puts(m, " kB\n");

All fields in this file are related to the task. New field related
to "current" will stick like an eyesore.