linux-kernel - Re: [PATCH 3/3] seq_file: convert seq buffer to vmalloc

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Message-ID: <CAMbhsRSNYUF+wMrhu+LMWpSuDrE6o3p_t7Ai5ZOqGDXHs6cTzg@mail.gmail.com>
Date:	Thu, 22 Sep 2011 14:19:58 -0700
From:	Colin Cross <ccross@...roid.com>
To:	Joe Perches <joe@...ches.com>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Alexey Dobriyan <adobriyan@...il.com>
Subject: Re: [PATCH 3/3] seq_file: convert seq buffer to vmalloc

On Thu, Sep 22, 2011 at 2:07 PM, Joe Perches <joe@...ches.com> wrote:
> On Thu, 2011-09-22 at 13:57 -0700, Colin Cross wrote:
>> seq_files are often used for debugging.  When things are going wrong
>> due to failed physically contiguous allocations, the exponentially
>> growing physically contiguous allocations in seq_read can make things
>> worse.  There is no need for physically contiguous memory, so switch
>> to virtually contiguous memory instead.
>
> vmalloc's are relatively expensive.
> Perhaps use kmalloc when appropriate instead?
Talking about allocation efficiency in the context of seq_files is
silly - for a KMALLOC_MAX_SIZE buffer (8MB), you are already going to
allocate 11 times, with increasingly large buffers, and are likely to
fail long before you ever get to KMALLOC_MAX_SIZE.

> []
>> -     /* don't ask for more than the kmalloc() max size */
>> -     if (size > KMALLOC_MAX_SIZE)
>> -             size = KMALLOC_MAX_SIZE;
>> -
>> -     buf = kmalloc(size, GFP_KERNEL);
>> +     buf = vmalloc(size);
>>       if (!buf)
>>               return -ENOMEM;
>
>        if (size > KMALLOC_MAX_SIZE)
>                buf = vmalloc(size, GFP_KERNEL)
>        else
>                buf = kmalloc(size, GFP_KERNEL);
KMALLOC_MAX_SIZE is far too big for this.  KMALLOC_MAX_SIZE is the
maximum allocation that is theoretically possible, but will fail if
you don't have any completely empty pageblocks.  If I were to put a
size here, it would probably be order 3, but even that can easily fail
on a system that has under memory pressure.

>> +     vfree(m->buf);
>
>        if (m->size > KMALLOC_MAX_SIZE)
>                vfree(m->buf);
>        else
>                kfree(m->buf);
>
>>       m->buf = buf;
>>       m->size = size;
>>
>> @@ -106,7 +103,7 @@ static int traverse(struct seq_file *m, loff_t offset)
>>               return 0;
>>       }
>>       if (!m->buf) {
>> -             m->buf = kmalloc(m->size = PAGE_SIZE, GFP_KERNEL);
>> +             m->buf = vmalloc(m->size = PAGE_SIZE);
>
> embedding the set of m->size like this is ugly.
I agree, but it was there in the original file.  I could clean it up,
but that should be in a separate patch.

> [do the same as above kmalloc/vmalloc based on size]
>
> etc.
>
>
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