Re: [PATCH] mm/vmstat: add header line to /proc/buddyinfo output

[David Hildenbrand]

On 18.09.25 10:31, Ye Liu wrote:
> 在 2025/9/18 16:16, David Hildenbrand 写道:
> > On 18.09.25 10:11, Ye Liu wrote:
> > > 在 2025/9/18 15:29, Michal Hocko 写道:
> > > > On Thu 18-09-25 15:17:40, Ye Liu wrote:
> > > > > From: Ye Liu <liuye@xxxxxxxxxx>
> > > > >
> > > > > Add a header line to /proc/buddyinfo that shows the order numbers
> > > > > for better readability and clarity.
> > > > >
> > > > > Before:
> > > > > Node 0, zone      DMA      0      0      0      0      0      0      0 ...
> > > > > Node 0, zone    DMA32      5      8      6      6      7      5      8 ...
> > > > > Node 0, zone   Normal   1113    351    138     65     38     31     25 ...
> > > > >
> > > > > After:
> > > > > Free pages per order       0      1      2      3      4      5      6 ...
> > > > > Node 0, zone      DMA      0      0      0      0      0      0      0 ...
> > > > > Node 0, zone    DMA32      5      8      6      6      7      5      8 ...
> > > > > Node 0, zone   Normal   1113    351    138     65     38     31     25 ...
> > > >
> > > > Why is this needed? And have you considered tha this might break
> > > > existing parsers of the file?
> > >
> > > Thanks for the review.
> > >
> > > The reason for this change is simply to make /proc/buddyinfo self-describing.
> > > Right now you have to know which column is which order; with a header it’s
> > > obvious. This is similar to what /proc/pagetypeinfo already does, e.g.:
> > >
> > >     Page block order: 9
> > >     Pages per block: 512
> > >
> > >     Free pages count per migrate type at order   0 1 2 3 ...
> > >     Node 0, zone DMA, type Unmovable             0 0 0 ...
> > >
> > > Regarding existing parsers: the patch does not change any of the existing
> > > “Node … zone …” lines, it only adds a single header line before them. Most
> > > parsers match “Node” lines and ignore everything else, so the risk should be
> > > low. If you know of any existing parser that this would break, please let
> > > me know so I can address it.
> >
> > What if there is a single one out there that has hardcoded to skip the first line only?
>
> I understand there may be no way to be fully compatible with all existing
> parsers. However, /proc/buddyinfo is essentially raw data intended for
> human and tool consumption, and parsers are expected to be robust against
> format changes.

Let's take a look at a random one: pcp-buddyinfo.

Reading the source code [1] of the /proc/buddyinfo parser in
refresh_proc_buddyinfo() I have my doubts:


    if ((fp = linux_statsfile("/proc/buddyinfo", buf, sizeof(buf))) == NULL)
        return -oserror();

    while (fgets(buf,sizeof(buf),fp) != NULL) {
        char node_name[128];
        char *zone_name;
        unsigned int values[SPLIT_MAX];

        i = read_node_name(buf, node_name);
        i+=6; /* erase ", zone" */
        read_buddyinfo(buf+i, read_buf, MAX_ORDER+1); /* read zone name and page order */
        zone_name=read_buf[0];
        for (i=0; i < MAX_ORDER; i++)
            values[i] = strtoul(read_buf[i+1], NULL, 10);
        for (i=0; i < proc_buddyinfo->nbuddys; i++) {
            if (strcmp(proc_buddyinfo->buddys[i].node_name, node_name)==0 &&
		strcmp(proc_buddyinfo->buddys[i].zone_name, zone_name)==0)
                break;
        }

Do you think that one would survive your changes?


https://github.com/performancecopilot/pcp/blob/main/src/pmdas/linux/proc_buddyinfo.c

--
Cheers

David / dhildenb