Re: [PATCH / RFC] makedumpfile header to show LZO/snappy/zlib

Petr Tesarik
2013-09-26T13:32:27Z

On Fri, 20 Sep 2013 09:04:40 -0400 (EDT)
Dave Anderson <anderson@redhat.com> wrote:

>
>
> ----- Original Message -----
> > Hello Dave,
> >
> > (2013/09/16 0:56), Dave Anderson wrote:
> > >
> > > With the advent of LZO and snappy compression support, it would be
> > > helpful if it could be readily determined which type of compression
> > > was used to create the dumpfile.
> > >
> > > The zlib/LZO/snappy indicator is only found down in the bowels of each
> > > page_desc structure:
> > >
> > > /* page flags */
> > > #define DUMP_DH_COMPRESSED_ZLIB 0x1 /* page is compressed with zlib
> > > */
> > > #define DUMP_DH_COMPRESSED_LZO 0x2 /* paged is compressed with lzo
> > > */
> > > #define DUMP_DH_COMPRESSED_SNAPPY 0x4
> > > /* paged is compressed with
> > > snappy */
> > >
> > > /* descriptor of each page for vmcore */
> > > typedef struct page_desc {
> > > off_t offset; /* the offset of the
> > > page data*/
> > > unsigned int size; /* the size of this dump
> > > page */
> > > unsigned int flags; /* flags */
> > > unsigned long long page_flags; /* page flags */
> > > } page_desc_t;
> > >
> > > The compressed kdump's header does contain a "status" flag:
> > >
> > > struct disk_dump_header {
> > > char signature[SIG_LEN]; /* = "KDUMP "
> > > */
> > > int header_version; /* Dump header version
> > > */
> > > struct new_utsname utsname; /* copy of
> > > system_utsname */
> > > struct timeval timestamp; /* Time stamp */
> > > unsigned int status; /* Above flags */
> > > ...
> > >
> > > which has these 3 flags:
> > >
> > > #define DUMP_HEADER_COMPLETED 0
> > > #define DUMP_HEADER_INCOMPLETED 1
> > > #define DUMP_HEADER_COMPRESSED 8
> > >
> > > But they are all remnants of the old "diskdump" facility, and none of them
> > > are ever used by makedumpfile.
> > >
> > > Any chance that the LZO/snappy/zlib method could also be put up in
> > > the disk_dump_header?
> > >
> > > Dave
> > >
> > > --- makedumpfile-1.5.4/makedumpfile.c.orig
> > > +++ makedumpfile-1.5.4/makedumpfile.c
> > > @@ -5162,6 +5162,16 @@ write_kdump_header(void)
> > > dh->bitmap_blocks = divideup(info->len_bitmap, dh->block_size);
> > > memcpy(&dh->timestamp, &info->timestamp, sizeof(dh->timestamp));
> > > memcpy(&dh->utsname, &info->system_utsname, sizeof(dh->utsname));
> > > + if (info->flag_compress & DUMP_DH_COMPRESSED_ZLIB)
> > > + dh->status |= DUMP_DH_COMPRESSED_ZLIB;
> > > +#ifdef USELZO
> > > + else if (info->flag_compress & DUMP_DH_COMPRESSED_LZO)
> > > + dh->status |= DUMP_DH_COMPRESSED_LZO)
> > > +#endif
> > > +#ifdef USESNAPPY
> > > + else if (info->flag_compress & DUMP_DH_COMPRESSED_SNAPPY)
> > > + dh->status |= DUMP_DH_COMPRESSED_SNAPPY)
> > > +#endif
> >
> > dh->status will not guarantee that *all pages* are compressed
> > by the check below:
> >
> > > #ifdef USELZO
> > > } else if (info->flag_lzo_support
> > > && (info->flag_compress &
> > > DUMP_DH_COMPRESSED_LZO)
> > > && ((size_out = info->page_size),
> > > lzo1x_1_compress(buf, info->page_size,
> > > buf_out,
> > > &size_out, wrkmem) ==
> > > LZO_E_OK)
> > > && (size_out < info->page_size)) {
> > ^^^^^^^^^^^^^^^^^^^^^^^^^^^
> > When there is no effect of compression, the page will not be compressed.
> >
> > > pd.flags = DUMP_DH_COMPRESSED_LZO;
> > > pd.size = size_out;
> > > memcpy(buf, buf_out, pd.size);
> > > #endif
> >
> > If we are on the same page, I'll accept your patch.
>
> Right, I understand.

I think the primary intention is to let crash determine early if it is
able to open a given dump or not. And that's a good idea!

From a semantic point of view, the status field in the header should be
a logical OR of all compression methods used in the coredump. AFAICS,
Dave's patch would actually work for the current implementation.

Did I miss something?

Petr T