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

[Petr Tesarik <ptesarik@suse.cz>]

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

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