Re: [PATCH v5 5/8] vmcore: allocate ELF note segment in the 2nd kernelvmalloc memory

[HATAYAMA Daisuke]

(2013/05/15 0:35), Vivek Goyal wrote:
> On Tue, May 14, 2013 at 10:57:35AM +0900, HATAYAMA Daisuke wrote:
> > The reasons why we don't allocate ELF note segment in the 1st kernel
> > (old memory) on page boundary is to keep backward compatibility for
> > old kernels, and that if doing so, we waste not a little memory due to
> > round-up operation to fit the memory to page boundary since most of
> > the buffers are in per-cpu area.
> >
> > ELF notes are per-cpu, so total size of ELF note segments depends on
> > number of CPUs. The current maximum number of CPUs on x86_64 is 5192,
> > and there's already system with 4192 CPUs in SGI, where total size
> > amounts to 1MB. This can be larger in the near future or possibly even
> > now on another architecture that has larger size of note per a single
> > cpu. Thus, to avoid the case where memory allocation for large block
> > fails, we allocate vmcore objects on vmalloc memory.
> >
> > This patch adds elfnotesegbuf and elfnotesegbuf_sz variables to keep
> > pointer to the ELF note segment buffer and its size. There's no longer
> > the vmcore object that corresponds to the ELF note segment in
> > vmcore_list. Accordingly, read_vmcore() has new case for ELF note
> > segment and set_vmcore_list_offsets_elf{64,32}() and other helper
> > functions starts calculating offset from sum of size of ELF headers
> > and size of ELF note segment.
> >
> > Signed-off-by: HATAYAMA Daisuke <d.hatayama@xxxxxxxxxxxxxx>
> > ---
> >
> >   fs/proc/vmcore.c |  225 ++++++++++++++++++++++++++++++++++++++++--------------
> >   1 files changed, 165 insertions(+), 60 deletions(-)
> >
> > diff --git a/fs/proc/vmcore.c b/fs/proc/vmcore.c
> > index 48886e6..795efd2 100644
> > --- a/fs/proc/vmcore.c
> > +++ b/fs/proc/vmcore.c
> > @@ -34,6 +34,9 @@ static char *elfcorebuf;
> >   static size_t elfcorebuf_sz;
> >   static size_t elfcorebuf_sz_orig;
> >
> > +static char *elfnotesegbuf;
> > +static size_t elfnotesegbuf_sz;
>
> How about calling these just elfnotes_buf and elfnotes_sz.
>
> [..]
> > +/* Merges all the PT_NOTE headers into one. */
> > +static int __init merge_note_headers_elf64(char *elfptr, size_t *elfsz,
> > +					   char **notesegptr, size_t *notesegsz,
> > +					   struct list_head *vc_list)
> > +{
> > +	int i, nr_ptnote=0, rc=0;
> > +	char *tmp;
> > +	Elf64_Ehdr *ehdr_ptr;
> > +	Elf64_Phdr phdr;
> > +	u64 phdr_sz = 0, note_off;
> > +	struct vm_struct *vm;
> > +
> > +	ehdr_ptr = (Elf64_Ehdr *)elfptr;
> > +
> > +	/* The first path calculates the number of PT_NOTE entries and
> > +	 * total size of ELF note segment. */
> > +	rc = process_note_headers_elf64(ehdr_ptr, &nr_ptnote, &phdr_sz, NULL);
> > +	if (rc < 0)
> > +		return rc;
> > +
> > +	*notesegsz = roundup(phdr_sz, PAGE_SIZE);
> > +	*notesegptr = vzalloc(*notesegsz);
> > +	if (!*notesegptr)
> > +		return -ENOMEM;
> > +
> > +	vm = find_vm_area(*notesegptr);
> > +	BUG_ON(!vm);
> > +	vm->flags |= VM_USERMAP;
> > +
> > +	/* The second path copies the ELF note segment in the ELF note
> > +	 * segment buffer. */
> > +	rc = process_note_headers_elf64(ehdr_ptr, NULL, NULL, *notesegptr);
>
> So same function process_note_headers_elf64() is doing two different
> things based on parameters passed. Please create two new functions
> to do two different things and name these appropriately.
>
> Say
>
> get_elf_note_number_and_size()
> copy_elf_notes()

I see. Similar to other functions, 32-bit and 64-bit versions are 
needed. So I give them symbols:

get_note_number_and_size_elf64()
copy_notes_elf64()

and elf32 counterpart.

> > +	if (rc < 0)
> > +		return rc;
> > +
> >   	/* Prepare merged PT_NOTE program header. */
> >   	phdr.p_type    = PT_NOTE;
> >   	phdr.p_flags   = 0;
> > @@ -304,23 +364,18 @@ static int __init merge_note_headers_elf64(char *elfptr, size_t *elfsz,
> >   	return 0;
> >   }
> >
> > -/* Merges all the PT_NOTE headers into one. */
> > -static int __init merge_note_headers_elf32(char *elfptr, size_t *elfsz,
> > -						struct list_head *vc_list)
> > +static int __init process_note_headers_elf32(const Elf32_Ehdr *ehdr_ptr,
> > +					     int *nr_ptnotep, u64 *phdr_szp,
> > +					     char *notesegp)
>
> Can you please describe function parameters at the beginning of function
> in a comment. Things are gettting little confusing now.
>
> What does notesegp signify? phdr_szp could be simply *phdr_sz,
> nr_ptnotesp could be *nr_notes. Please simplify the naming a bit.
> Seems too twisted to me.

I see. I'll reflect that in addition to your other comments.

--
Thanks.
HATAYAMA, Daisuke

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