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Re: [PATCH v3 04/14] lib/vsprintf.c: expand field_width to 24 bits

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On Thu, 2015-12-03 at 12:54 -0800, Joe Perches wrote:
> On Thu, 2015-12-03 at 21:51 +0100, Rasmus Villemoes wrote:
> > Maurizio Lombardi reported a problem [1] with the %pb extension: It
> > doesn't work for sufficiently large bitmaps, since the size is
> > stashed
> > in the field_width field of the struct printf_spec, which is
> > currently
> > an s16. Concretely, this manifested itself in
> > /sys/bus/pseudo/drivers/scsi_debug/map being empty, since the
> > bitmap
> > printer got a size of 0, which is the 16 bit truncation of the
> > actual
> > bitmap size.
> > 
> > We do want to keep struct printf_spec at 8 bytes so that it can
> > cheaply be passed by value. The qualifier field is only used for
> > internal bookkeeping in format_decode, so we might as well use a
> > local
> > variable for that. This gives us an additional 8 bits, which we can
> > then use for the field width.
> > 
> > To stay in 8 bytes, we need to do a little rearranging and make the
> > type member a bitfield as well. For consistency, change all the
> > members to bit fields. gcc doesn't generate much worse code with
> > these
> > changes (in fact, bloat-o-meter says we save 300 bytes - which I
> > think
> > is a little surprising).
> > 
> > I didn't find a BUILD_BUG/compiletime_assertion/... which would
> > work
> > outside function context, so for now I just open-coded it.
> > 
> > [1] http://thread.gmane.org/gmane.linux.kernel/2034835
> 
> Thanks for keeping at this Rasmus.
> This seems quite reasonable.

I like most of the stuff here, though, Joe, can we avoid open-coded
BUILD_BUG_ON()?


-- 
Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Intel Finland Oy

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[PATCH v3 00/14] printf stuff for 4.5 Rasmus Villemoes <linux@rasmusvillemoes.dk> - 2015-12-03 22:00 +0100
  [PATCH v3 01/14] lib/vsprintf.c: pull out padding code from dentry_name() Rasmus Villemoes <linux@rasmusvillemoes.dk> - 2015-12-03 22:00 +0100
  Re: [PATCH v3 04/14] lib/vsprintf.c: expand field_width to 24 bits Joe Perches <joe@perches.com> - 2015-12-03 22:00 +0100
    Re: [PATCH v3 04/14] lib/vsprintf.c: expand field_width to 24 bits Andy Shevchenko <andriy.shevchenko@linux.intel.com> - 2015-12-03 22:40 +0100
      Re: [PATCH v3 04/14] lib/vsprintf.c: expand field_width to 24 bits Andrew Morton <akpm@linux-foundation.org> - 2015-12-04 00:40 +0100
        Re: [PATCH v3 04/14] lib/vsprintf.c: expand field_width to 24 bits Joe Perches <joe@perches.com> - 2015-12-04 01:10 +0100
          Re: [PATCH v3 04/14] lib/vsprintf.c: expand field_width to 24 bits Rasmus Villemoes <linux@rasmusvillemoes.dk> - 2015-12-04 10:10 +0100
        Re: [PATCH v3 04/14] lib/vsprintf.c: expand field_width to 24 bits Rasmus Villemoes <linux@rasmusvillemoes.dk> - 2015-12-04 10:10 +0100
      Re: [PATCH v3 04/14] lib/vsprintf.c: expand field_width to 24 bits Joe Perches <joe@perches.com> - 2015-12-04 00:50 +0100
  [PATCH v3 08/14] lib/test_printf.c: don't BUG Rasmus Villemoes <linux@rasmusvillemoes.dk> - 2015-12-03 22:00 +0100
  [PATCH v3 03/14] lib/vsprintf.c: eliminate potential race in string() Rasmus Villemoes <linux@rasmusvillemoes.dk> - 2015-12-03 22:00 +0100
  [PATCH v3 06/14] lib/vsprintf.c: warn about too large precisions and field widths Rasmus Villemoes <linux@rasmusvillemoes.dk> - 2015-12-03 22:00 +0100
  [PATCH v3 09/14] lib/test_printf.c: check for out-of-bound writes Rasmus Villemoes <linux@rasmusvillemoes.dk> - 2015-12-03 22:00 +0100
  [PATCH v3 02/14] lib/vsprintf.c: move string() below widen_string() Rasmus Villemoes <linux@rasmusvillemoes.dk> - 2015-12-03 22:00 +0100
  [PATCH v3 14/14] lib/test_printf.c: test dentry printing Rasmus Villemoes <linux@rasmusvillemoes.dk> - 2015-12-03 22:00 +0100
    Re: [PATCH v3 14/14] lib/test_printf.c: test dentry printing Andrew Morton <akpm@linux-foundation.org> - 2015-12-04 01:20 +0100
      Re: [PATCH v3 14/14] lib/test_printf.c: test dentry printing Rasmus Villemoes <linux@rasmusvillemoes.dk> - 2015-12-04 09:20 +0100
        Re: [PATCH v3 14/14] lib/test_printf.c: test dentry printing Andrew Morton <akpm@linux-foundation.org> - 2015-12-04 09:50 +0100
  [PATCH v3 04/14] lib/vsprintf.c: expand field_width to 24 bits Rasmus Villemoes <linux@rasmusvillemoes.dk> - 2015-12-03 22:00 +0100

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