On Thu, 26 Apr 2012 17:21:36 +0200 Jes Sorensen <Jes.Sorensen@xxxxxxxxxx> wrote: > On 04/26/12 17:18, Doug Ledford wrote: > > On 04/26/2012 11:12 AM, Jes.Sorensen@xxxxxxxxxx wrote: > >> > From: Jes Sorensen <Jes.Sorensen@xxxxxxxxxx> > >> > > >> > fbdef49811c9e2b54e2064d9af68cfffa77c6e77 incorrectly tried to fix sign > >> > extension of the bitmap offset. However mdinfo->bitmap_offset is a u32 > >> > and needs to be converted to a 32 bit signed integer before the sign > >> > extension. > >> > > >> > Signed-off-by: Jes Sorensen <Jes.Sorensen@xxxxxxxxxx> > > I was scratching my head over this patch, saying to myself "But won't > > that cause us to truncate large values of bitmap_offset?" And it will, > > but I see your point now, that's *exactly* the problem if we don't do > > the sign conversion before the extension, the actual bitmap_offset > > should really be signed in order to support negative offsets, but since > > it isn't, when we save a negative offset into bitmap_offset it appears > > as a really large positive offset, and then when we sign extend to long, > > it keeps the large size positive offset instead of picking up the > > negative offset. Gotcha. So, I see why this works, but do you think it > > should be fixed this way, or by converting bitmap_offset to type int32 > > instead of uint32? > > Heh, I have to admit I cheated too and asked Richard Henderson for help > as I couldn't figure out why the sign conversion failed. Otherwise I > would probably still have been scratching my head over it :) > > I noticed other parts of the code already handled it this way, so my fix > is consistent with that, but we could do both. Neil? The reason that "bitmap_offset" in 'mdp_superblock_1' is '__u32' is simply that there is no '__s32'. I wouldn't be against changing it, but I think you've concluded that it keeps the byte swapping simpler if we don't. But then I read recently that Rob Pike thinks byte swapping is always wrong http://commandcenter.blogspot.com/2012/04/byte-order-fallacy.html so maybe we shouldn't be todo that.... I'll just take your patches as they are I think - they look good. Thanks, NeilBrown
Attachment:
signature.asc
Description: PGP signature
