On Fri, Nov 10, 2017 at 09:17:33AM +0000, Horia Geantă wrote: > > > I must be missing something. In the case rem == 0, let's say > > the original value of np is npo. Then at the start of the loop, > > np = npo - 1, and at the last iteration, k = npo - 2, so we do > IIUC at the start of the loop np = npo (and not npo - 1), since np is no > longer decremented in the rem == 0 case: > - np--; > + if (rem) > + np--; > > > > > sg_set_buf(&sg[npo - 1], xbuf[npo - 2], PAGE_SIZE); > > > and accordingly last iteration is for k = npo - 1: > sg_set_buf(&sg[npo], xbuf[npo - 1], PAGE_SIZE); > > > While the sg_init_table call sets the end-of-table at > > > > sg_init_table(sg, npo + 1); > > > while this marks sg[npo] as last SG table entry. OK, we're both sort of right. You're correct that this generates a valid SG list in that the number of entries match the end-of-table marking. But the thing that prompted to check this patch in the first place is the semantics behind it. For the case rem == 0, it means that buflen is a multiple of PAGE_SIZE. In that case, the code with your patch will create an SG list that's one page longer than buflen. AFAICS it should be harmless but it would still be better if we can generate an SG list that's exactly buflen long rather than one page longer. Cheers, -- Email: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx> Home Page: http://gondor.apana.org.au/~herbert/ PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
