On Thu, Feb 03, 2022 at 04:59:56PM -0800, John Hubbard wrote: > On 2/3/22 16:45, Jason Gunthorpe wrote: > > On Thu, Feb 03, 2022 at 12:44:57PM -0800, John Hubbard wrote: > > > On 2/3/22 05:01, Jason Gunthorpe wrote: > > > ... > > > > > > In the new branch if (pages), you set page = ERR_PTR(-EFAULT) and goto > > > > > > out. However, at the label out, the value of page is not used, but the > > > > > > return uses the variables i and ret. > > > > > > > > > > Yes, I think that the complaint is accurate. The intent of this code is > > > > > to return either number of pages so far (i) or ret (which should be zero > > > > > in this case), because we are just stopping early, rather than calling > > > > > this an actual error. > > > > > > > > IIRC GUP shouldn't return 0, it should return an error code, not zero. > > > > > > > > Jason > > > > > > Errors work for single pages, but GUP is a multi-page API call. If it > > > returned an error part way through the list of pages, then callers would > > > have no way of knowing how many pages to release. > > > > Yes, but that is returning a positive error code, I said it should not > > return zero. > > > > When it hits an error with pages already loaded it returns that number > > and the caller will then do gup once more with the VA pointing at the > > problematic page. Then GUP can return the error code because it has 0 > > pages on the next iteration. > > > > It should not return 0 here when it got an error. > > This is perhaps better API design, but it's not what exists now. I think it is what exists today, 0 certainly is not implemented as 'need retry' anywhere I found. So why do we return 0, if it means an error, instead of returning the actual errno? > The call sites today handle 0 pages ret value correctly, This isn't correct though: if (get_user_pages(vaddr, 1, write ? FOLL_WRITE : 0, &page, NULL) <= 0) return -EFAULT; If GUP wanted the caller to permanently fail with -EFAULT, it should have directly returned EFAULT. 0 means 'to be retried', whatever that means, and there is no retry in the above. IOW, the above does not handle a 0 return correctly, according to the comment. Jason
