David Howells wrote on Mon, Aug 11, 2025 at 03:37:43PM +0100: > Dominique Martinet via B4 Relay wrote: > > It's apparently possible to get an iov forwarded all the way up to the > > By "forwarded" I presume you mean "advanced"? Thanks, swapped words in v2 > > This should have been because we're only in the 2nd slot and there's > > another one after this, but iterate_folioq should not try to map a > > folio that skips the whole size, and more importantly part here does > > not end up zero (because 'PAGE_SIZE - skip % PAGE_SIZE' ends up > > PAGE_SIZE and not zero..), so skip forward to the "advance to next > > folio" code. > > Note that things get complicated because folioqs form a segmented list that > can be under construction as it advances. So if there's no next folioq > segment at the time you advance to the end of the current one, it will end up > parked at the end of the last folio or with slot==nr_slots because there's > nowhere for it to advance to. Hmm, I've already sent a v2 with other things fixed but now you made me look at the "we're at the end of the iov_iter" case I think this won't work well either? folioq_folio() always returns something, and the advance code only advances if folioq->next is set and doesn't bail out if it's unset. There should be a `if (slot == folioq_nr_slots(folioq)) break` check somewhere as well? Or is the iov_iter guaranteed to always 1/ have some data and 2/ either be big enough or have remaining data in a step? I can believe the former but wouldn't trust the later... > Note that extract_folioq_to_sg() already does this as does > iov_iter_extract_folioq_pages(). Yes we're not quite consistent here, some functions like the plain iov_iter_advance will get you on an invalid slot to check for folioq->next on next invocations while others point at the end of the last folio in the queue (like iov_iter_extract_folioq_pages(), and iov_folioq_get_pages() before patch 2); I think either pattern is valid; I've changed iov_folioq_get_pages() because it was a bit weird to have an iov_iter with offset > count and iov_iter_advance wouldn't do this, but I agree either should work, we just probably want to be more consistent. Thanks, -- Dominique Martinet | Asmadeus
