On Fri, Sep 09, 2016 at 12:55:21AM +0100, Al Viro wrote: > Again, what I propose is a new iov_iter flavour. Backed by pipe_buffer array, > used only for reads (i.e. copy to, not copy from). Three states for element: > pagecache one, copied data, empty. Semantics: > * copy_page_to_iter(): grab a reference to page and stick it into > the next element (making it a pagecache one) with offset and len coming > directly from arguments. Start with checking if we are asking for the next chunk of the same page and simply adjust ->length if so. > * copy_to_iter(): if the last element is a 'copied data' with empty > space remaining - copy to the end. Otherwise allocate a new page and stick > it into the next element (making it 'copied data'), then copy into it. If > still not all data copied, do the same for the next element, etc. Of course, > if there's no elements left, we are done copying. > * zero_iter(): ditto, with s/copy/fill with zeroes/ > * iov_iter_get_pages(): allocate pages, stick them into the next > slots (making those 'copied data'). That might need some changes, though - > I'm still looking through the users. The tricky part is decision when to > update the lengths. ... setting lengths to PAGE_SIZE. * a new primitive to be used instead of iov_iter_advance() in success case of ->direct_IO() from generic_file_read_iter() - equivalent to iov_iter_advance() for all existing iov_iter flavours. For this one: iov_iter_advance() + truncate ->length on the element we'd ended up on + free pages on all subsequent elements, converting them to "empty". > * iov_iter_get_pages_alloc(): not sure, hadn't really looked yet. Usual "allocate array, then as in iov_iter_get_pages()" > * iov_iter_alignment(): probably just returns 0. > * iov_iter_advance(): probably like bvec variant. Probably needs to scream bloody murder if we are seeking _not_ to the end of the last element. * ->count handling: capacity. IOW, number of unused elements times PAGE_SIZE + if the current element is 'copied data' the amount of data left in this one. That will need a careful review - any ->read_iter() making assumptions about iov_iter_count() after copy_page_to_iter() might need to be adjusted (i.e. we can't assume that iov_iter_count() decreases exactly by the amount returned by copy_page_to_iter()). * copying from such iov_iter: BUG() * fault-in: nop * iov_iter_npages(): elements left * dup_iter(): BUG() for now * csum_and_copy_to_iter(): similar to copy_to_iter(), with obvious modifications for actually calculating csum. Or just BUG() - we are not likely to use it at the moment. Looks like that should suffice... And it looks like that ought to take shmem ->splice_read() out as well, so that's one fewer caller of splice_to_pipe() to switch... -- To unsubscribe from this list: send the line "unsubscribe linux-xfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
