On Thu, Nov 18, 2021 at 03:55:12PM +0000, Matthew Wilcox wrote:
> On Wed, Nov 17, 2021 at 09:07:07AM -0800, Darrick J. Wong wrote:
> > I've started using 'next', or changing the code to make 'end' be the
> > last element in the range the caller wants to act upon. The thing is,
> > those are all iterators, so 'next' fits, whereas it doesn't fit so well
> > for range zeroing where that might have been all the zeroing we wanted
> > to do.
>
> Yeah, it doesn't really work so well for one of the patches in this
> series:
>
> if (buffer_new(bh)) {
> ...
> folio_zero_segments(folio,
> to, block_end,
> block_start, from);
>
> ("zero between block_start and block_end, except for the region
> specified by 'from' and 'to'"). Except that for some reason the
> ranges are specified backwards, so it's not obvious what's going on.
> Converting that to folio_zero_ranges() would be a possibility, at the
> expense of complexity in the caller, or using 'max' instead of 'end'
> would also add complexity to the callers.
The call above looks like it is preparing to copy some data into the
middle of a buffer by zero-initializing the bytes before and the bytes
after that middle region.
Admittedly my fs-addled brain actually finds this hot mess easier to
understand:
folio_zero_segments(folio, to, blocksize - 1, block_start, from - 1);
but I suppose the xend method involves less subtraction everywhere.
>
> > Though. 'xend' (shorthand for 'excluded end') is different enough to
> > signal that the reader should pay attention. Ok, how about xend then?
>
> Done!
>
> @@ -367,26 +367,26 @@ static inline void memzero_page(struct page *page, size_t
> offset, size_t len)
> * folio_zero_segments() - Zero two byte ranges in a folio.
> * @folio: The folio to write to.
> * @start1: The first byte to zero.
> - * @end1: One more than the last byte in the first range.
> + * @xend1: One more than the last byte in the first range.
> * @start2: The first byte to zero in the second range.
> - * @end2: One more than the last byte in the second range.
> + * @xend2: One more than the last byte in the second range.
> */
> static inline void folio_zero_segments(struct folio *folio,
> - size_t start1, size_t end1, size_t start2, size_t end2)
> + size_t start1, size_t xend1, size_t start2, size_t xend2)
> {
> - zero_user_segments(&folio->page, start1, end1, start2, end2);
> + zero_user_segments(&folio->page, start1, xend1, start2, xend2);
> }
>
> /**
> * folio_zero_segment() - Zero a byte range in a folio.
> * @folio: The folio to write to.
> * @start: The first byte to zero.
> - * @end: One more than the last byte in the first range.
> + * @xend: One more than the last byte to zero.
> */
> static inline void folio_zero_segment(struct folio *folio,
> - size_t start, size_t end)
> + size_t start, size_t xend)
> {
> - zero_user_segments(&folio->page, start, end, 0, 0);
> + zero_user_segments(&folio->page, start, xend, 0, 0);
Works for me,
Reviewed-by: Darrick J. Wong <djwong@xxxxxxxxxx>
--D
> }
>
> /**
>
