On Wed, May 14, 2025 at 4:43 PM Ackerley Tng <ackerleytng@xxxxxxxxxx> wrote:
> ...
> +/**
> + * kvm_gmem_zero_range() - Zeroes all sub-pages in range [@start, @end).
> + *
> + * @mapping: the filemap to remove this range from.
> + * @start: index in filemap for start of range (inclusive).
> + * @end: index in filemap for end of range (exclusive).
> + *
> + * The pages in range may be split. truncate_inode_pages_range() isn't the right
> + * function because it removes pages from the page cache; this function only
> + * zeroes the pages.
> + */
> +static void kvm_gmem_zero_range(struct address_space *mapping,
> + pgoff_t start, pgoff_t end)
> +{
> + struct folio_batch fbatch;
> +
> + folio_batch_init(&fbatch);
> + while (filemap_get_folios(mapping, &start, end - 1, &fbatch)) {
> + unsigned int i;
> +
> + for (i = 0; i < folio_batch_count(&fbatch); ++i) {
> + struct folio *f;
> + size_t nr_bytes;
> +
> + f = fbatch.folios[i];
> + nr_bytes = offset_in_folio(f, end << PAGE_SHIFT);
> + if (nr_bytes == 0)
> + nr_bytes = folio_size(f);
> +
> + folio_zero_segment(f, 0, nr_bytes);
folio_zero_segment takes byte offset and number of bytes within the
folio. This invocation needs to operate on the folio range that is
overlapping with [Start, end) and instead it's always starting from 0
and ending at an unaligned offset within the folio. This will result
in zeroing more than requested or lesser than requested or both
depending on the request and folio size.
> + }
> +
> + folio_batch_release(&fbatch);
> + cond_resched();
> + }
> +}
> +
