On 13/02/2024 21:55, Zi Yan wrote:
> From: Zi Yan <z...@nvidia.com>
>
> To minimize the number of pages after a huge page truncation, we do not
> need to split it all the way down to order-0. The huge page has at most
> three parts, the part before offset, the part to be truncated, the part
> remaining at the end. Find the greatest common divisor of them to
> calculate the new page order from it, so we can split the huge
> page to this order and keep the remaining pages as large and as few as
> possible.
>
> Signed-off-by: Zi Yan <z...@nvidia.com>
> ---
> mm/truncate.c | 21 +++++++++++++++++++--
> 1 file changed, 19 insertions(+), 2 deletions(-)
>
> diff --git a/mm/truncate.c b/mm/truncate.c
> index 725b150e47ac..49ddbbf7a617 100644
> --- a/mm/truncate.c
> +++ b/mm/truncate.c
> @@ -21,6 +21,7 @@
> #include <linux/task_io_accounting_ops.h>
> #include <linux/shmem_fs.h>
> #include <linux/rmap.h>
> +#include <linux/gcd.h>
> #include "internal.h"
>
> /*
> @@ -210,7 +211,8 @@ int truncate_inode_folio(struct address_space *mapping,
> struct folio *folio)
> bool truncate_inode_partial_folio(struct folio *folio, loff_t start, loff_t
> end)
> {
> loff_t pos = folio_pos(folio);
> - unsigned int offset, length;
> + unsigned int offset, length, remaining;
> + unsigned int new_order = folio_order(folio);
>
> if (pos < start)
> offset = start - pos;
> @@ -221,6 +223,7 @@ bool truncate_inode_partial_folio(struct folio *folio,
> loff_t start, loff_t end)
> length = length - offset;
> else
> length = end + 1 - pos - offset;
> + remaining = folio_size(folio) - offset - length;
>
> folio_wait_writeback(folio);
> if (length == folio_size(folio)) {
> @@ -235,11 +238,25 @@ bool truncate_inode_partial_folio(struct folio *folio,
> loff_t start, loff_t end)
> */
> folio_zero_range(folio, offset, length);
>
> + /*
> + * Use the greatest common divisor of offset, length, and remaining
> + * as the smallest page size and compute the new order from it. So we
> + * can truncate a subpage as large as possible. Round up gcd to
> + * PAGE_SIZE, otherwise ilog2 can give -1 when gcd/PAGE_SIZE is 0.
> + */
> + new_order = ilog2(round_up(gcd(gcd(offset, length), remaining),
> + PAGE_SIZE) / PAGE_SIZE);
Given you have up to 2 regions remaining, isn't it possible that you want a
different order for both those regions (or even multiple orders within the same
region)? I guess you just choose gcd for simplicity?
> +
> + /* order-1 THP not supported, downgrade to order-0 */
> + if (new_order == 1)
> + new_order = 0;
I guess this would need to change if supporting order-1 file folios?
> +
> +
> if (folio_has_private(folio))
> folio_invalidate(folio, offset, length);
> if (!folio_test_large(folio))
> return true;
> - if (split_folio(folio) == 0)
> + if (split_huge_page_to_list_to_order(&folio->page, NULL, new_order) ==
> 0)
I know you are discussing removing this patch, but since you created
split_folio_to_order() wouldn't that be better here?
Thanks,
Ryan
> return true;
> if (folio_test_dirty(folio))
> return false;
