On 14/02/2024 16:19, Zi Yan wrote: > On 14 Feb 2024, at 5:43, Ryan Roberts wrote: > >> On 13/02/2024 21:55, Zi Yan wrote: >>> From: Zi Yan <ziy@xxxxxxxxxx> >>> >>> 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 <ziy@xxxxxxxxxx> >>> --- >>> 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? > > Right. You raise the same concern as Hugh[1]. I am minimizing the call of > split_huge_page_to_list_to_order() and you and Hugh want to minimize the > number of folios after the split. Yours will give better outcome after split, > but requires either multiple calls or a more sophisticated implementation > of page split[2]. We probably can revisit this once splitting to any order > gets wider use. Yeah, fair enough. Sorry hadn't read Hugh's original feedback. > >>> + >>> + /* 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? > > Right. > >>> + >>> + >>> 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? > > Sure. Will change the patch locally. > > [1] https://lore.kernel.org/linux-mm/9dd96da-efa2-5123-20d4-4992136ef3ad@xxxxxxxxxx/ > [2] https://lore.kernel.org/linux-mm/0AC0520E-1BD2-497E-A7ED-05394400BFC9@xxxxxxxxxx/ > > -- > Best Regards, > Yan, Zi