Re: [PATCH v4 6/7] mm: truncate: split huge page cache page to a non-zero order if possible.

On 14 Feb 2024, at 5:43, Ryan Roberts wrote:

On 13/02/2024 21:55, Zi Yan wrote:

...

From: Zi Yan ziy@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 ziy@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?

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.

...

• /* 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@google.... [2] https://lore.kernel.org/linux-mm/0AC0520E-1BD2-497E-A7ED-05394400BFC9@nvidia...

-- Best Regards, Yan, Zi