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

[Ryan Roberts <ryan.roberts@arm.com>]

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?

> +
> +	/* 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;