Re: [PATCH RFC v2 2/3] mm/page_alloc: refactor the initial compaction handling

[Joshua Hahn]

On Fri, 19 Dec 2025 18:38:52 +0100 Vlastimil Babka <vbabka@xxxxxxx> wrote:

Hi Vlastimil,

I hope you are doing well, sorry for the late reply. The patch overall looks
good to me, but I have a few very small nits.

> The initial direct compaction done in some cases in
> __alloc_pages_slowpath() stands out from the main retry loop of
> reclaim + compaction.
> 
> We can simplify this by instead skipping the initial reclaim attempt via
> a new local variable compact_first, and handle the compact_prority to
> match the original behavior.
> 
> Suggested-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
> ---
>  mm/page_alloc.c | 106 +++++++++++++++++++++++++++++---------------------------
>  1 file changed, 54 insertions(+), 52 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 9e7b0967f1b5..cb8965fd5e20 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -4512,6 +4512,11 @@ static bool oom_reserves_allowed(struct task_struct *tsk)
>  	return true;
>  }
>  
> +static inline bool gfp_thisnode_noretry(gfp_t gfp_mask)
> +{
> +	return (gfp_mask & __GFP_NORETRY) && (gfp_mask & __GFP_THISNODE);
> +}

NIT: Is there a reason why this was turned into its own function? The checks
seem short enough to open-code and it seems like there's only one caller as
far as I can tell. Actually I think there are some better candidates of
turning boolean checks into functions, like the one below:

[...snip...]

> +	/*
> +	 * For costly allocations, try direct compaction first, as it's likely
> +	 * that we have enough base pages and don't need to reclaim. For non-
> +	 * movable high-order allocations, do that as well, as compaction will
> +	 * try prevent permanent fragmentation by migrating from blocks of the
> +	 * same migratetype.
> +	 */
> +	if (can_compact && (costly_order || (order > 0 &&
> +					ac->migratetype != MIGRATE_MOVABLE))) {
> +		compact_first = true;
> +		compact_priority = INIT_COMPACT_PRIORITY;
> +	}
> +

It has indeed become shorter thanks to this patch, but I think if we want to
make the code more "readable" we can stay consistent and move these into thier
own boolean checks, or just leave them open-coded. No strong preference here,
just wanted to offer my 2c and hear what you think.

[...snip...]

> +		/*
> +		 * For the initial compaction attempt we have lowered its
> +		 * priority. Restore it for further retries. With __GFP_NORETRY
> +		 * there will be a single round of reclaim+compaction with the
> +		 * lowered priority.
> +		 */
> +		if (!(gfp_mask & __GFP_NORETRY)) {
> +			compact_priority = DEF_COMPACT_PRIORITY;
> +		}

NIT: I think these braces are unecessary : -)

Otherwise, LGTM! Thank you for your work, it looks a lot cleaner.

Reviewed-by: Joshua Hahn <joshua.hahnjy@xxxxxxxxx>