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SubjectRe: [PATCH v2 for v5.9] mm/page_alloc: handle a missing case for memalloc_nocma_{save/restore} APIs
On Mon, 28 Sep 2020 17:50:46 +0900 js1304@gmail.com wrote:

> From: Joonsoo Kim <iamjoonsoo.kim@lge.com>
>
> memalloc_nocma_{save/restore} APIs can be used to skip page allocation
> on CMA area, but, there is a missing case and the page on CMA area could
> be allocated even if APIs are used. This patch handles this case to fix
> the potential issue.
>
> Missing case is an allocation from the pcplist. MIGRATE_MOVABLE pcplist
> could have the pages on CMA area so we need to skip it if ALLOC_CMA isn't
> specified.

Changelog doesn't describe the end-user visible effects of the bug.
Please send this description?

> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -3367,9 +3367,16 @@ struct page *rmqueue(struct zone *preferred_zone,
> struct page *page;
>
> if (likely(order == 0)) {
> - page = rmqueue_pcplist(preferred_zone, zone, gfp_flags,
> + /*
> + * MIGRATE_MOVABLE pcplist could have the pages on CMA area and
> + * we need to skip it when CMA area isn't allowed.
> + */
> + if (!IS_ENABLED(CONFIG_CMA) || alloc_flags & ALLOC_CMA ||
> + migratetype != MIGRATE_MOVABLE) {
> + page = rmqueue_pcplist(preferred_zone, zone, gfp_flags,
> migratetype, alloc_flags);
> - goto out;
> + goto out;
> + }
> }
>
> /*

We still really really don't want to be adding overhead to the page
allocation hotpath for a really really obscure feature from a single
callsite.

Do we have an understanding of how many people's kernels are enabling
CONFIG_CMA?

I previously suggested retrying the allocation in
__gup_longterm_locked() but you said "it cannot ensure that we
eventually get the non-CMA page". Please explain why?

What about manually emptying the pcplists beforehand?

Or byassing the pcplists for this caller and calling __rmqueue() directly?

> @@ -3381,7 +3388,7 @@ struct page *rmqueue(struct zone *preferred_zone,
>
> do {
> page = NULL;
> - if (alloc_flags & ALLOC_HARDER) {
> + if (order > 0 && alloc_flags & ALLOC_HARDER) {
> page = __rmqueue_smallest(zone, order, MIGRATE_HIGHATOMIC);
> if (page)
> trace_mm_page_alloc_zone_locked(page, order, migratetype);

What does this hunk do?

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