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SubjectRe: [PATCH 02/16] mm: sl[au]b: Add knowledge of PFMEMALLOC reserve pages
On Mon, 16 Apr 2012, Mel Gorman wrote:

> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 280eabe..0fa2c72 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1463,6 +1463,7 @@ failed:
> #define ALLOC_HARDER 0x10 /* try to alloc harder */
> #define ALLOC_HIGH 0x20 /* __GFP_HIGH set */
> #define ALLOC_CPUSET 0x40 /* check for correct cpuset */
> +#define ALLOC_PFMEMALLOC 0x80 /* Caller has PF_MEMALLOC set */
>
> #ifdef CONFIG_FAIL_PAGE_ALLOC
>
> @@ -2208,16 +2209,22 @@ gfp_to_alloc_flags(gfp_t gfp_mask)
> } else if (unlikely(rt_task(current)) && !in_interrupt())
> alloc_flags |= ALLOC_HARDER;
>
> - if (likely(!(gfp_mask & __GFP_NOMEMALLOC))) {
> - if (!in_interrupt() &&
> - ((current->flags & PF_MEMALLOC) ||
> - unlikely(test_thread_flag(TIF_MEMDIE))))
> + if ((current->flags & PF_MEMALLOC) ||
> + unlikely(test_thread_flag(TIF_MEMDIE))) {
> + alloc_flags |= ALLOC_PFMEMALLOC;
> +
> + if (likely(!(gfp_mask & __GFP_NOMEMALLOC)) && !in_interrupt())
> alloc_flags |= ALLOC_NO_WATERMARKS;
> }
>
> return alloc_flags;
> }
>
> +bool gfp_pfmemalloc_allowed(gfp_t gfp_mask)
> +{
> + return !!(gfp_to_alloc_flags(gfp_mask) & ALLOC_PFMEMALLOC);
> +}
> +
> static inline struct page *
> __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
> struct zonelist *zonelist, enum zone_type high_zoneidx,
> @@ -2407,8 +2414,16 @@ nopage:
> got_pg:
> if (kmemcheck_enabled)
> kmemcheck_pagealloc_alloc(page, order, gfp_mask);
> - return page;
>
> + /*
> + * page->pfmemalloc is set when the caller had PFMEMALLOC set or is
> + * been OOM killed. The expectation is that the caller is taking
> + * steps that will free more memory. The caller should avoid the
> + * page being used for !PFMEMALLOC purposes.
> + */
> + page->pfmemalloc = !!(alloc_flags & ALLOC_PFMEMALLOC);
> +
> + return page;
> }
>
> /*

I think this is slightly inconsistent if the page allocation succeeded
without needing ALLOC_NO_WATERMARKS, meaning that page was allocated above
the min watermark. That's possible if the slowpath's first call to
get_page_from_freelist() succeeds without needing
__alloc_pages_high_priority(). So perhaps we need to do something like

got_pg_memalloc:
...
page->pfmemalloc = !!(alloc_flags & ALLOC_PFMEMALLOC);
got_pg:
if (kmemcheck_enabled)
kmemcheck_pagealloc_alloc(page, order, gfp_mask);
return page;

and use got_pg_memalloc everywhere we currently use got_pg other than the
when it succeeds with ALLOC_NO_WATERMARKS.

> @@ -2459,6 +2474,8 @@ retry_cpuset:
> page = __alloc_pages_slowpath(gfp_mask, order,
> zonelist, high_zoneidx, nodemask,
> preferred_zone, migratetype);
> + else
> + page->pfmemalloc = false;
>
> trace_mm_page_alloc(page, order, gfp_mask, migratetype);
>


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