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SubjectRe: [PATCH 2/2] mm: do not loop over ALLOC_NO_WATERMARKS without triggering reclaim
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On 11/16/2015 02:22 PM, mhocko@kernel.org wrote:
> From: Michal Hocko <mhocko@suse.com>
>
> __alloc_pages_slowpath is looping over ALLOC_NO_WATERMARKS requests if
> __GFP_NOFAIL is requested. This is fragile because we are basically
> relying on somebody else to make the reclaim (be it the direct reclaim
> or OOM killer) for us. The caller might be holding resources (e.g.
> locks) which block other other reclaimers from making any progress for
> example. Remove the retry loop and rely on __alloc_pages_slowpath to
> invoke all allowed reclaim steps and retry logic.
>
> We have to be careful about __GFP_NOFAIL allocations from the
> PF_MEMALLOC context even though this is a very bad idea to begin with
> because no progress can be gurateed at all. We shouldn't break the
> __GFP_NOFAIL semantic here though. It could be argued that this is
> essentially GFP_NOWAIT context which we do not support but PF_MEMALLOC
> is much harder to check for existing users because they might happen
> deep down the code path performed much later after setting the flag
> so we cannot really rule out there is no kernel path triggering this
> combination.
>
> Acked-by: Mel Gorman <mgorman@suse.de>
> Signed-off-by: Michal Hocko <mhocko@suse.com>
> ---
> mm/page_alloc.c | 32 ++++++++++++++++++--------------
> 1 file changed, 18 insertions(+), 14 deletions(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index b153fa3d0b9b..df7746280427 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -3046,32 +3046,36 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
> * allocations are system rather than user orientated
> */
> ac->zonelist = node_zonelist(numa_node_id(), gfp_mask);
> - do {
> - page = get_page_from_freelist(gfp_mask, order,
> - ALLOC_NO_WATERMARKS, ac);
> - if (page)
> - goto got_pg;
> -
> - if (gfp_mask & __GFP_NOFAIL)
> - wait_iff_congested(ac->preferred_zone,
> - BLK_RW_ASYNC, HZ/50);

I've been thinking if the lack of unconditional wait_iff_congested() can affect
something negatively. I guess not?

> - } while (gfp_mask & __GFP_NOFAIL);
> + page = get_page_from_freelist(gfp_mask, order,
> + ALLOC_NO_WATERMARKS, ac);
> + if (page)
> + goto got_pg;
> }
>
> /* Caller is not willing to reclaim, we can't balance anything */
> if (!can_direct_reclaim) {
> /*
> - * All existing users of the deprecated __GFP_NOFAIL are
> - * blockable, so warn of any new users that actually allow this
> - * type of allocation to fail.
> + * All existing users of the __GFP_NOFAIL are blockable, so warn
> + * of any new users that actually allow this type of allocation
> + * to fail.
> */
> WARN_ON_ONCE(gfp_mask & __GFP_NOFAIL);
> goto nopage;
> }
>
> /* Avoid recursion of direct reclaim */
> - if (current->flags & PF_MEMALLOC)
> + if (current->flags & PF_MEMALLOC) {
> + /*
> + * __GFP_NOFAIL request from this context is rather bizarre
> + * because we cannot reclaim anything and only can loop waiting
> + * for somebody to do a work for us.
> + */
> + if (WARN_ON_ONCE(gfp_mask & __GFP_NOFAIL)) {
> + cond_resched();
> + goto retry;
> + }
> goto nopage;
> + }
>
> /* Avoid allocations with no watermarks from looping endlessly */
> if (test_thread_flag(TIF_MEMDIE) && !(gfp_mask & __GFP_NOFAIL))
>



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