Re: [PATCH 3/8] mm, page_alloc: don't retry initial attempt in slowpath

From: Vlastimil Babka
Date: Mon Jul 18 2016 - 07:34:38 EST

On 07/18/2016 01:29 PM, Michal Hocko wrote:
On Mon 18-07-16 13:22:57, Vlastimil Babka wrote:
After __alloc_pages_slowpath() sets up new alloc_flags and wakes up kswapd, it
first tries get_page_from_freelist() with the new alloc_flags, as it may
succeed e.g. due to using min watermark instead of low watermark. It makes
sense to to do this attempt before adjusting zonelist based on
alloc_flags/gfp_mask, as it's still relatively a fast path if we just wake up
kswapd and successfully allocate.

This patch therefore moves the initial attempt above the retry label and
reorganizes a bit the part below the retry label. We still have to attempt
get_page_from_freelist() on each retry, as some allocations cannot do that
as part of direct reclaim or compaction, and yet are not allowed to fail
(even though they do a WARN_ON_ONCE() and thus should not exist). We can reuse
the call meant for ALLOC_NO_WATERMARKS attempt and just set alloc_flags to
ALLOC_NO_WATERMARKS if the context allows it. As a side-effect, the attempts
from direct reclaim/compaction will also no longer obey watermarks once this
is set, but there's little harm in that.

Kswapd wakeups are also done on each retry to be safe from potential races
resulting in kswapd going to sleep while a process (that may not be able to
reclaim by itself) is still looping.

Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>

Same here, my ack still holds
Acked-by: Michal Hocko <mhocko@xxxxxxxx>

Sorry, forgot to add them before sending. Thanks for both!