Re: [patch 12/12] mm: page_alloc: do not lock up low-order allocations upon OOM

Michal Hocko
2015-03-26T15:32:59Z

On Wed 25-03-15 02:17:16, Johannes Weiner wrote:
> When both page reclaim and the OOM killer fail to free memory, there
> are no more options for the allocator to make progress on its own.
>
> Don't risk hanging these allocations. Leave it to the allocation site
> to implement the fallback policy for failing allocations.

The changelog communicates the impact of this patch _very_ poorly. The
potential regression space is quite large. Every syscall which is not
allowed to return ENOMEM and it relies on an allocation would have to be
audited or a common mechanism to catch them deployed.

I really believe this is a good thing _longterm_ but I still do not
think it is the upstream material anytime soon without extensive testing
which is even not mentioned here.

> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
> ---
> mm/page_alloc.c | 19 ++++++-------------
> 1 file changed, 6 insertions(+), 13 deletions(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 9e45e97aa934..f2b1a17416c4 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -2331,12 +2331,10 @@ void warn_alloc_failed(gfp_t gfp_mask, int order, const char *fmt, ...)
>
> static inline struct page *
> __alloc_pages_may_oom(gfp_t gfp_mask, unsigned int order, int alloc_flags,
> - const struct alloc_context *ac, unsigned long *did_some_progress)
> + const struct alloc_context *ac)
> {
> struct page *page = NULL;
>
> - *did_some_progress = 0;
> -
> /*
> * This allocating task can become the OOM victim itself at
> * any point before acquiring the lock. In that case, exit
> @@ -2376,13 +2374,9 @@ __alloc_pages_may_oom(gfp_t gfp_mask, unsigned int order, int alloc_flags,
> goto out;
> }
>
> - if (out_of_memory(ac->zonelist, gfp_mask, order, ac->nodemask, false)) {
> - *did_some_progress = 1;
> - } else {
> + if (!out_of_memory(ac->zonelist, gfp_mask, order, ac->nodemask, false))
> /* Oops, these shouldn't happen with the OOM killer disabled */
> - if (WARN_ON_ONCE(gfp_mask & __GFP_NOFAIL))
> - *did_some_progress = 1;
> - }
> + WARN_ON_ONCE(gfp_mask & __GFP_NOFAIL);
>
> /*
> * Allocate from the OOM killer reserves.
> @@ -2799,13 +2793,12 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
> }
>
> /* Reclaim has failed us, start killing things */
> - page = __alloc_pages_may_oom(gfp_mask, order, alloc_flags, ac,
> - &did_some_progress);
> + page = __alloc_pages_may_oom(gfp_mask, order, alloc_flags, ac);
> if (page)
> goto got_pg;
>
> - /* Retry as long as the OOM killer is making progress */
> - if (did_some_progress)
> + /* Wait for user to order more dimms, cuz these are done */
> + if (gfp_mask & __GFP_NOFAIL)
> goto retry;
>
> noretry:
> --
> 2.3.3
>

--
Michal Hocko
SUSE Labs