Michal Hocko wrote:
> CONFIG_MMU=n doesn't have oom reaper so let's stick to the original
> ALLOC_NO_WATERMARKS approach but be careful because they still might
> deplete all the memory reserves so keep the semantic as close to the
> original implementation as possible and give them access to memory
> reserves only up to exit_mm (when tsk->mm is cleared) rather than while
> tsk_is_oom_victim which is until signal struct is gone.
Currently memory allocations from __mmput() can use memory reserves but
this patch changes __mmput() not to use memory reserves. You say "keep
the semantic as close to the original implementation as possible" but
this change is not guaranteed to be safe.
> @@ -2943,10 +2943,19 @@ bool __zone_watermark_ok(struct zone *z, unsigned int order, unsigned long mark,
> * the high-atomic reserves. This will over-estimate the size of the
> * atomic reserve but it avoids a search.
> */
> - if (likely(!alloc_harder))
> + if (likely(!alloc_harder)) {
> free_pages -= z->nr_reserved_highatomic;
> - else
> - min -= min / 4;
> + } else {
> + /*
> + * OOM victims can try even harder than normal ALLOC_HARDER
> + * users
> + */
> + if (alloc_flags & ALLOC_OOM)
ALLOC_OOM is ALLOC_NO_WATERMARKS if CONFIG_MMU=n.
I wonder this test makes sense for ALLOC_NO_WATERMARKS.
> + min -= min / 2;
> + else
> + min -= min / 4;
> + }
> +
>
> #ifdef CONFIG_CMA
> /* If allocation can't use CMA areas don't use free CMA pages */
> @@ -3603,6 +3612,22 @@ gfp_to_alloc_flags(gfp_t gfp_mask)
> return alloc_flags;
> }
>
> +static bool oom_reserves_allowed(struct task_struct *tsk)
> +{
> + if (!tsk_is_oom_victim(tsk))
> + return false;
> +
> + /*
> + * !MMU doesn't have oom reaper so we shouldn't risk the memory reserves
> + * depletion and shouldn't give access to memory reserves passed the
> + * exit_mm
> + */
> + if (!IS_ENABLED(CONFIG_MMU) && !tsk->mm)
> + return false;
Branching based on CONFIG_MMU is ugly. I suggest timeout based next OOM
victim selection if CONFIG_MMU=n. Then, we no longer need to worry about
memory reserves depletion and we can treat equally.
> +
> + return true;
> +}
> +
> bool gfp_pfmemalloc_allowed(gfp_t gfp_mask)
> {
> if (unlikely(gfp_mask & __GFP_NOMEMALLOC))
> @@ -3770,6 +3795,7 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
> unsigned long alloc_start = jiffies;
> unsigned int stall_timeout = 10 * HZ;
> unsigned int cpuset_mems_cookie;
> + bool reserves;
>
> /*
> * In the slowpath, we sanity check order to avoid ever trying to
> @@ -3875,15 +3901,24 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
> if (gfp_mask & __GFP_KSWAPD_RECLAIM)
> wake_all_kswapds(order, ac);
>
> - if (gfp_pfmemalloc_allowed(gfp_mask))
> - alloc_flags = ALLOC_NO_WATERMARKS;
> + /*
> + * Distinguish requests which really need access to whole memory
> + * reserves from oom victims which can live with their own reserve
> + */
> + reserves = gfp_pfmemalloc_allowed(gfp_mask);
> + if (reserves) {
> + if (tsk_is_oom_victim(current))
> + alloc_flags = ALLOC_OOM;
If reserves == true due to reasons other than tsk_is_oom_victim(current) == true
(e.g. __GFP_MEMALLOC), why dare to reduce it?
> + else
> + alloc_flags = ALLOC_NO_WATERMARKS;
> + }
If CONFIG_MMU=n, doing this test is silly.
if (tsk_is_oom_victim(current))
alloc_flags = ALLOC_NO_WATERMARKS;
else
alloc_flags = ALLOC_NO_WATERMARKS;
>
> /*
> * Reset the zonelist iterators if memory policies can be ignored.
> * These allocations are high priority and system rather than user
> * orientated.
> */
> - if (!(alloc_flags & ALLOC_CPUSET) || (alloc_flags & ALLOC_NO_WATERMARKS)) {
> + if (!(alloc_flags & ALLOC_CPUSET) || reserves) {
> ac->zonelist = node_zonelist(numa_node_id(), gfp_mask);
> ac->preferred_zoneref = first_zones_zonelist(ac->zonelist,
> ac->high_zoneidx, ac->nodemask);
> @@ -3960,7 +3995,7 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
> goto got_pg;
>
> /* Avoid allocations with no watermarks from looping endlessly */
> - if (test_thread_flag(TIF_MEMDIE) &&
> + if (tsk_is_oom_victim(current) &&
> (alloc_flags == ALLOC_NO_WATERMARKS ||
> (gfp_mask & __GFP_NOMEMALLOC)))
> goto nopage;
And you are silently changing to "!costly __GFP_DIRECT_RECLAIM allocations never fail
(even selected for OOM victims)" (i.e. updating the too small to fail memory allocation
rule) by doing alloc_flags == ALLOC_NO_WATERMARKS if CONFIG_MMU=y.
Applying this change might disturb memory allocation behavior. I don't like this patch.
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