Re: [PATCH RFC] mm: allow __GFP_NOFAIL allocation up to BLK_MAX_BLOCK_SIZE to support LBS

[Michal Hocko <mhocko@suse.com>]

On Mon 03-11-25 10:25:40, Michal Hocko wrote:
> On Mon 03-11-25 10:01:54, Vlastimil Babka wrote:
> > Maybe we could keep the warning for >=PMD_ORDER as that would still mean
> > someone made an error?
> 
> I am not sure TBH. For those large requests (anything that is costly
> order) it is essentially a loop around allocator inside the allocator.
> I would be really much more worried about order-3 which still triggers
> the oom killer and could kill half of the system without much progress.
> For oder-2 you at least have task_struct which spans 2 pages but I do
> not think we have any guaranteed order-3 page for each task to guarantee
> anything when killing those.

Essentially something like this
diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index 25923cfec9c6..2df477d97cee 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -1142,6 +1142,14 @@ bool out_of_memory(struct oom_control *oc)
 	if (!(oc->gfp_mask & __GFP_FS) && !is_memcg_oom(oc))
 		return true;
 
+	/*
+	 * unlike for other !costly requests killing a task is not
+	 * really guaranteed to free any order-3 pages. Warn about
+	 * that to see whether that happens often enough to special
+	 * case.
+	 */
+	WARN_ON(oc->order == 3 && (oc->gfp_mask & __GFP_NOFAIL));
+
 	/*
 	 * Check if there were limitations on the allocation (only relevant for
 	 * NUMA and memcg) that may require different handling.
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index d1d037f97c5f..ca8795156b14 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3993,6 +3993,7 @@ __alloc_pages_may_oom(gfp_t gfp_mask, unsigned int order,
 	/* Coredumps can quickly deplete all memory reserves */
 	if (current->flags & PF_DUMPCORE)
 		goto out;
+
 	/* The OOM killer will not help higher order allocs */
 	if (order > PAGE_ALLOC_COSTLY_ORDER)
 		goto out;
@@ -4612,11 +4613,6 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
 	int reserve_flags;
 
 	if (unlikely(nofail)) {
-		/*
-		 * We most definitely don't want callers attempting to
-		 * allocate greater than order-1 page units with __GFP_NOFAIL.
-		 */
-		WARN_ON_ONCE(order > 1);
 		/*
 		 * Also we don't support __GFP_NOFAIL without __GFP_DIRECT_RECLAIM,
 		 * otherwise, we may result in lockup.
-- 
Michal Hocko
SUSE Labs