mm/page_alloc: don't warn about large allocations with __GFP_NOFAIL

Filesystems use __GFP_NOFAIL to allocate block-sized folios for metadata
reads at critical points, since they cannot afford to go read-only, shut
down, or enter an inconsistent state due to memory pressure.

Currently, attempting to allocate page units greater than order-1 with the
__GFP_NOFAIL flag triggers a WARN_ON() in __alloc_pages_slowpath(). 
However, filesystems supporting large block sizes (blocksize > PAGE_SIZE)
can easily require allocations larger than order-1.

As Matthew Wilcox noted in [1], if we have a filesystem with 64KiB
sectors, there will be many clean folios in the page cache that are 64KiB
or larger.  He also explained in [2] why kvmalloc isn't a valid approach
here.

With gfp flags and order already included in the OOM report, both
Vlastimil Babka and Michal Hocko suggested that we can take the risk of
removing this warning first and then observe whether a large number of
related OOM reports appear.

If that happens, we can consider adding special handling in other places.

Link: https://lkml.kernel.org/r/20251105085652.4081123-1-libaokun@huaweicloud.com
Signed-off-by: Baokun Li <libaokun1@huawei.com>
Suggested-by: Matthew Wilcox <willy@infradead.org>
Link: https://lore.kernel.org/all/aQPX1-XWQjKaMTZB@casper.infradead.org [1]
Link: https://lore.kernel.org/all/aQTHMI3t5mNXp0M1@casper.infradead.org [2]
Suggested-by: Vlastimil Babka <vbabka@suse.cz>
Link: https://lore.kernel.org/all/188a95ba-6384-4319-bb74-c0d9ec6c4079@suse.cz
Suggested-by: Michal Hocko <mhocko@suse.com>
Link: https://lore.kernel.org/all/aQotQBjnDDeL_wHx@tiehlicka
Acked-by: Michal Hocko <mhocko@suse.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Brendan Jackman <jackmanb@google.com>
Cc: ErKun Yang <yangerkun@huawei.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: "zhangyi (F)" <yi.zhang@huawei.com>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/page_alloc.c

@@ -4683,11 +4683,6 @@ __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.