Regression caused by commit 4687fdbb805a ("mm/filemap: Support VM_HUGEPAGE for file mappings")

[Roman Gushchin]

Hello!

The commit 4687fdbb805a ("mm/filemap: Support VM_HUGEPAGE for file
mappings") causes a regression in our production for containers
which are running short on memory. In some cases they are getting
stuck for hours in a vicious reclaim cycle. Reverting this commit
fixes the problem.

As I understand, the intention of the commit is to allocate large folios
whenever possible, and the idea is to ignore device-specific readahead
settings and the mmap_miss logic to achieve that, which makes total
sense.

However under a heavy memory pressure there must be a mechanism to
revert to order-0 folios, otherwise the memory pressure is inevitable
increased. Maybe mmap_miss heuristics should still be applied? Any other
ideas how to fix it?

Also, a side question: I wonder if it makes sense to allocate 1-2
PMD-sized folios if mapping_large_folio_support() is not there?

Thanks!