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Howlett" , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Usama Arif , Peter Xu , Jason Gunthorpe Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Cedric Le Goater , Saravanan D , stable@vger.kernel.org References: <20260827-hugepfn-allowable-orders-v1-1-94819c8807c8@kernel.org> From: Baolin Wang In-Reply-To: <20260827-hugepfn-allowable-orders-v1-1-94819c8807c8@kernel.org> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 9F9DD14000C X-Stat-Signature: 8ej75x5xacxypxxk3pmigs3pky3hrb4z X-Rspam-User: X-HE-Tag: 1788164156-822223 X-HE-Meta: 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 F+Z+yB0P Rz2hRAFrvhe63ibMkvy5+vbNAcjPNhFJTqRMsGR2EFmRAbzjSYfhEQ2NGIgq95qcMnh3HHRHjW5CiKNTS9gJHpsRb1zDBEA07ciRVjkaNCM9oCk32KZaEBcgvwJjRuxSK9Lw1h2/KMX1x1imbHgnQzCTHFZ9idDzYPtGNHJ+TaMMyZh8JJbOCQZV2C0Kv0iFzCbMzMwby7LgTqcNpyMNFQXnGvz3uMFt6LFDL9SgA4fRoyWiaRvs1soqL5+pkV9YN7eL81Lj4/kw+NLKAVyLwZkjV6U5UMaVuBOTbTgqltvSCeyXZAEwj5vxyVJeMGsUBqe7slb2DX3F8gbOjMXar2INKNe5nIYiMp9BJTrgHgY03gFZ57o/RXZfp3x1YF8nrBDyejvQiRV6zJ4w= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 8/28/26 3:55 AM, Lorenzo Stoakes (ARM) wrote: > The sysfs THP tuneables at /sys/kernel/mm/transparent_huge_pages/ rather > confusingly only control the behaviour of THP in some instances. > > They are not applicable to MADV_COLLAPSE operations, nor to DAX mappings. > > Long-term, THP is predicated upon compaction being able to obtain large > folios to populate THP ranges. > > However, vm_normal_folio() returns NULL for PFN map mappings, thus their > reference count is maintained by the driver, not core mm. > > As a consequence, the folios are not subject to reclaim nor compaction, so > are not truly part of the THP mechanism at all. > > However, since commit 5dd40721f147 ("mm: allow THP orders for PFNMAPs") > introduced the ability to establish huge PFN maps, they have been subject > to THP tuneables. > > This is incorrect - if a huge PFN map is available (defined by > vma->vm_ops->huge_fault being non-NULL for a VMA_PFNMAP_BIT VMA), then it > should be mapped huge upon fault-in. > > Correct this by explicitly checking for this while ensuring that smaps > continues to accurately report THPeligible statistics. > > While here, abstract the entire file-backed THP check in > vma_can_map_huge_file(), with sensible separation of logic into helper > functions. > > Note that drm_gem_shmem_mmap() and panthor_gem_mmap() establish huge PFN > maps of shmem folios, however they are marked unevictable in > drm_gem_get_pages(), and in any case would fail the reference check in > __remove_mapping() even if they weren't. > > Failing to map huge PFN maps has resulted in significant real-world > performance degradation, see links for details. > > Reported-by: Cedric Le Goater > Closes: https://lore.kernel.org/linux-mm/20260805055544.1568534-1-clg@redhat.com/ > Reported-by: Saravanan D > Closes: https://lore.kernel.org/linux-mm/20260821070520.25759-1-saravanand@crusoe.ai/ > Fixes: 5dd40721f147 ("mm: allow THP orders for PFNMAPs") > Cc: stable@vger.kernel.org > Signed-off-by: Lorenzo Stoakes (ARM) > --- LGTM. Logic looks clearer now. Thanks. Reviewed-by: Baolin Wang