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refcount: [PATCH] mm/hmm: Fix a hmm_range_fault() livelock / starvation problem To: =?UTF-8?Q?Thomas_Hellstr=C3=B6m?= , Matthew Brost Cc: Andrew Morton , intel-xe@lists.freedesktop.org, Ralph Campbell , Christoph Hellwig , Jason Gunthorpe , Jason Gunthorpe , Leon Romanovsky , linux-mm@kvack.org, stable@vger.kernel.org, dri-devel@lists.freedesktop.org References: <20260130144529.79909-1-thomas.hellstrom@linux.intel.com> <20260130100013.fb1ce1cd5bd7a440087c7b37@linux-foundation.org> <57fd7f99-fa21-41eb-b484-56778ded457a@nvidia.com> <2d96c9318f2a5fc594dc6b4772b6ce7017a45ad9.camel@linux.intel.com> <0025ee21-2a6c-4c6e-a49a-2df525d3faa1@nvidia.com> <9a9853a320a30802ff35803a574aab037aa2fd92.camel@linux.intel.com> Content-Language: en-US From: John Hubbard In-Reply-To: <9a9853a320a30802ff35803a574aab037aa2fd92.camel@linux.intel.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-ClientProxiedBy: SJ0PR03CA0059.namprd03.prod.outlook.com (2603:10b6:a03:33e::34) To 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https://docs.kernel.org/core-api/pin_user_pages.html > > But it would not work if some part of core mm grabs a page refcount and > *expects* that to pin a page in the sense that it should not be > migrated. But you're suggesting that's actually the case? Yes. The migration code itself, in fact, uses folio_get() with the expectation that this blocks migration: migrate_vma_collect_pmd(): /* * By getting a reference on the folio we pin it and that blocks * any kind of migration. Side effect is that it "freezes" the * pte. * * We drop this reference after isolating the folio from the lru * for non device folio (device folio are not on the lru and thus * can't be dropped from it). */ folio = page_folio(page); folio_get(folio); I'm experiencing a bit of local-mind-boggle at decoupling refcount from pincount, but part of that is probably just bias towards "but it's always been that way!". haha :) thanks, -- John Hubbard