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Wed, 4 Mar 2026 09:00:47 -0500 (EST) Date: Wed, 4 Mar 2026 14:00:42 +0000 From: Kiryl Shutsemau To: Usama Arif Cc: Andrew Morton , npache@redhat.com, david@kernel.org, ziy@nvidia.com, linux-mm@kvack.org, matthew.brost@intel.com, joshua.hahnjy@gmail.com, hannes@cmpxchg.org, rakie.kim@sk.com, byungchul@sk.com, gourry@gourry.net, ying.huang@linux.alibaba.com, apopple@nvidia.com, riel@surriel.com, shakeel.butt@linux.dev, linux-kernel@vger.kernel.org, kernel-team@meta.com, Usama Arif Subject: Re: [PATCH] mm/migrate_device: fix folio refcount leak on folio_split_unmapped failure Message-ID: References: <20260304120132.3973445-1-usamaarif642@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260304120132.3973445-1-usamaarif642@gmail.com> X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: 88BEB1C0014 X-Stat-Signature: eco5ae5zwrkjowe8aowrxkufrhb3ucfz X-Rspam-User: X-HE-Tag: 1772632852-790295 X-HE-Meta: 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 H7VgCHhY k9HgXa9FpdRWJPfYWU+CsnzYaQc3CO8W0dJcXDUC6GLe0gH/B/O/nRZgoXbeVeXMNpAA76w90Z8CxM+IMiLVD5sEQ4fmblkMyQMB0cgC/bfgxvGG/3CoTmqfnALZpTldpZpB7RVb+m0c65LFjil5L49PSPIohLlMCnN8djnOPus8Z8CQltLO0OG0ukrGxUXJcJ2ZOSfBn7TqVHJdu7YhY2T03f8KAPaeKorUI7OkTzGitGkzaXfjKYi/UeugUxgHRfRSnPRB3hwvW0nwLSGi9DWfPWeFVoBfQBWfr/O0D28vz8HvmjAJ1k/Si8UrFaxBqAYQy1Y8gU95QBAxFBK9N2zMVVUitNjaBYez1x/glxW8yZW5PtLplG0SCZ/o86+Yh+YwgPoYOiAW+E2b9tXIQYHhOHcZEXAK/dUvM9W9Z1La5JMk= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Wed, Mar 04, 2026 at 04:01:32AM -0800, Usama Arif wrote: > From: Usama Arif > > migrate_vma_split_unmapped_folio() takes an extra reference via > folio_get() before calling folio_split_unmapped(). On success, the > split consumes this reference: __folio_freeze_and_split_unmapped() > expects the +1 in its folio_ref_freeze() check, and distributes it > across the resulting sub-folios via folio_ref_unfreeze(...+1), which > are later balanced by folio_put() calls in __migrate_device_finalize(). Without this explanation folio_get() looks very random. And I still can't say I understand reference management for the folios here. Who takes reference for the folio if it !THP that gets return in the _finalize()? Can we get reference for THP and !THP at the same spot? I think we should avoid spacial-casing THP where possible. -- Kiryl Shutsemau / Kirill A. Shutemov