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DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1774577743; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=8tx/Qjve1iVx/sjsNymGDpKKmMZv1TUyDs/EfcYY5GE=; b=Hnot0p5K89CAv1AQSFPCVNypjXRfAocNMIZyWJnznY4gpq+GWy2CVy+0V7yyIEu0F+HuLG VCDbIV/SqybnVZSHtV9JQkcdTWMzMp9IQ8Hs+qWemVt0yevh5Dk4OIgO/2Wqfo3fp+W2Sb aJvabR1V9LfLrBzcx+XgaoHKYXMXuJs= From: Usama Arif To: Andrew Morton , david@kernel.org, Lorenzo Stoakes , willy@infradead.org, linux-mm@kvack.org Cc: fvdl@google.com, hannes@cmpxchg.org, riel@surriel.com, shakeel.butt@linux.dev, kas@kernel.org, baohua@kernel.org, dev.jain@arm.com, baolin.wang@linux.alibaba.com, npache@redhat.com, Liam.Howlett@oracle.com, ryan.roberts@arm.com, Vlastimil Babka , lance.yang@linux.dev, linux-kernel@vger.kernel.org, kernel-team@meta.com, maddy@linux.ibm.com, mpe@ellerman.id.au, linuxppc-dev@lists.ozlabs.org, hca@linux.ibm.com, gor@linux.ibm.com, agordeev@linux.ibm.com, borntraeger@linux.ibm.com, svens@linux.ibm.com, linux-s390@vger.kernel.org, Usama Arif Subject: [v3 15/24] fs/dax: handle split_huge_pmd failure in dax_iomap_pmd_fault Date: Thu, 26 Mar 2026 19:08:57 -0700 Message-ID: <20260327021403.214713-16-usama.arif@linux.dev> In-Reply-To: <20260327021403.214713-1-usama.arif@linux.dev> References: <20260327021403.214713-1-usama.arif@linux.dev> X-Mailing-List: linuxppc-dev@lists.ozlabs.org List-Id: List-Help: List-Owner: List-Post: List-Archive: , List-Subscribe: , , List-Unsubscribe: Precedence: list MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT dax_iomap_pmd_fault() splits a huge PMD when the PMD fault falls back to PTE-level handling. The split is necessary so that the subsequent PTE fault does not misinterpret the huge PMD entry as a page table pointer. In practice this cannot fail today: DAX VMAs are always file-backed, and __split_huge_pmd() only allocates a PTE page table (the operation that can return -ENOMEM) for anonymous VMAs. For file-backed VMAs the split path simply zaps the PMD and returns 0. Use WARN_ON_ONCE to document this invariant and check the return value for __must_check compliance introduced in the next patch. Signed-off-by: Usama Arif --- fs/dax.c | 9 ++++++++- 1 file changed, 8 insertions(+), 1 deletion(-) diff --git a/fs/dax.c b/fs/dax.c index a5237169b4679..ed1859e8a916f 100644 --- a/fs/dax.c +++ b/fs/dax.c @@ -2039,7 +2039,14 @@ static vm_fault_t dax_iomap_pmd_fault(struct vm_fault *vmf, unsigned long *pfnp, dax_unlock_entry(&xas, entry); fallback: if (ret == VM_FAULT_FALLBACK) { - split_huge_pmd(vmf->vma, vmf->pmd, vmf->address); + /* + * split_huge_pmd() cannot fail for file-backed (DAX) VMAs + * since splitting only zaps the PMD without allocating a + * PTE page table. + */ + if (WARN_ON_ONCE(split_huge_pmd(vmf->vma, vmf->pmd, + vmf->address))) + ret = VM_FAULT_OOM; count_vm_event(THP_FAULT_FALLBACK); } out: -- 2.52.0