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[35.187.245.222]) by smtp.gmail.com with ESMTPSA id 98e67ed59e1d1-35902bdaffdsm686832a91.17.2026.02.24.12.57.58 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 24 Feb 2026 12:58:00 -0800 (PST) Date: Tue, 24 Feb 2026 20:57:56 +0000 From: Pranjal Shrivastava To: Leon Romanovsky Cc: Ashish Mhetre , robin.murphy@arm.com, joro@8bytes.org, will@kernel.org, iommu@lists.linux.dev, linux-kernel@vger.kernel.org, linux-tegra@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH RFC] iommu/dma: Validate page before accessing P2PDMA state Message-ID: References: <20260224104257.1641429-1-amhetre@nvidia.com> <20260224123221.GM10607@unreal> Precedence: bulk X-Mailing-List: iommu@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20260224123221.GM10607@unreal> On Tue, Feb 24, 2026 at 02:32:21PM +0200, Leon Romanovsky wrote: > On Tue, Feb 24, 2026 at 10:42:57AM +0000, Ashish Mhetre wrote: > > When mapping scatter-gather entries that reference reserved > > memory regions without struct page backing (e.g., bootloader created > > carveouts), is_pci_p2pdma_page() dereferences the page pointer > > returned by sg_page() without first verifying its validity. > > I believe this behavior started after commit 88df6ab2f34b > ("mm: add folio_is_pci_p2pdma()"). Prior to that change, the > is_zone_device_page(page) check would return false when given a > non‑existent page pointer. > Doesn't folio_is_pci_p2pdma() also check for zone device? I see[1] that it does: static inline bool folio_is_pci_p2pdma(const struct folio *folio) { return IS_ENABLED(CONFIG_PCI_P2PDMA) && folio_is_zone_device(folio) && folio->pgmap->type == MEMORY_DEVICE_PCI_P2PDMA; } I believe the problem arises due to the page_folio() call in folio_is_pci_p2pdma(page_folio(page)); within is_pci_p2pdma_page(). page_folio() assumes it has a valid struct page to work with. For these carveouts, that isn't true. Potentially something like the following would stop the crash: diff --git a/include/linux/memremap.h b/include/linux/memremap.h index e3c2ccf872a8..e47876021afa 100644 --- a/include/linux/memremap.h +++ b/include/linux/memremap.h @@ -197,7 +197,8 @@ static inline void folio_set_zone_device_data(struct folio *folio, void *data) static inline bool is_pci_p2pdma_page(const struct page *page) { - return IS_ENABLED(CONFIG_PCI_P2PDMA) && + return IS_ENABLED(CONFIG_PCI_P2PDMA) && page && + pfn_valid(page_to_pfn(page)) && folio_is_pci_p2pdma(page_folio(page)); } But my broader question is: why are we calling a page-based API like is_pci_p2pdma_page() on non-struct-page memory in the first place? Could we instead add a helper to verify if the sg_page() return value is actually backed by a struct page? If it isn't, we should arguably skip the P2PDMA logic entirely and fall back to a dma_map_phys style path. Isn't handling these "pageless" physical ranges the primary reason dma_map_phys exists? +mm list Thanks, Praan [1] https://elixir.bootlin.com/linux/v6.19.3/source/include/linux/memremap.h#L179 > If any fix is needed, the is_pci_p2pdma_page() must be changed and not iommu. > > Thanks > > > > > This causes a kernel paging fault when CONFIG_PCI_P2PDMA is enabled > > and dma_map_sg_attrs() is called for memory regions that have no > > associated struct page: > > > > Unable to handle kernel paging request at virtual address fffffc007d100000 > > ... > > Call trace: > > iommu_dma_map_sg+0x118/0x414 > > dma_map_sg_attrs+0x38/0x44 > > > > Fix this by adding a pfn_valid() check before calling > > is_pci_p2pdma_page(). If the page frame number is invalid, skip the > > P2PDMA check entirely as such memory cannot be P2PDMA memory anyway. > > > > Signed-off-by: Ashish Mhetre > > --- > > drivers/iommu/dma-iommu.c | 4 ++++ > > 1 file changed, 4 insertions(+) > > > > diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c > > index 5dac64be61bb..5f45f33b23c2 100644 > > --- a/drivers/iommu/dma-iommu.c > > +++ b/drivers/iommu/dma-iommu.c > > @@ -1423,6 +1423,9 @@ int iommu_dma_map_sg(struct device *dev, struct scatterlist *sg, int nents, > > size_t s_length = s->length; > > size_t pad_len = (mask - iova_len + 1) & mask; > > > > + if (!pfn_valid(page_to_pfn(sg_page(s)))) > > + goto post_pci_p2pdma; > > + > > switch (pci_p2pdma_state(&p2pdma_state, dev, sg_page(s))) { > > case PCI_P2PDMA_MAP_THRU_HOST_BRIDGE: > > /* > > @@ -1449,6 +1452,7 @@ int iommu_dma_map_sg(struct device *dev, struct scatterlist *sg, int nents, > > goto out_restore_sg; > > } > > > > +post_pci_p2pdma: > > sg_dma_address(s) = s_iova_off; > > sg_dma_len(s) = s_length; > > s->offset -= s_iova_off; > > -- > > 2.25.1 > > > > >