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[34.140.140.54]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-477bf1e868bsm188617465e9.4.2025.11.24.04.37.34 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 24 Nov 2025 04:37:35 -0800 (PST) Date: Mon, 24 Nov 2025 12:37:31 +0000 From: Mostafa Saleh To: Will Deacon Cc: linux-mm@kvack.org, iommu@lists.linux.dev, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, corbet@lwn.net, joro@8bytes.org, robin.murphy@arm.com, akpm@linux-foundation.org, vbabka@suse.cz, surenb@google.com, mhocko@suse.com, jackmanb@google.com, hannes@cmpxchg.org, ziy@nvidia.com, david@redhat.com, lorenzo.stoakes@oracle.com, Liam.Howlett@oracle.com, rppt@kernel.org, Qinxin Xia Subject: Re: [PATCH v2 3/4] drivers/iommu-debug-pagealloc: Track IOMMU pages Message-ID: References: <20251106163953.1971067-1-smostafa@google.com> <20251106163953.1971067-4-smostafa@google.com> Precedence: bulk X-Mailing-List: linux-doc@vger.kernel.org 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: On Thu, Nov 13, 2025 at 11:00:29AM +0000, Will Deacon wrote: > On Thu, Nov 06, 2025 at 04:39:52PM +0000, Mostafa Saleh wrote: > > Using the new calls, use an atomic refcount to track how many times > > a page is mapped in any of the IOMMUs. > > > > For unmap we need to use iova_to_phys() to get the physical address > > of the pages. > > > > We use the smallest supported page size as the granularity of tracking > > per domain. > > This is important as it possible to map pages and unmap them with > > larger sizes (as in map_sg()) cases. > > > > Signed-off-by: Mostafa Saleh > > Tested-by: Qinxin Xia > > --- > > drivers/iommu/iommu-debug-pagealloc.c | 74 +++++++++++++++++++++++++++ > > 1 file changed, 74 insertions(+) > > > > diff --git a/drivers/iommu/iommu-debug-pagealloc.c b/drivers/iommu/iommu-debug-pagealloc.c > > index a6a2f844b09d..0e14104b971c 100644 > > --- a/drivers/iommu/iommu-debug-pagealloc.c > > +++ b/drivers/iommu/iommu-debug-pagealloc.c > > @@ -27,16 +27,90 @@ struct page_ext_operations page_iommu_debug_ops = { > > .need = need_iommu_debug, > > }; > > > > +static struct page_ext *get_iommu_page_ext(phys_addr_t phys) > > +{ > > + struct page *page = phys_to_page(phys); > > + struct page_ext *page_ext = page_ext_get(page); > > + > > + return page_ext; > > +} > > + > > +static struct iommu_debug_metadate *get_iommu_data(struct page_ext *page_ext) > > +{ > > + return page_ext_data(page_ext, &page_iommu_debug_ops); > > +} > > + > > +static void iommu_debug_inc_page(phys_addr_t phys) > > +{ > > + struct page_ext *page_ext = get_iommu_page_ext(phys); > > + struct iommu_debug_metadate *d = get_iommu_data(page_ext); > > + > > + WARN_ON(atomic_inc_return(&d->ref) <= 0); > > Is it worth dumping some information about the page in addition to the > WARN_ON()? That way, you might be able to benefit from other debug > options (e.g. PAGE_OWNER) if they are enabled. These WARN_ON are for overflows, which should never happen. I initially thought about using the refcount_t, but it didn’t seem suitable as refcount_add() expects that the refcount is already “1” indicating that an object was already created which doesn’t fit in the semantics of what this is. Similar for refcount_dec(). In the next patch there is a WARN_ON for the refcount check to capture the mis-behaving context, I will add a debug print with the leaked physical address in that case as this is the important one. > > > + page_ext_put(page_ext); > > +} > > + > > +static void iommu_debug_dec_page(phys_addr_t phys) > > +{ > > + struct page_ext *page_ext = get_iommu_page_ext(phys); > > + struct iommu_debug_metadate *d = get_iommu_data(page_ext); > > + > > + WARN_ON(atomic_dec_return(&d->ref) < 0); > > nit: I can't see why you need memory ordering guarantees for the refcount, > so you could use the relaxed variants for the inc/dec operations. Will do. > > > + page_ext_put(page_ext); > > +} > > + > > +/* > > + * IOMMU page size might not match the CPU page size, in that case, we use > > + * the smallest IOMMU page size to refcount the pages in the vmemmap. > > + * That is important as both map and unmap has to use the same page size > > + * to update the refcount to avoid double counting the same page. > > + * And as we can't know from iommu_unmap() what was the original page size > > + * used for map, we just use the minimum supported one for both. > > + */ > > +static size_t iommu_debug_page_size(struct iommu_domain *domain) > > +{ > > + return 1UL << __ffs(domain->pgsize_bitmap); > > +} > > + > > void __iommu_debug_map(struct iommu_domain *domain, phys_addr_t phys, size_t size) > > { > > + size_t off; > > + size_t page_size = iommu_debug_page_size(domain); > > Since this is a debug feature, is it worth checking other properties of > the arguments too? For example, that phys is non-zero and that phys + > size doesn't overflow? > Makes sense, I will add some more checks. > > + for (off = 0 ; off < size ; off += page_size) { > > + if (!pfn_valid(__phys_to_pfn(phys + off))) > > + continue; > > + iommu_debug_inc_page(phys + off); > > + } > > } > > > > void __iommu_debug_unmap(struct iommu_domain *domain, unsigned long iova, size_t size) > > { > > + size_t off; > > + size_t page_size = iommu_debug_page_size(domain); > > + > > + for (off = 0 ; off < size ; off += page_size) { > > + phys_addr_t phys = iommu_iova_to_phys(domain, iova + off); > > + > > + if (!phys || !pfn_valid(__phys_to_pfn(phys + off))) > > + continue; > > Hmm, it looks weird to add 'off' to both 'iova' _and_ the resulting > physical address. Is that correct? > Yes, that's a bug, I will fix it. > > + iommu_debug_dec_page(phys); > > + } > > } > > > > void __iommu_debug_remap(struct iommu_domain *domain, unsigned long iova, size_t size) > > { > > + size_t off; > > + size_t page_size = iommu_debug_page_size(domain); > > + > > + for (off = 0 ; off < size ; off += page_size) { > > + phys_addr_t phys = iommu_iova_to_phys(domain, iova + off); > > + > > + if (!phys || !pfn_valid(__phys_to_pfn(phys + off))) > > + continue; > > + > > + iommu_debug_inc_page(phys); > > + } > > You can make the bulk of this code common with the unmap function. Will do. Thank, Mostafa > > Will