Donet Tom <donet...@linux.ibm.com> writes: > On 3/3/25 18:32, Aneesh Kumar K.V wrote: >> Donet Tom <donet...@linux.ibm.com> writes: >> >>> A vmemmap altmap is a device-provided region used to provide >>> backing storage for struct pages. For each namespace, the altmap >>> should belong to that same namespace. If the namespaces are >>> created unaligned, there is a chance that the section vmemmap >>> start address could also be unaligned. If the section vmemmap >>> start address is unaligned, the altmap page allocated from the >>> current namespace might be used by the previous namespace also. >>> During the free operation, since the altmap is shared between two >>> namespaces, the previous namespace may detect that the page does >>> not belong to its altmap and incorrectly assume that the page is a >>> normal page. It then attempts to free the normal page, which leads >>> to a kernel crash. >>> >>> In this patch, we are aligning the section vmemmap start address >>> to PAGE_SIZE. After alignment, the start address will not be >>> part of the current namespace, and a normal page will be allocated >>> for the vmemmap mapping of the current section. For the remaining >>> sections, altmaps will be allocated. During the free operation, >>> the normal page will be correctly freed. >>> >>> Without this patch >>> ================== >>> NS1 start NS2 start >>> _________________________________________________________ >>> | NS1 | NS2 | >>> --------------------------------------------------------- >>> | Altmap| Altmap | .....|Altmap| Altmap | ........... >>> | NS1 | NS1 | | NS2 | NS2 | >>> >> ^^^ this should be allocated in ram? >> > > Yes, it should be allocated from RAM. However, in the current > implementation, an altmap page gets allocated. This is because the > NS2 vmemmap section's start address is unaligned. There is an > altmap_cross_boundary() check. Here, from the vmemmap section > start, we identify the namespace start and check if the namespace start > is within the boundary. Since it is within the boundary, it returns false, > causing an altmap page to be allocated. During the PTE update, the > vmemmap start address is aligned down to PAGE_SIZE, and the PTE is > updated. As a result, the altmap page is shared between the current > and previous namespaces. > > If we had aligned the vmemmap start address, the > altmap_cross_boundary() function would return true because the > vmemmap section's start address belongs to the previous > namespace. Therefore normal page gets allocated. During the > PTE set operation, since the address is already aligned, the > PTE will updated. >
So the nvdimm driver should ensure that alignment right? I assume other things will also require that to be properly aligned.? -aneesh
