Problem: *** Stephen reported vread() will skip vm_map_ram areas when reading out /proc/kcore with drgn utility. Please see below link to get more about it: /proc/kcore reads 0's for vmap_block https://lore.kernel.org/all/87ilk6gos2.fsf@xxxxxxxxxx/T/#u Root cause: *** The normal vmalloc API uses struct vmap_area to manage the virtual kernel area allocated and associate a vm_struct to store more information and passed out. However, area reserved through vm_map_ram() interface doesn't allocate vm_struct to bind with. So the current code in vread() will skip the vm_map_ram area through 'if (!va->vm)' conditional checking. Solution: *** There are two types of vm_map_ram area. One is the whole vmap_area being reserved and mapped at one time; the other is the whole vmap_area with VMAP_BLOCK_SIZE size being reserved at one time, while mapped into split regions with smaller size several times via vb_alloc(). I will call the 2nd type vb region. In patch 1 and 2, add flags into struct vmap_area to mark these two types of vm_map_ram area, meanwhile add bitmap field used_map into struct vmap_block to mark those vb regions being used to differentiate with dirty and free regions in vmap_block. With the help of above vmap_area->flags and vmap_block->used_map, we can recognize them in vread() and handle them respectively in patch 3. Besides, *** In patch 5, let's ignore vmap area with VM_UNINITIALIZED set in vm->flags, because this kind of area is created by calling __vmalloc_node_range(), VM_UNINITIALIZED set indicating it has vm_struct associated with, but is still during the page allocating and mapping process. In patch 6 and 7, change area flag from VM_ALLOC to VM_IOREMAP in two places. This will show them as 'ioremap' in /proc/vmallocinfo, and exclude them from /proc/kcore. Testing *** Only did the basic testing. Changelog *** v1->v2: - Change alloc_vmap_area() to pass in va_flags so that we can pass and set vmap_area->flags for vm_map_ram area. With this, no extra action need be added to acquire vmap_area_lock when doing the vmap_area->flags setting. Uladzislau reviewed v1 and pointed out the issue. - Improve vb_vread() to cover the case where reading is started from a dirty or free region. RFC->v1: - Add a new field flags in vmap_area to mark vm_map_ram area. It cold be risky reusing the vm union in vmap_area in RFC. I will consider reusing the union in vmap_area to save memory later. Now just take the simpler way to let's focus on resolving the main problem. - Add patch 4~7 for optimization. Baoquan He (7): mm/vmalloc.c: add used_map into vmap_block to track space of vmap_block mm/vmalloc.c: add flags to mark vm_map_ram area mm/vmalloc.c: allow vread() to read out vm_map_ram areas mm/vmalloc: explicitly identify vm_map_ram area when shown in /proc/vmcoreinfo mm/vmalloc: skip the uninitilized vmalloc areas powerpc: mm: add VM_IOREMAP flag to the vmalloc area sh: mm: set VM_IOREMAP flag to the vmalloc area arch/powerpc/kernel/pci_64.c | 2 +- arch/sh/kernel/cpu/sh4/sq.c | 2 +- include/linux/vmalloc.h | 1 + mm/vmalloc.c | 106 +++++++++++++++++++++++++++++------ 4 files changed, 92 insertions(+), 19 deletions(-) -- 2.34.1
