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 details. /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 pass out. However, area reserved through vm_map_ram() interface doesn't allocate vm_struct to associate with. So the current code in vread() will skip the vm_map_ram area through 'if (!va->vm)' conditional checking. Solution: *** To mark the area reserved through vm_map_ram() interface, add field 'flags' into struct vmap_area. Bit 0 indicates this is vm_map_ram area created through vm_map_ram() interface, bit 1 marks out the type of vm_map_ram area which makes use of vmap_block to manage split regions via vb_alloc/free(). And also add bitmap field 'used_map' into struct vmap_block to mark those further subdivided 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 and handle vm_map_ram areas successfully. All these are done in patch 1~3. Meanwhile, do some improvement on areas related to vm_map_ram areas in patch 4, 5. And also change area flag from VM_ALLOC to VM_IOREMAP in patch 6, 7 because this will show them as 'ioremap' in /proc/vmallocinfo, and exclude them from /proc/kcore. Testing *** - I did basic testing on kvm guest, and run below commands to access kcore file to do statistics: makedumpfile --mem-usage /proc/kcore - Stephen helped to test v4 and confirm the problem he spotted and reported has been resolved with the v4 patchset. Changelog *** v4->v5: - Add a 'unlock' label and goto jumping in while loop of vmap_ram_vread() function, this improve the codes execution. This is done in patch 3. - Add Stephen's Reported-by and Tested-by tags in patch 3. v3->v4: - Fix typo in pach 2 catched by Lorenzo. - Add WARN_ON(flags == VMAP_BLOCK) in vread() to address Dan's concern that VMAP_BLOCK could be set alone in vmap->flags. - Add checking on the returned value from xa_load() in vmap_ram_vread(), Uladzislau commented on the risk of this place. - Fix a bug in s_show() which is changed in patch 5. The old change will cause 'va->vm is NULL but the VMAP_RAM flag is not set' case wrongly handled. Please see below link for details. - https://lore.kernel.org/all/Y8aAmuUY9OxrYlLm@kili/T/#u - Add Uladzislau and Lorenzo's Reviewed-by. v2->v3: - Benefited from find_unlink_vmap_area() introduced by Uladzislau, do not need to worry about the va->vm and va->flags reset during freeing. - Change to identify vm_map_area with VMAP_RAM in vmap->flags in vread(). - Rename the old vb_vread() to vmap_ram_vread(). - Handle two kinds of vm_map_area area reading in vmap_ram_vread() respectively. - Fix bug of the while loop code block in vmap_block reading, found by Lorenzo. - Improve conditional check related to vm_map_ram area, suggested by Lorenzo. 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 | 127 ++++++++++++++++++++++++++++++----- 4 files changed, 112 insertions(+), 20 deletions(-) -- 2.34.1
