[PATCH v3 0/7] mm/vmalloc.c: allow vread() to read out vm_map_ram areas

Baoquan He
2023-01-13T03:20:30Z

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@oracle.com/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
***
Only did the basic testing on kvm guest, and run below command to
access kcore file to do statistics:

makedumpfile --mem-usage /proc/kcore

Changelog
***
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 | 114 ++++++++++++++++++++++++++++++-----
4 files changed, 101 insertions(+), 18 deletions(-)

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2.34.1