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Subject[PATCH v2 0/7] mm/vmalloc.c: allow vread() to read out vm_map_ram areas
Date
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@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 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

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