linux-kernel - [PATCH RFC 0/3] mm/vmalloc.c: allow vread() to read out vm_map

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Date:   Wed,  9 Nov 2022 11:35:31 +0800
From:   Baoquan He <bhe@...hat.com>
To:     linux-kernel@...r.kernel.org
Cc:     linux-mm@...ck.org, akpm@...ux-foundation.org,
        stephen.s.brennan@...cle.com, urezki@...il.com, hch@...radead.org,
        Baoquan He <bhe@...hat.com>
Subject: [PATCH RFC 0/3] mm/vmalloc.c: allow vread() to read out vm_map_ram areas

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 by '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.

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 regions being used to differentiate with dirty
and free regions.

With the help of above vmap_area->flags and vmap_block->used_map, we can
recognize them in vread() and handle them respectively.

Test:
***
I don't know what system has vm_map_ram() area. So just pass compiling
test and execute "makedumpfile --mem-usage /proc/kcore" to guarantee it
won't impact the old kcore reading.

	[root@...-x3950x6-01 ~]# free -h
	               total        used        free      shared  buff/cache   available
	Mem:           3.9Ti       3.6Gi       3.9Ti       7.0Mi       497Mi       3.9Ti
	Swap:          8.0Gi          0B       8.0Gi
	[root@...-x3950x6-01 ~]# makedumpfile --mem-usage /proc/kcore
	The kernel version is not supported.
	The makedumpfile operation may be incomplete.

	TYPE		PAGES			EXCLUDABLE	DESCRIPTION
	----------------------------------------------------------------------
	ZERO		327309          	yes		Pages filled with zero
	NON_PRI_CACHE	81750           	yes		Cache pages without private flag
	PRI_CACHE	83981           	yes		Cache pages with private flag
	USER		12735           	yes		User process pages
	FREE		1055688908      	yes		Free pages
	KERN_DATA	17464385        	no		Dumpable kernel data 

	page size:		4096            
	Total pages on system:	1073659068      
	Total size on system:	4397707542528    Byte

Baoquan He (3):
  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

 include/linux/vmalloc.h |  1 +
 mm/vmalloc.c            | 81 +++++++++++++++++++++++++++++++++++++----
 2 files changed, 75 insertions(+), 7 deletions(-)

-- 
2.34.1