Re: [PATCH] mm/page_owner: use vmalloc instead of kmalloc

Miles Chen
2018-10-26T08:50:45Z

On Thu, 2018-10-25 at 12:27 -0700, Matthew Wilcox wrote:
> On Fri, Oct 26, 2018 at 02:34:41AM +0800, miles.chen@mediatek.com wrote:
> > The kbuf used by page owner is allocated by kmalloc(),
> > which means it can use only normal memory and there might
> > be a "out of memory" issue when we're out of normal memory.
> >
> > Use vmalloc() so we can also allocate kbuf from highmem
> > on 32bit kernel.
>
> ... hang on, there's a bigger problem here.
>
> static const struct file_operations proc_page_owner_operations = {
> .read = read_page_owner,
> };
>
> read_page_owner(struct file *file, char __user *buf, size_t count, loff_t *ppos)
> {
> ...
> return print_page_owner(buf, count, pfn, page,
> page_owner, handle);
> }
>
> static ssize_t
> print_page_owner(char __user *buf, size_t count, unsigned long pfn,
> struct page *page, struct page_owner *page_owner,
> depot_stack_handle_t handle)
> {mount -t debugfs none /sys/kernel/debug/
> ...
> kbuf = kmalloc(count, GFP_KERNEL);
>
> So I can force the kernel to make an arbitrary size allocation, triggering
> OOMs and forcing swapping if I can get a file handle to this file.
> The only saving grace is that (a) this is a debugfs file and (b) it's
> root-only (mode 0400). Nevertheless, I feel some clamping is called
> for here. Do we really need to output more than 4kB worth of text here?
>
I did a test on my device, the allocation count is 4096 and around 6xx
bytes are used each print_page_owner() is called. It looks like that
clamping the reading count to PAGE_SIZE is ok.

The following output from print_page_owner() is 660 bytes long, I think
PAGE_SIZE should be enough to print the information we need.

Page allocated via order 0, mask 0x6200ca(GFP_HIGHUSER_MOVABLE)
PFN 262199 type Movable Block 512 type Movable Flags 0x4003c(referenced|
uptodate|dirty|lru|swapbacked)
get_page_from_freelist+0x1580/0x1650
__alloc_pages_nodemask+0xcc/0xfa4
shmem_alloc_page+0xa4/0xc8
shmem_alloc_and_acct_page+0x138/0x2b8
shmem_getpage_gfp.isra.54+0x164/0xfc8
shmem_write_begin+0x84/0xcc
generic_perform_write+0xe8/0x210
__generic_file_write_iter+0x1d4/0x230
generic_file_write_iter+0x184/0x2e8
new_sync_write+0x144/0x1c4
vfs_write+0x194/0x278
ksys_write+0x64/0xd4
xwrite+0x34/0x84
do_copy+0xf4/0x168
flush_buffer+0x68/0xec
__gunzip+0x370/0x448