Re: [PATCH v4 1/3] mm/vmap: keep track of free blocks for vmap allocation

[Uladzislau Rezki <urezki@gmail.com>]

Hello, Andrew.

> An earlier version of this patch was accused of crashing the kernel:
> 
> https://lists.01.org/pipermail/lkp/2019-April/010004.html
> 
> does the v4 series address this?
I tried before to narrow down that crash but i did not succeed, so
i have never seen that before on my test environment as well as
during running lkp-tests including trinity test case:

test-url: http://codemonkey.org.uk/projects/trinity/

But after analysis of the Call-trace and slob_alloc(): 

<snip>
[    0.395722] Call Trace:
[    0.395722]  slob_alloc+0x1c9/0x240
[    0.395722]  kmem_cache_alloc+0x70/0x80
[    0.395722]  acpi_ps_alloc_op+0xc0/0xca
[    0.395722]  acpi_ps_get_next_arg+0x3fa/0x6ed
<snip>

<snip>
    /* Attempt to alloc */
    prev = sp->lru.prev;
    b = slob_page_alloc(sp, size, align);
    if (!b)
        continue;

    /* Improve fragment distribution and reduce our average
     * search time by starting our next search here. (see
     * Knuth vol 1, sec 2.5, pg 449) */
    if (prev != slob_list->prev &&
            slob_list->next != prev->next)
        list_move_tail(slob_list, prev->next); <- Crash is here in __list_add_valid()
    break;
}
<snip>

i see that it tries to manipulate with "prev" node that may be removed
from the list by slob_page_alloc() earlier if whole page is used. I think
that crash has to be fixed by the below commit:

https://www.spinics.net/lists/mm-commits/msg137923.html

it was introduced into 5.1-rc3 kernel.

Why ("mm/vmalloc.c: keep track of free blocks for vmap allocation")
was accused is probably because it uses "kmem cache allocations with struct alignment"
instead of kmalloc()/kzalloc(). Maybe because of bigger size requests
it became easier to trigger the BUG. But that is theory.

--
Vlad Rezki