Re: [PATCH] mm/vmalloc: Check absolute error return from vmap_[p4d|pud|pmd|pte]_range()

[Anshuman Khandual <anshuman.khandual@arm.com>]

On 06/13/2019 03:03 PM, Roman Penyaev wrote:
> On 2019-06-13 10:12, Anshuman Khandual wrote:
>> vmap_pte_range() returns an -EBUSY when it encounters a non-empty PTE. But
>> currently vmap_pmd_range() unifies both -EBUSY and -ENOMEM return code as
>> -ENOMEM and send it up the call chain which is wrong. Interestingly enough
>> vmap_page_range_noflush() tests for the absolute error return value from
>> vmap_p4d_range() but it does not help because -EBUSY has been merged with
>> -ENOMEM. So all it can return is -ENOMEM. Fix this by testing for absolute
>> error return from vmap_pmd_range() all the way up to vmap_p4d_range().
> 
> I could not find any real external caller of vmap API who really cares
> about the errno, and frankly why they should?  This is allocation path,

map_vm_area() which is an exported symbol suppose to provide the right
error code regardless whether it's current users care for it or not.

> allocation failed - game over.  When you step on -EBUSY case something
> has gone completely wrong in your kernel, you get a big warning in
> your dmesg and it is already does not matter what errno you get.

Its true that vmap_pte_range() does warn during error conditions. But if
we really dont care about error return code then we should just remove
specific error details (ENOMEM/EBUSY) and instead replace them with simple
boolean false/true or (0/1/-1) return values at each level. Will that be
acceptable ? What we have currently is wrong where vmap_pmd_range() could
just wrap EBUSY as ENOMEM and send up the call chain.