Re: [PATCH v6 4/4] mm: vmalloc: convert vread() to vread_iter()

[David Hildenbrand <david@redhat.com>]

On 22.03.23 15:55, Lorenzo Stoakes wrote:
> Having previously laid the foundation for converting vread() to an iterator
> function, pull the trigger and do so.
> 
> This patch attempts to provide minimal refactoring and to reflect the
> existing logic as best we can, for example we continue to zero portions of
> memory not read, as before.
> 
> Overall, there should be no functional difference other than a performance
> improvement in /proc/kcore access to vmalloc regions.
> 
> Now we have eliminated the need for a bounce buffer in read_kcore_iter(),
> we dispense with it, and try to write to user memory optimistically but
> with faults disabled via copy_page_to_iter_nofault(). We already have
> preemption disabled by holding a spin lock.
> 
> If this fails, we fault in and retry a single time. This is a conservative
> approach intended to avoid spinning on vread_iter() if we repeatedly
> encouter issues reading from it.

I have to ask again: Can you comment why that is ok? You might end up 
signaling -EFAULT to user space simply because swapping/page 
migration/whatever triggered at the wrong time.

That could break existing user space or which important part am I missing?

-- 
Thanks,

David / dhildenb